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SuperBProgress Reports
Physics
Accelerator
Detector
Computing
March 31, 2010
Abstract
This report describes the present status of the detector design
for SuperB. It is one of four separateprogress reports that, taken
collectively, describe progress made on the SuperB Project since
thepublication of the SuperB Conceptual Design Report in 2007 and
the Proceedings of SuperBWorkshop VI in Valencia in 2008.
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Contents
1 Introduction 11.1 The Physics Motivation . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . 11.2 The SuperB
Project Elements . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . 11.3 The Detector Design Progress Report . . . . . . .
. . . . . . . . . . . . . . . . . . . 2
2 Overview 22.1 Physics Performance . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . 32.2 Challenges on
Detector Design . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . 62.3 Open Issues . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . 72.4 Detector R&D . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . 8
3 Silicon Vertex Tracker 93.1 Detector concept . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.1 SVT and Layer0 . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 93.1.2 Performance Studies . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . 103.1.3 Background
Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . 12
3.2 Layer0 options under study . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . 123.2.1 Striplets . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.2.2
Hybrid Pixels . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . 143.2.3 MAPS . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . 143.2.4 Pixel Module
Integration . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. 15
3.3 A MAPS-based all-pixel SVT using a deep P-well process . . .
. . . . . . . . . . . . 163.4 R&D Activities . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4 Drift Chamber 194.1 Backgrounds . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . 204.2 Mechanical
Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . 204.3 Drift Chamber Geometry . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . 204.4 Gas Mixture . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . 214.5 Cell Design and Layout . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . 224.6 R&D work . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . 22
5 Particle Identification 245.1 Detector concept . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
5.1.1 Charged particle identification at SuperB . . . . . . . .
. . . . . . . . . . . . 245.1.2 BABAR DIRC . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . 24
5.2 Barrel PID at SuperB . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . 255.2.1 Performance optimization . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 255.2.2
Design and R&D status . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . 27
5.3 Forward PID at SuperB . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . 285.3.1 Motivation for a forward PID
detector . . . . . . . . . . . . . . . . . . . . . . 285.3.2
Forward PID requirements . . . . . . . . . . . . . . . . . . . . .
. . . . . . . 305.3.3 Status of the forward PID R&D effort . .
. . . . . . . . . . . . . . . . . . . . 30
6 Electromagnetic Calorimeter 336.1 Barrel Calorimeter . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
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6.2 Forward Endcap Calorimeter . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . 346.3 Backward Endcap Calorimeter . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 366.4 R&D
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . 39
6.4.1 Barrel Calorimeter . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . 396.4.2 Forward Calorimeter . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . 416.4.3 Backward
Calorimeter . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . 41
7 Instrumented Flux Return 427.1 Performance optimization . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
7.1.1 Identification Technique . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . 427.1.2 Baseline Design Requirements . .
. . . . . . . . . . . . . . . . . . . . . . . . . 427.1.3 Design
optimization and performance studies . . . . . . . . . . . . . . .
. . . 43
7.2 R&D . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . 447.2.1 R&D tests and
results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. 447.2.2 Prototype . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . 45
7.3 Baseline Detector Design . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . 467.3.1 Flux Return . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
8 Electronics, Trigger, DAQ and Online 478.1 Overview of the
Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . 47
8.1.1 Trigger Strategy . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . 478.1.2 Trigger Rates and Event Size
Estimation . . . . . . . . . . . . . . . . . . . . 498.1.3 Dead
Time and Buffer Queue Depth Considerations . . . . . . . . . . . .
. . 49
8.2 Electronics, Trigger and DAQ . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . 498.2.1 Fast Control and Timing
System . . . . . . . . . . . . . . . . . . . . . . . . . 508.2.2
Clock, Control and Data Links . . . . . . . . . . . . . . . . . . .
. . . . . . . 518.2.3 Common Front-End Electronics . . . . . . . .
. . . . . . . . . . . . . . . . . . 528.2.4 Readout Module . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 538.2.5
Experiment Control System . . . . . . . . . . . . . . . . . . . . .
. . . . . . . 538.2.6 Level 1 Hardware Trigger . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . 54
8.3 Online System . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . 558.3.1 ROM Readout and Event
Building . . . . . . . . . . . . . . . . . . . . . . . . 568.3.2
High Level Trigger Farm . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . 568.3.3 Data Logging . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . 568.3.4 Event Data
Quality Monitoring and Display . . . . . . . . . . . . . . . . . .
. 578.3.5 Run Control System . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . 578.3.6 Detector Control System . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . 578.3.7 Other
Components . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . 578.3.8 Software Infrastructure . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . 58
8.4 Front-End Electronics . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . 588.4.1 SVT Electronics . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 588.4.2 DCH
Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . 598.4.3 PID Electronics . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . 598.4.4 EMC Electronics . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
618.4.5 IFR Electronics . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . 62
8.5 R&D . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . 638.6 Conclusions . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. 64
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9 Software and Computing 649.1 The baseline model . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
9.1.1 The requirements . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . 659.2 Computing tools and services for
the Detector and Physics TDR studies . . . . . . . 66
9.2.1 Fast simulation of the SuperB detector . . . . . . . . . .
. . . . . . . . . . . . 669.2.2 Bruno: the SuperB full simulation
tool . . . . . . . . . . . . . . . . . . . . . . 69
9.3 Simulation output: Hits and MonteCarlo Truth . . . . . . . .
. . . . . . . . . . . . . 709.4 Staged simulation . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 709.5
Interplay with FastSim . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 70
9.5.1 The distributed production environment . . . . . . . . . .
. . . . . . . . . . . 719.5.2 The software development and
collaborative tools . . . . . . . . . . . . . . . . 77
10 Mechanical Integration 79
11 Budget and Schedule 7911.1 Detector Costs . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8011.2
Schedule . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . 85
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1.2 The SuperB Project Elements 1
1 Introduction
1.1 The Physics Motivation
The Standard Model successfully explains thewide variety of
experimental data that hasbeen gathered over several decades with
ener-gies ranging from under a GeV up to severalhundred GeV. At the
start of the millennium,the flavor sector was perhaps less explored
thanthe gauge sector, but the PEP-II and KEK-Basymmetric B
Factories, and their associatedexperiments BABAR and Belle, have
produceda wealth of important flavor physics highlightsduring the
past decade [1]. The most notableexperimental objective, the
establishment of theCabibbo-Kobayashi-Maskawa phase as consis-tent
with experimentally observed CP-violatingasymmetries in B meson
decay, was cited in theaward of the 2008 Nobel Prize to Kobayashi
&Maskawa [2].
The B Factories have provided a set of unique,over-constrained
tests of the Unitarity Triangle.These have, in the main, been found
to be con-sistent with Standard Model predictions. The Bfactories
have done far more physics than orig-inally envisioned; BABAR alone
has publishedmore than 400 papers in refereed journals todate.
Measurements of all three angles of theUnitarity Triangle - sin2α
and γ, in addition tosin 2β; the establishment of D0D̄0 mixing;
theuncovering of intriguing clues for potential NewPhysics in B→
K(?)l+l− and B→ Kπ and de-cays; and unveiling an entirely
unexpected, newspectroscopy, are some examples of
importantexperimental results beyond those initially
con-templated.
With the LHC now beginning operations, themajor experimental
discoveries of the next fewyears will probably be at the energy
frontier,where we would hope not only to complete theStandard Model
by observing the Higgs parti-cle, but to find signals of New
Physics which arewidely expected to lie around the 1 TeV
energyscale. If found, the New Physics phenomena willneed data from
very sensitive heavy flavor ex-
periments if they are to be understood in detail.Determining the
nature of the New Physics in-volved requires the information on
rare b, c andτ decays, and on CP violation in b and c quarkdecays
that only a very high luminosity asym-metric B Factory can provide
[3]. On the otherhand, if such signatures of New Physics are
notobserved at the LHC, then the excellent sensi-tivity provided at
the luminosity frontier by Su-perB provides another avenue to
observing NewPhysics at mass scales up to 10 TeV or morethrough
observation of rare processes involvingB and D mesons and studies
of LFV in τ decays.
1.2 The SuperB Project Elements
It is generally agreed that the physics being ad-dressed by a
next-generation B factory requiresa data sample that is some 50-100
times largerthan the existing combined sample from BABARand Belle,
or at least 50-75 ab−1. Acquiring suchan integrated luminosity in a
5 year time framerequires that the collider run at a luminosity
ofat least 1036cm−2s−1.
For a number of years, an Italian led, INFNhosted, collaboration
of scientists from Canada,Italy, Israel, France, Norway, Spain,
Poland, UKand the US have worked together to design andpropose a
high luminosity 1036 asymmetric BFactory project, called SuperB to
be built at ornear the Frascati laboratory [4]. The project,which
is managed by a project board, includesdivisions for the
accelerator, the detector, thecomputing, and the site &
facilities.
The accelerator portion of the projectemploys lessons learned
from modern low-emittance synchrotron light sources andILC/CLIC
R&D, and an innovative new ideafor the intersection region of
the storage rings[5], called crab waist, to reach luminosities
over50 times greater than those obtained by earlierB factories at
KEK and SLAC. There is nowan attractive, cost-effective accelerator
design,including polarization, which is being furtherrefined and
optimized [6]. It is designedto incorporate many PEP-II
components.This facility promises to deliver fundamen-
SuperB Detector Progress Report
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2 2 Overview
tal discovery-level science at the luminosityfrontier.
There is also an active international proto-collaboration
working effectively on the designof the detector. The detector team
draws heav-ily on its deep experience with the BABAR de-tector,
which has performed in an outstandingmanner both in terms of
scientific productivityand operational efficiency. BABAR serves as
thefoundation of the upgraded SuperB detector.
To date, the project has been very favorablyreviewed by several
international committees.This international community now awaits a
de-cision by the Italian government on its supportof the SuperB
project.
1.3 The Detector Design ProgressReport
This document describes the design and devel-opment of the
SuperB detector, which is basedon a major upgrade of BABAR. This is
oneof several descriptive ”Design Progress Reports(DPR)” being
produced by the SuperB projectduring the first part of 2010 to
motivate andsummarize the development, and present statusof each
major division of the project (Physics,Accelerator, Detector, and
Computing) so as topresent a snapshot of the entire project at a
in-termediate stage between the CDR, which waswritten in 2007, and
the TDR that is being de-veloped during the next year.
This ”Detector DPR” begins with a briefoverview of the detector
design, the challengesinvolved in detector operations at the
luminos-ity frontier, the approach being taken to opti-mize the
remaining general design choices, andthe R&D program that is
underway to developand validate the system and subsystem
designs.Each of the detector subsystems and the generaldetector
systems are then described in more de-tail, followed by a
description of the integrationand assembly of the full detector.
Finally, thepaper concludes with a discussion of detectorcosts and
a schedule overview.
References
[1] C. Amsler et al. (Particle Data Group),Physics Letters B667,
1 (2008).
[2]
http://nobelprize.org/nobel_prizes/physics/laureates/2008/press.html,and
http://www-public.slac.stanford.edu/babar/Nobel2008.htm.
[3] D. Hitlin et al. ”Proceedings of SuperBWorkshop VI: New
Physics at the SuperFlavor Factory”; arXiv:0810.1312v2
[hep-ph].
[4] M. Bona et al. ”SuperB : A High-Luminosity Heavy Flavour
Fac-tory: Conceptual Design Report”;arXiv:0709.0451v2 [hep-ex],
INFN/AE- 07/2, SLAC-R-856, LAL 07-15.
[5] P. Raimondi, 2nd LNF Workshop on Su-perB , Frascati, Italy,
March 16-18 2006,and Proceedings of the 2007 Particle Ac-celerator
Conference (PAC 2007) Albu-querque, New Mexico, USA, June
25-29.
[6] Design Progress Report for the SuperB Ac-celerator (2010) in
preparation.
2 Overview
The SuperB detector concept is based on theBABAR detector, with
those modifications re-quired to operate at a luminosity of
1036
or more, and with a reduced center-of massboost. Further
improvements needed to copewith higher beam-beam and other
beam-relatedbackgrounds, as well as to improve detector
her-meticity and performance, are also discussed,as is the
necessary R&D required to implementthis upgrade. Cost estimates
and the scheduleare described in Section 11.
The current BABAR detector consists of atracking system with a 5
layer double-sided sili-con strip vertex tracker (SVT) and a 40
layerdrift chamber (DCH) inside a 1.5T magnetic
SuperB Detector Progress Report
http://nobelprize.org/nobel_prizes/
physics/laureates/2008/press.htmlhttp://nobelprize.org/nobel_prizes/
physics/laureates/2008/press.htmlhttp://www-public.slac.
stanford.edu/babar/Nobel2008.htmhttp://www-public.slac.
stanford.edu/babar/Nobel2008.htm
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2.1 Physics Performance 3
field, a Cherenkov detector with fused silica barradiators
(DIRC), an electromagnetic calorime-ter (EMC) consisting of 6580
CsI(Tl) crystalsand an instrumented flux-return (IFR) com-prised of
both limited streamer tube (LST) andresistive plate chamber (RPC)
detectors for K0Ldetection and µ identification.
The SuperB detector concept reuses a num-ber of components from
BABAR: the flux-returnsteel, the superconducting coil, the barrel
of theEMC and the fused silica bars of the DIRC.The flux-return
will be augmented with addi-tional absorber to increase the number
of inter-actions lengths for muons to roughly 7λ. TheDIRC camera
will be replaced by multi-channelplate (MCP) photon detectors in
focusing con-figuration with fused silica optics to reduce
theimpact of beam related backgrounds and im-prove performance. The
forward EMC will fea-ture cerium-doped LSO (lutetium
orthosilicate)or LYSO (lutetium yttrium orthosilicate) crys-tals,
hereafter referred to as L(Y)SO crystals,which have a much shorter
scintillation timeconstant, a lower Moliére radius and better
ra-diation hardness than the current CsI(Tl) crys-tals, again for
reduced sensitivity to beam back-grounds and better position
resolution.
The tracking detectors for SuperB will benew. The current SVT
cannot operate at L =1036, and the DCH has reached the end of its
de-sign lifetime and must be replaced. To maintainsufficient ∆t
resolution for time-dependent CPviolation measurements with the
SuperB boostof βγ = 0.24, the vertex resolution will be im-proved
by reducing the radius of the beam pipe,placing the inner-most
layer of the SVT at a ra-dius of roughly 1.2 cm. This innermost
layerof the SVT will be constructed of either siliconstriplets or
MAPS or other pixelated sensors,depending on the estimated
occupancy frombeam-related backgrounds. Likewise the cellsize and
geometry of the DCH will be driven byoccupancy considerations. The
hermeticity ofthe SuperB detector, and thus its performancefor
certain physics channels will be improved byincluding a backwards
”veto-quality” EMC de-tector comprising a lead-scintillator stack.
The
justification for inclusion of a forward PID isless clear on
balance and remains under study.The baseline design concept is a
fast Cherenkovlight- based time-of-flight system.
[WE NEED A NEW FIGURE.]The SuperB detector concept is shown
in
Fig. 1. The top portion of this elevation viewshows the minimal
set of new detector compo-nents, with the most reuse of current
BABARdetector components; the bottom half showsthe configuration of
new components requiredto cope with higher beam backgrounds and
toachieve greater hermeticity.
2.1 Physics Performance
The SuperB detector design as described in theConceptual Design
Report [1] left open a num-ber of questions that have a large
impact on theoverall detector geometry. The main ones in-clude
estimating the effect of a PID device infront of the forward EMC,
the need of an EMCin the backward region, the position of the
in-nermost layer of the SVT and its internal geom-etry, the SVT-DCH
transition radius, and theamount and distribution of absorber in
the IFR.
The study of these options has been per-formed by evaluating the
physics reach of a setof benchmark decay channels or the overall
per-formance in the reconstruction of charged andneutral particles.
To accomplish this task a fastsimulation specifically developed for
SuperB hasbeen used (sec. 9), combined with a completeset of
analysis tools inherited for the most partfrom the BaBar
experiment. The main sourcesof machine background have also been
simulatedwith GEANT4 to estimate the rates and occu-pancies as a
function of the position. The mainresults of the ongoing
performance studies aresummarized in this section.
Time-dependent measurements are an impor-tant part of the SuperB
physics program. Tokeep a time resolution comparable to what
wasmeasured at BABAR, the SuperB reduced boostmust be compensated
with a much better vertexresolution by placing the innermost layer
of theSVT (Layer0) as close as possible to the IP. Themain factor
limiting the minimum distance from
SuperB Detector Progress Report
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4 2 Overview
Figure 1: Concept for the SuperB detector. The upper half shows
the baseline concept, and thebottom half adds a number of detector
optional configurations.
the IP is the hit rate from e+e− → e+e−e+e−background events. In
this context the perfor-mances of the hybrid pixels (1.08% X0,
14µmhit reso.) and striplets (0.40% X0, 8µm hitreso.) have been
compared. Simulation stud-ies of B0 → ΦK0S decays have shown that
witha boost βγ = 0.28 the hybrid pixels and thestriplets reach a
sin 2βeff per event error equalto BABAR at a distance of 1.5 cm and
2.0 cm,respectively. With βγ = 0.24 the error in-creases by 7-8%.
Similar conclusions also applyto B0 → π+π−. These results will help
decid-ing what is the most appropriate technology andposition for
the Layer0.
The BABAR SVT five-layer design was moti-vated by the request of
standalone tracking forlow-pT tracks and redundancy in case
severalmodules failed during operations. The defaultSuperB SVT
design consisting of a Layer0 plusa BABAR-like SVT detector has
been comparedwith two alternative models made of a total
of 5 or 4 layers. Studies of track parametersresolutions and B →
D∗K kinematic variablesand reconstruction efficiency have shown
thatwhen the number of layers is reduced the low-pT track
efficiency decreases significantly, whilethe tracks quality is
basically unaffected. Theseresults support a six-layer layout.
Studies have also shown that the best over-all SVT+DCH tracking
performance would beachieved when the outer radius of the SVT
iskept small (14 cm as in BABAR or even less) andthe inner wall of
the DCH is as close to the SVTas possible. However, though in the
SuperB de-tector there is not a fixed support tube as therewas in
BABAR, space between SVT and DCHmust be left to allocate the
cryostats for thesuper-conducting magnets in the interaction
re-gion. This constraint is expected to limit theminimum DCH inner
radius to about 20-25 cm.
The impact of a forward PID device has beenestimated analyzing
the physics reach in chan-
SuperB Detector Progress Report
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2.1 Physics Performance 5
nels such as B → K(∗)νν̄ by weighting the ad-vantage of having a
better PID information inthe forward region with the drawbacks
arisingfrom more material in front of the EM calorime-ter and a
slightly shorter DCH. Three detec-tor configurations have been
compared: BABAR,the SuperB baseline (no forward PID device),and the
configuration with the addition of atime-of-flight (TOF) detector
between the DCHand the forward EMC. The results for the de-cay mode
B → Kνν̄ with the tag side recon-structed in the semileptonic modes
are reportedin Fig. 2. The study shows that moving fromBABAR to the
SuperB detector instrumentedwith the TOF device the precision
S/
√S +B
increases by about 13%, of which 7-8% arisesfrom the increase of
the overall detector accep-tance because of the reduced boost and
5-6%is due to the improved pion/kaon separation inthe forward
region. The machine backgroundswere not included in the simulation.
The analy-sis will be repeated keeping them into account.
]-1Integrated Lumi[ab10 20 30 40 50 60 70
S/s
qrt
(S+B
)
1
2
3
4
5
6
νν+K→+Gains in Signal B
= 0.56)βγBaBar (
= 0.28)βγSuperB base-line (
base-line+TOF
νν+K→+Gains in Signal B
Figure 2: S/√S +B of B → Kνν̄ as a function
of the integrated luminosity in threedetector
configurations.
The backward calorimeter under considera-tion is designed to be
used in veto mode. Itsimpact on physics can be estimated by
study-ing the sensitivity of rare B decays with oneor more
neutrinos in the final state, which ben-efit from having a more
hermetic detection ofneutrals to reduce the background
contamina-tion. One of the most important benchmark
channels of this kind is B → τν. Preliminarystudies, not
including the machine backgrounds,indicate that when the backward
calorimeter isinstalled the statistical precision S/
√S +B is
enhanced by about 10%. The results are sum-marized in Fig. 3.
The top plot shows howS/√S +B changes as a function of the cut
on
Eextra (the total energy of charged and neutralparticles that
cannot be directly associated withthe reconstructed daughters of
the signal or tagB) with and without the backward EMC. Thesignal is
peaked at zero. The bottom plot showsthe ratio of S/
√S +B as a function of the Eextra
cut. The analysis will be repeated including themain sources of
machine background, which canaffect the Eextra distributions
significantly. Thepossibility of using the backward calorimeter asa
PID time-of-flight device is under study.
, GeVextraCut on E0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
-1 @
75
abS
+BS
2
4
6
8
10
12SuperB With BwdNo Bwd
=50000gensigN
=10000000
genbkgN
, GeVextraCut on E0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
w/o
S+BS
/w
ith
S+BS
0.850.9
0.951
1.051.1
1.151.2
1.25SuperB With Bwd/No Bwd
Figure 3: Top: S/√S +B as a function of the
cut on Eextra with (circles) and with-out (squares) the backward
EMC.Bottom: ratio of S/
√S +B as a func-
tion of the Eextra cut.
The presence of a forward PID or backwardEMC affects the maximum
extension of theDCH and therefore the tracking and the
dE/dxperformance in those regions. The impact of theTOF PID
detector is practically negligible be-
SuperB Detector Progress Report
-
6 2 Overview
cause it only takes a few centimeters from theDCH. On the other
hand, the effect of a forwardRICH device (∼ 20 cm DCH length
reduction)or the backward EMC (∼ 30 cm) is somewhatlarger. For
example, it is found a σ(p)/p in-crease of about 25% and 35% for
tracks withpolar angle of 23◦ and 150◦, respectively. Evenin this
case, however, the overall impact is gen-erally quite limited
because only a small fractionof tracks cross the extreme forward
and back-ward regions.
The IFR system will be upgraded by replac-ing the BABAR’s RPCs
and LSTs with layersof much faster extruded plastic scintillator
cou-pled to WLS fibers read out by APDs operatedin Geiger mode. The
identification of muonsand K0L is optimized with a GEANT4
simula-tion by tuning the amount of iron absorber andthe
distribution of the active detector layers.The current baseline
design has an iron thick-ness of 92 cm segmented with 8 layers of
scin-tillator. Preliminary estimates indicate a muonefficiency
larger than 90% for p > 1.5 GeV/cwhen the pion misidentification
rate is 2%.
2.2 Challenges on Detector Design
Besides the production of the short lived par-ticles that are
the main object of investigationof SuperBmany other phenomena
connected tothe collider operation generate long lived parti-cles
interacting with the SuperBdetector. Theselatter particles form the
machine background.
The problem of the machine backgroundis one of the leading
challenges of the Su-perBproject: each subsystem must be designedso
that its performances are minimally degradedbecause of the
occupancy produced by the back-ground hits moreover the detectors
must be pro-tected against deteriorations arising from radi-ation
damage.
In effect, what is required is to achieve detec-tor performances
and operational lifetimes sim-ilar or better than those achieved in
BABAR butat a two order of magnitude higher luminosity.
Backgrounds particles produced by beam gasscattering and by the
synchrotron radiationnear the interaction point (IP) are expected
to
be manageable since the relevant SuperBdesignparameter (mainly
the beam current) is fairlyclose to the PEP-II one.
Touschek backgrounds are expected to belarger than in BABAR
since the extremely lowdesign emittances of the SuperBbeams.
Pre-liminary simulations indicate that a system ofbeam collimators
upstream the IP can reducethe particles losses at tolerable
levels.
The main source of concern arise from thebackground particles
produced at the IP byQED processes whose cross section is order
of200 mb that corresponds at nominal luminosityto a rate order of
200 GHz. The main process isthe radiative Bhabha one (i.e.: e+e− →
e+e−γ)in which one of the incoming beam particleslooses a
significant fraction of its energy by theemission of a photon. Both
the photon andthe radiating beam particles emerge from theIP
traveling almost collinearly with respect tothe beam-line. The
magnetic elements down-stream the IP over-steers these primaries
parti-cles into the vacuum chamber walls producingelectro-magnetic
showers whose final productsare the backgrounds particles seen by
the sub-system. The particles of these electromagneticshowers can
also excite the giant nuclear res-onance of the material around the
beam lineexpelling neutrons from the nucleus. A carefuloptimization
of the mechanical aperture of thevacuum chambers and of the optical
functionsis needed to keep a large stay clear for the off-energy
primaries particles hence reducing thebackground rate.
The first Geant4 full Monte Carlo simula-tions of this process
at SuperB indicates that ashield around the beam line will be
required tokeep the electrons, positrons, photons and neu-trons
away from the detector both to keep occu-pancies and radiation
damage at a comfortablelevel.
Besides the “radiative Bhabha” the “quasielastic Bhabha” process
was also considered.The cross section for producing a primary
par-ticle reconstructed by the detector via this pro-cess is order
of 100 nb that correspond to a rateorder of 100 kHz. It is
reasonable to assume
SuperB Detector Progress Report
-
2.3 Open Issues 7
that this will be the driving term of the levelone trigger rate.
Single beam contributions tothe trigger rate are in fact expected
to be of thesame order of the BABAR one being the nomi-nal beam
currents and the other relevant designparameters comparables.
The final issue related to high luminosityis the production of
electron positron pairs atthe IP by the two photons process e+e−
→e+e−e+e− whose total cross section evaluatedat leading order with
the Monte Carlo genera-tor DIAG36 [2] is 7.3 mb that corresponds
atnominal luminosity to a rate of 7.3 GHz. Thepairs produced by
this process are characterizedby a very soft transverse momentum
spectrum.The solenoidal magnetic field in the trackingvolume
confines most of these background par-ticles inside the beam pipe.
The particles hav-ing a transverse momentum greather enough toreach
the beam pipe ( pt > 2.5 MeV/c) and witha momentum polar angle
inside the layer 0 ac-ceptance are produced with a rate ∼ 0.5 GHz
atnominal luminosity. This background will drivethe segmentation
size and the read-out architec-ture of the SVT layer 0. The
background traksurface rate on the SVT layer 0 as a function ofits
radius is reported on Fig.4. An effort to sim-
< 1.3)Helix diameter (cm) @ 1.5 T (-1.3 < 0.5 1 1.5 2 2.5
3
2 s
/cm
µCu
mul
ativ
e pa
rticl
es /
1
-110
1
10
210
Figure 4: Pairs background track rate for unitsurface as a
function of the SVTlayer 0 radius. Multiple track hits nottaken
into account.
ulate all these backgrounds with a Geant4 basedcode is underway
at present. A fairly accurate
model of the detector model and of the beamline elements is
available to the collaboration.Several configurations have been
simulated andstudied providing some preliminaries guidelinesto the
detector and machine teams. The final-ization of the interaction
region and detectordesign will require further developments of
theGeant4 background simulation tools on the de-tector response
side.
2.3 Open Issues
The basic geometry, structure and physics per-formance of the
SuperB detector is predeter-mined, in the main, by the retention of
theoverall magnet, return steel, and support struc-ture from the
BaBar detector, and a number ofits largest, and most expensive,
subsystems. Infact, even though this fixes both the basic
ge-ometry, and much of the physics performance,it does not really
constrain the expected per-formance of the SuperB detector in any
impor-tant respect. BaBar was already a fully opti-mized B-factory
detector for physics, and anyimprovements in performance that could
comefrom changing the overall layout or rebuildingthe large
subsystems would be modest over-all. The primary challenge for
SuperB is toretain physics performance similar to BaBar inthe
higher background environment described insubsection 2.2, while
operating at much higher( x50) data taking rates.
Within this overall constraint, optimizationof the geometrical
layout and new detector el-ements for the most important physics
chan-nels remains of substantial interest. The pri-mary tools for
sorting through the options are(1) simulation, performed under the
auspices ofa ”Detector Geometry Working Group”, thatstudies basic
tracking, PID, and neutrals per-formance of different detector
configurations, in-cluding their impact on each other, and
studiesthe physics reach for a number of benchmarkchannels; and (2)
detector R&D, including pro-totyping, developing new subsystem
technolo-gies, and understanding the costs, and robust-ness of
systems, as well as their impacts on eachother. The first item,
discussed in subsection
SuperB Detector Progress Report
-
8 2 Overview
2.1, clearly provides guidance to the second, asdiscussed in
subsection 2.4 and the subsystemchapters which follow, and vice
versa.
At the level of the overall detector, the imme-diate issue is to
define the detector envelopes.Optimization can and will continue
for sometime yet within each detector system. The stud-ies
performed to date leave us with the defaultdetector proposal, with
only a few open optionsremaining at the level of the detector
geometryenvelopes. These open issues are: (1) whetherthere is a
forward PID detector, and, if so, atwhat z location does the DCH
end and the EMCbegin, and (2) whether there is, or is not, a
back-ward EMC. These open issues are expected tobe resolved by the
Technical Board within thenext few months following further studies
by theDetector Geometry Working Group, in collabo-ration with the
relevant system groups.
2.4 Detector R&D
The SuperB detector concept rests, for the mostpart, on well
validated basic detector technol-ogy. Nonetheless, each of the
detectors has maychallenges due to the high rates and
demandingperformance requirements with R&D initiativesongoing
in all detector systems to improve thespecific performance, and
optimize the overalldetector design. These are described in
moredetail in each subsystem section.
The SVT innermost layer has to provide goodspace resolution
while coping with high back-ground. Although silicon striplets are
a viableoption at moderate background levels, a pixelsystem would
certainly be more robust againstbackground. Keeping the material in
a pixelsystem low enough not to deteriorate the vertex-ing
performance is challenging, and there is con-siderable activity to
develop thin hybrid pixelsor, even better, monolithic active
pixels. Thesedevices may be part of planned upgrade pathand
installed as a second generation layer 0. Ef-forts are directed
towards the development ofsensors, high rate readout electronics,
coolingsystems and mechanical support with low ma-terial
content.
In the DCH, many parameters must be op-timized for SuperB
running, such as the gasmixture and the cell layout. Precision
measure-ments of fundamental gas parameters are ongo-ing, as well
as studies with small cell chamberprototypes and simulation of the
properties ofdifferent gas mixtures and cell layouts. A possi-ble
improvement of the performance of the DCHis the innovative Cluster
Counting method, inwhich single cluster of charge are resolved
time-wise and counted, improving the resolution onthe track
specific ionization and the space accu-racy. This technique
requires significant R&Dto be proven feasible in the
experiment.
Though the Barrel PID system takes over ma-jor components from
BaBar, the camera andreadout are a significant step forward
requiringextensive R&D. The challenges include the per-formance
of pixelated PMTs for DIRC, the de-sign of the fused silica optical
system, the cou-pling of the fused silica optics to the existingbar
boxes, the mechanical design of the cam-era, and the choice of
electronics. Many of theindividual components of the new camera
arenow under active study by members of the PIDgroup , and runs are
underway with a single barprototype located in a cosmic ray
telescope. Afull scale (1/12 azimuth ) prototype incorporat-ing the
complete optical design is planned forcosmic ray tests during the
next two years.
End cap PID devices are less well understood,and whether or not
they are well motivatedfor the overall detector remains to be
demon-strated. Present R&D is centered on develop-ing a good
conceptual understanding of differ-ent proposed concepts, on
simulating how theirperformance effects the physics performance
ofthe detector, and on conceptual R&D for com-ponents of
specific devices to validate conceptsand highlight the technical
and cost issues.
The EMC barrel is a well understood deviceat the lower
luminosity of BaBar. Though therewill be some technical issues
associated with re-furbishing, the main R&D needed at present
isto understand the effects of pile-up in simula-tion, so as to be
able to design the appropriatefront-end shaping time for the
readout.
SuperB Detector Progress Report
-
9
The forward and backward EMCs are bothnew devices, using cutting
edge technology.Both will require one or more full beam
tests,hopefully at the same time, within the next yearor two.
Prototypes for these test are being de-signed and constructed.
Systematic studies of IFR system componentshave been performed
in a variety of bench andcosmic ray tests, leading to the present
proposeddesign. This design will be beam tested in a fullscale
prototype currently being prepared for aFermilab beam. This device
will demonstratethe muon identification capabilities as a func-tion
of different iron configurations, and willalso be able to study
detector efficiency and spa-tial resolution.
At present, the Electronics, DAQ, and Trig-ger (ETD), have been
designed for the baseluminosity of 1x1036cm−2sec−1, with
adequateheadroom. Further R&D must continue in orderto
understand the requirements at a luminosityup to 4 times greater,
and to insure that there isa smooth upgrade path when the present
designis inadequate. On a broad scale, as discussedin the system
chapter, each of the many com-ponents of ETD have numerous
technical chal-lenges that will require substantial R&D as
thedesign advances.
References
[1] SuperB Conceptual Design
Report,http://www.pi.infn.it/SuperB/CDRarXiv:0709.0451v2
[hep-ex]
[2] F. A. Berends, P. H. Daverveldt andR. Kleiss, “Monte Carlo
Simulation of TwoPhoton Processes. 2. Complete Lowest Or-der
Calculations for Four Lepton Produc-tion Processes in electron
Positron Colli-sions,” Comput. Phys. Commun. 40, 285(1986).
3 Silicon Vertex Tracker
3.1 Detector concept
3.1.1 SVT and Layer0
The main task of the Silicon Vertex Tracker is toprovide precise
position information on chargedtracks to perform measurement of
time- depen-dent CP asymmetries in B0 decays, which formthe basis
of the SuperB scientic program, as itdid for the frst generation of
asymmetric B Fac-tories. In addition, charged particles with
trans-verse momenta lower than 100 MeV/c will notreach the central
tracking chamber, so for theseparticles the SVT must provide the
completetracking information.
These goals have been reached in the BABARdetector with a five
layer of silicon strip de-tectors, shown schematically in Fig. 5.
TheBaBar SVT showed excellent performance forthe whole life of the
experiment, thanks to a ro-bust design that took into account the
physicsrequirements as well as enough safety margin,to coope with
the machine background, and re-dundancy considerations. The SuperB
SVT de-sign is based on the BaBar vertex detector lay-out with the
addition of a an innermost layervery close to the IP (Layer0). This
new layer isneeded, with the reduced beam energy asymme-try, to
improve the vertex resolution and to keepa time resolution for CP
measurement compa-rable to what was measured at BaBar.
Physicsstudies and background conditions, as explainedin detail in
the next two sections, set stringentrequirements on the Layer0
design: radius ofabout 1.5 cm, high granularity (50 × 50µm2pitch),
low material budget (about 1% X0), ad-equate radiation
resistance.
For the Technical Design Report preparationseveral options are
under study for the Layer0technology, with different levels of
maturity, ex-pected performance and safety margin againstbackground
conditions: striplets modules basedon high resistivity sensors with
short strips, hy-brid pixels and other thin pixel sensors based
onCMOS Monolithic Active Pixel Sensor (MAPS).
SuperB Detector Progress Report
http://www.pi.infn.it/SuperB/CDR
-
10 3 Silicon Vertex Tracker
580 mm
350 mrad520 mrad
ee +-
Beam Pipe
Space Frame
Fwd. support cone
Bkwd.supportcone
Front end electronics
Babar
SuperB Beam Pipe
SuperB Layer0
Figure 5: Longitudinal section of the SVT
The current baseline confguration for theLayer0 is based on the
striplets option, bee-ing the one that gives the better physics
perfor-mance, as detailed in next section. Neverthelessoptions with
pixel sensors, more robust in highbackground conditions, are beeing
developed,with specifc R&D programs, in order to meetall the
Layer0 requirements (i.e. low pitch andmaterial budget, high
readout speed and radia-tion hardness). This will allow the
replacementof the Layer0 striplets modules in a ”secondphase” of
the experiment. For this purpousethe SuperB interaction region and
the SVT me-chanics will be designed to ensure a rapid accessto the
detector for a fast replacement procedureof the Layer0.
The external SVT layers (1-5), with a ra-dius between 3 and 15
cm, will be build withthe same technology used for the BaBar
SVT(double sided silicon strip sensor), that is ade-quate with the
machine background conditionsexpected at that location in the
SuperB accel-erator scheme (i.e. with low beam currents).
The SVT angular acceptance, constrained bythe interaction region
design, will be 300 mradin both the forward and backward
directions,corresponding to a solid angle coverage of 95%in the
center-of-mass frame.
3.1.2 Performance Studies
The SuperB interaction region design is charac-terized by the
small size of the transversal sec-tion of the beams, at the level
of few µm forσx, and hundreds of nm for σy. Therefore itwill be
possible to reduce the radial dimensionof the beampipe tube, to 1
cm, while prevent-ing the beams to scatter into the beampipe
ma-terial within the detector coverage angle. Thetotal amount of
radial material of the berilliumbeampipe, which includes a few µm
of gold foil,and a water cooling channel, has been estimatedto be
less than 0.5% X0. For the proposedvalue for the center of mass
boost in SuperB,βγ = 0.28 (7 GeV e− beam against a 4 GeV e+
beam), the average B vertex separation alongthe z coordinate,
〈∆z〉 ' βγcτB = 125µm, isreduced almost by a half with respect to
theBABAR experiment, where βγ = 0.55. In or-der to maintain a
suitable resolution on ∆t fortime-dependent analyses, the proper
time differ-ence between the two B decays, it is necessaryto
improve the vertex resolution (about a factor2 better) with respect
to the current BABARperformances: typically 50− 80 µm in z for
ex-clusively reconstructed modes and 100−150 µmfor inclusively
reconstructed modes. The vertexprecision requirements for physics,
have beenachieved in the BABAR experiment, thanks tothe
performances of the silicon vertex tracker(SVT), a five-layer
double-sided silicon detec-
SuperB Detector Progress Report
-
3.1 Detector concept 11
tor. The configuration of the SuperB interac-tion region allows
to measure the first hit of thetracks near the production vertex,
by adding avertex detector layer (Layer0) very close to thebeampipe
and keeping the BABAR SVT layoutfor the outer layers. This
six-layer vertex de-tector solution would improve significantly
thetrack parameter determination, matching themore demanding
requirements on the vertexresolution, while maintaining the
stand-alonetracking capabilities for low momentum parti-cles.
The choice among the various options underconsideration for the
Layer0 (striplets, CMOSMAPS and hybrid pixels) has to take into
ac-count the physics requirements for the vertexresolution,
depending on the pitch and the to-tal amount of material of the
modules. In ad-dition, to assure optimal performance for
trackreconstruction, the sensor occupancy has to bemaintained under
the level of a few percent, im-posing further requirements on the
sensor seg-mentation and on the front-end electronics. Ra-diation
hardness should also be taken into ac-count, although it is
expected not to be par-ticularly demanding compared to LHC
detectorspecifications.
In order to simulate the resolution on the Bdecay vertices and
on ∆t for different Layer0configurations, we have used the fast
simula-tion program FastSim [?], which reproduces thedetector
response according to analytical pa-rameterizations. Several
studies have been per-formed where we have reconstructed
exclusivelyone B of the event (Breco) and evaluated theother B
(Btag) decay vertex using the chargedtracks of the rest of the
event after rejectinglong-lived particles and tracks not
compatiblewith the candidate vertex. We have consid-ered different
B decay modes as Breco, suchas B → π+π−, φK0S and also decay
modeswhere the impact of the Layer0 on the decayvertex
determination is less effective, such asB → K0SK0S , K0Sπ0. For
each decay mode wehave studied the resolution on ∆t and the
per-event error on the physical interesting quantitysin(2βeff
).
Figure 6: Resolution on the proper time differ-ence of the two B
mesons (βγ = 0.28),for different Layer0 radii, as a functionof
Layer0 thickness (in X0 %).
The main result is that the resolution on ∆tat SuperB allows a
comparable or even betterper-event error on sin(2βeff ) for each B
decaymode that we have considered. The conclusionis valid for all
the technologies that we have con-sidered for Layer0, (i.e., MAPS,
striplets, Hy-brid Pixels) and for reasonable values of theLayer0
radius and amount of radial material.As an example, in Fig. 6 is
reported the resolu-tion on ∆t for different Layer0 radii as a
func-tion of the Layer0 thickness (in X0 %) comparedto the BaBar
reference value. The dashed linerepresents the BABAR reference
value using thenominal value of the boost, βγ = 0.55, accord-ing to
FastSim.
We have also studied the impact of a possibleLayer0 inefficiency
on the sensitivity on sin(2β).The source of inefficiency could be
related toseveral causes, for example a much higher back-ground
rate than expected causing dead time inthe readout of the detector.
In Fig. 7 is reportedthe sin(2βeff ) per-event error for the B →
φK0Sdecay mode as a function of the Layer0 hit effi-ciency for the
different options (i.e. differentmaterial budget). The Layer0
radius in thestudy is about 1.6 cm. As it is evident fromthe plot,
the striplet solution allows for better
SuperB Detector Progress Report
-
12 3 Silicon Vertex Tracker
Hybrid pixels
Striplets
MAPS
Figure 7: sin2βeff per event error as a funcionof the Layer0
effciency for the dif-ferent options (i.e. different
materialbudget).
performances with respect to the BaBar refer-ence value even in
case of small inefficiency, andit has better performances compared
to otherLayer0 solutions. The main advantage of thestriplet
solution is the smaller amount of radialmaterial (about 0.5 % X0)
compared to the Hy-brid pixel (about 1% X0) and the MAPS solu-tions
(about 0.7 % X0). Infact, for particles ofmomentum up to few GeV/c
the multiple scat-tering effect is the dominant source of
uncer-tainty in the determination of their trajectoryand a low
material budget detector reduces thiseffect. A striplet-based
Layer0 solution wouldhave also a better intrinsic hit resolution
(about8 µm) with respect to the MAPS and the Hy-brid Pixel (about
14 µm with a digital readout)solutions. For those reasons a Layer0
based onstriplets has been chosen as the baseline solutionfor
SuperB, capable to cope with the machinebackground according to the
present estimates.
3.1.3 Background Conditions
Background considerations influence several as-pects of the
Silicon Vertex Tracker design:readout segmentation, electronics
shaping time,data transmission rate and radiation hardness.Severe
requirements are expecially imposed on
the Layer0 design. The different sources ofbackground have been
simulated with a detailedGeant4 detector and beamline description
to es-timate their impact on the experiment [1]. Thebackground
expected in the external layers ofthe SVT (radius > 3 cm) is
dominated by termsthat scale with beam currents and is similarto
background seen in the present BaBar SVT.The main background at the
Layer0 radius isdominated by luminosity terms, in particularby e+e−
→ e+e−e+e− pair production, beingradiative Bhabha events an order
of magnitudesmaller. Despite the huge cross section of thepair
production process, the rate of tracks hit-ting Layer0 is strongly
suppressed by the effectof the 1.5 Tesla magnetic field inside the
de-tector. Particles produced, with low transversemomentum, loop in
the detector magnetic fieldand only a small fraction reaches the
SVT lay-ers, with a strong radial dependence.
According to these studies the track rate atthe Layer0 at 1.5 cm
is at the level of about 5MHz/cm2, mainly due to electrons in the
MeVenergy range. The equivalent fluence corre-sponds toabout
3.5x1012n/cm2/yr and the doserate to whitstand is 3 Mrad/yr. It
seems ade-quate working with a safety factor of five on
thisbackground estimate.
3.2 Layer0 options under study
The present status for the development of thevarious Layer0
options under study for the Tech-nical Design Report preparation is
described inthis section.
3.2.1 Striplets
Double-sided silicon strips detectors (DSSD),200µm thick, with
50µm readout pitch repre-sent a proven and robust technology
meetingthe requirements on the SVT Layer 0 design, asdescribed in
the CDR [1]. In this design, shortstrips will be placed at an angle
of ±45◦ to thedetector edge on the two sides of the sensor, asshown
in Fig 8.
The strips will be connected to the readoutelectronics through a
a multilayer flexible cir-
SuperB Detector Progress Report
-
3.2 Layer0 options under study 13
Figure 8: Schematic view of the two sides of thestriplets
detector.
cuit glued to the sensor. A standard technologywith copper
traces is already available, althoughan aluminum microcable
technology is being ex-plored to reduce the impact on material of
theinterconnections.
Figure 9: Mechanical structure of a stripletsLayer 0 module.
The data-driven, high-bandwidth FSSR2readout chip [3], is a good
match to the Layer 0striplet design and is also suitable for the
read-out of the outer layers strip sensors. It has128 analog
channels providing a sparsified dig-ital output with address,
timestamp and pulse
height information for all hits. The selectableshaper peaking
time can be programmed downto 65 ns. The chip has been realized in
a 0.25µmCMOS technology for high radiation tolerance.The readout
architecture has been designed tooperate with a 132 ns clock that
will define thetimestamp granularity and the readout window.A
faster readout clock (70 MHz) is used in thechip, with a token pass
logic, to scan for thepresence of hits in the digital section, and
totransmit them off-chip, using a selectable num-ber of output data
lines. With six output lines,the chip can achieve an output data
rate of 840Mb/s. With a 1.83 cm strip length the expectedoccupancy
in the 132 ns time window is about12%, considering a hit rate of
100MHz/cm2,that includes the cluster multiplicity and a fac-tor 5
safety margin on the simulated backgroundtrack rate. The FSSR2
readout efficiency hasnever been measured with this occupancy.
Firstresults from ongoing Verilog simulations indi-cate the
efficiency is 90% or less. As shown inFig. 7 the physics impact
such an efficiency ismodest. Nonetheless it may be possible to
re-design the digital readout of the FSSR2 to in-crease the readout
efficiency at high occupancy.A total equivalent noise charge of 600
e− rms isexpected, including the effects of the strip andflex
circuit capacitance, as well as the metal se-ries resistance. The
signal to noise for a 200µmdetector is about 26, providing a good
noisemargin. It is also foreseen to conduct a mar-ket survey to
evaluate whether different readoutchips, possibly with a triggered
readout archi-tecture, may provide better performance.
Because of the unfavorable aspect ratio of thesensors, the
readout electronics needs to be ro-tated and placed along the beam
axis, outside ofthe sensitive volume of the detector, held by
acarbon fiber mechanical structure, as shown inFig.9. The 8 modules
forming Layer 0 will bemounted on flanges containing the cooling
cir-cuits. For the baseline design with striplets, theLayer 0
material budget will be about 0.46%X0for perpendicular tracks,
assuming a silicon sen-sor thickness of 200µm, a light module
supportstructure ( 100µm Silicon equivalent), similar
SuperB Detector Progress Report
-
14 3 Silicon Vertex Tracker
to the one used for the BABAR SVT modules,and the multilayer
flex contribution (3 flex lay-ers/module, 45µm Silicon
equivalent/layer).A reduction in the material budget to
about0.35%X0 is possible if kapton/aluminum micro-cable technology
can be employed with a tracepitch of about 50µm.
3.2.2 Hybrid Pixels
Hybrid pixels are a mature and viable solutionbut still requires
some R&D to meet Layer0 re-quirements (reduction in the
front-end pitch andin the total material budget, with respect to
hy-brid pixel systems developed for LHC experi-ments) A front-end
chip for hybrid pixel sensorwith 50 × 50 µm2 pitch and a fast
readout isunder development. The adopted readout ar-chitecture has
been previously developed by theSLIM5 Collaboration [4] for CMOS
Deep NWellMAPS [5],[6]: the data-push architecture fea-tures data
sparsification on pixel and timestampinformation for the hits. This
readout has beenrecently optimized for the target Layer0 rate of100
MHz/cm2 with promising results: VHDLsimulation of a full size
matrix (1.3 cm2) giveshit efficiency above 98% operating the
matrixwith a 60 MHz readout clock. A first proto-type chip with 4k
pixels has been submitted inSeptember 2009 with the ST
Microelectronics130 nm process and is currently under test.
Thefront-end chip, connected by bump-bonding toan high resistivity
pixel sensor matrix, will bethen characterized with beams in Autumn
2010.
3.2.3 MAPS
CMOS MAPS are a newer and more challengingtechnology. Their main
advantage with respectto hybrid pixels is that they could be very
thin,having the sensor and the readout incorporatedin a single CMOS
layer, only a few tenth of mi-crons thick. As the readout speed is
anotherrelevant aspect for application in the SuperBLayer0 we
proposed a new design approach toCMOS MAPS [5] which for the first
time madeit possible to build a thin pixel matrix featur-ing a
sparsified readout with timestamp infor-
Figure 10: The DNW MAPS concept.
mation for the hits [6]. In this new design thedeep N-well (DNW)
of a triple well commercialCMOS process is used as charge
collecting elec-trode and is extended to cover a large fractionof
the elementary cell (Fig. 3.2.3). Use of a largearea collecting
electrode allows the designer toinclude also PMOS transistors in
the front-end,therefore taking full adavantage of the proper-ties
of a complementary MOS technology forthe design of high performance
analog and dig-ital bocks. However, in order to avoid a
signif-icant degradation in charge collection effciency,the area
covered by PMOS devices and their N-wells, acting as parasitic
collection centers, hasto be small with respect to the DNW
sensorarea. Note that use of a charge preamplifier asthe input
stage of the channel makes the chargesensitivity independent of the
detector capaci-tance. The full signal processing chain
imple-mented at the pixel level (charge preamplifier,shaper,
discriminator and latch) is partly real-ized in the p-well
physically overlapped with thearea of the sensitive element,
allowing the de-velopment a complex in-pixel logic with
similarfunctionalities as in hybrid pixels.
Several prototype chips (the “APSEL” series)have been realized
with the STMicroelectron-ics, 130 nm triple well technology and
provedthe proposed approach is very promising forthe realization of
a thin pixel detector. TheAPSEL4D chip, a 4k pixel matrix with 50×
50µm2 pitch, with a new DNW cell and the spar-sified readout has
been characterized duringthe SLIM5 testbeam showing encouraging
re-
SuperB Detector Progress Report
-
3.2 Layer0 options under study 15
sults [7]. Hit efficiency of 92% has been mea-sured, a value
compatible with the present sen-sor layout that is designed with a
fill factor (i.e.the ratio of the electrode over the total
n-wellarea) of about 90%. Margins to improve the de-tection
efficiency with a different sensor layoutare beeing currently
investigated [8]
Several issues still need to be solved todemonstrate the ability
to build a working de-tector with this technology and require
someR&D. Among others the scalability to largermatrix size and
the radiation hardness of thetechnology are under evaluation for
the TDRpreparation.
3.2.4 Pixel Module Integration
To minimize the detrimental effect of multiplescattering the
reduction of the material is cru-cial for all the components of the
pixel modulein the active area.
The pixel module support structure needs toinclude a cooling
system to evacuate the powerdissipated by the front-end
electronics, about2W/cm2, present in the active area. The pro-posed
module support will be realized with alight carbon fiber support
with integrated mi-crochannels for the coolant fluid (total
materialbudget for support and cooling below 0.3 % X0).Measurements
on first support prototypes real-ized with this cooling technique
indicate that acooling system based on microchannels can bea viable
solution to the thermal and structuralproblem of Layer0 [10].
The pixel module will also need a light multi-layer bus
(Al/kapton based with total materialbudget of about 0.2 % X0), with
power/signalinputs and high trace density for high dataspeed to
connect the front-end chips in the ac-tive area to the HDI hybrid,
in the periphery ofthe module. With the data push
architecturepresently under study and the high backgroundrate
expected data with a 160 MHz clock needto be transfered on this
bus. With triggeredreadout architecture (beeing also
investigated)the complexity of the pixel bus, and
materialassociated, will be reduced.
Figure 11: Schematic drawing of the full Layer0made of 8 pixel
modules mountedaround the beam pipe with a pin-wheel
arrangement.
Considering the various pixel module compo-nents (sensor and
front-end with 0.4% X0, sup-port with cooling, and multilayer bus
with de-coupling capacitors) the total material in the ac-tive area
for the Layer0 module design based onhybrid pixel is about 1% X0.
For a pixel moduledesign based on CMOS MAPS, where the
con-tribution of the sensor and the integrated read-out electronics
become almost negligible, 0.05%X0, the total material budget is
about 0.65% X0.A schematic drawing of the full Layer0 madeof 8
pixel modules mounted around the beampipe with a pinwheel
arrangement is shown inFig. 3.2.4.
Due to the high background rate at theLayer0 location
radiation-hard fast links be-tween the pixel module and the DAQ
systemlocated outside the detector should be adopted.For all Layer0
options (that currently sharea similar data push architecture) the
untrig-gered data rate is 16 Gbit/s/readout section, as-suming a
background hit rate of 100Mhz/cm2.Triggered data rate is reduced to
about 1Gbit/s/readout section.
The HDI. positioned at the end of the mod-ule, outside the
active area, will be designed tohost several IC components: some
glue logic,buffers, fast serializers, drivers. The compo-
SuperB Detector Progress Report
-
16 3 Silicon Vertex Tracker
nents should be radiation hard for the applica-tion at the
Layer0 location (several Mrad/yr).
The baseline option for the link between theLayer0 modules and
the DAQ boards is cur-rently based on a mixed solution. A fast
cop-per link is forseen between the HDI and an in-termediate
transition board, positioned in anarea of moderate radiation levels
(several tens ofkrad/yr). On this transition card the logic withLV1
buffers will store the data until the recep-tion of the LV1 trigger
signal and only triggereddata will be send to the DAQ boards with
an op-tical link of 1 Gibt/s. The various pixel moduleinterfaces
will be characterized in a test set-upfor the TDR preparation.
3.3 A MAPS-based all-pixel SVT using adeep P-well process
Another alternative under evaluation is to havea all-pixel SVT
using MAPS pixels with a pixelsize of 50x50 µm. This approach uses
the180 nm INMAPS process which incorporates adeep P-well. A
perceived limitation of standardMAPS is not having full CMOS
capability as theadditional N-wells from the PMOS
transistorsparasitically collect charge, thus reducing thecharge
collected by the readout diode. Avoid-ing the usage of PMOS
transistors however doeslimit the capability of the readout
circuitry sig-nificantly. A limited use of PMOS is allowedwith the
DNW MAPS design (APSEL chips),which anyway accounts for a small
degradationin the collection efficiency. Therefore, a specialdeep
P-well layer was developed to overcomethe problems mentioned above.
The deep P-well protects charge generated in the epitaxiallayer
from being collected by parasitic N-wellsfor the PMOS. This then
ensures that all chargeis being collected by the readout diode and
max-imises charge collection efficiency. This is illus-trated in
Figure 12. This enhancement allowsthe use of full CMOS circuitry in
a MAPS andopens completely new possibilities for
in-pixelprocessing. The TPAC chip [11] for CALICE-UK [12, 13] has
been designed using the IN-MAPS process. The basic TPAC pixel has
asize of 50 × 50 µm and comprises a preampli-
fier, a shaper and a comparator [11]. The pixelonly stores hit
information in a Hit Flag. Thepixel is running without a clock and
the timinginformation is provided by the logic queryingthe Hit
Flag. For the SuperB application thepixel design was slightly
modified. Instead ofjust a comparator, a peak-hold latch was
addedto store the analog information as well. Thechip is organised
in columns with a commonADC at the end of each column. The ADC
isrealised as a Wilkinson ADC using a 5 MHzclock rate. The
simulated power consumptionfor each individual pixel is 12 µW. The
columnlogic constantly queries the pixels, but only digi-tises the
information for the pixels with a ”HitFlag”. This allows one to
save both space andreduce the power usage and since the speed ofthe
chip is limited by the ADC also increases thereadout speed. Both
the address of the pixel be-ing hit and its ADC output are stored
in a FIFOat the end of the column. To further increasethe readout
speed, the ADC uses a pipelined ar-chitecture with 4 analog input
lines to increasethroughput of the ADC. One of the main
bot-tlenecks is getting the data off the chip. It isenvisaged to
use the Level 1 trigger informa-tion to reject most of the events
and to reducethe data rate on-chip before moving it off-chip.This
will significantly reduce the data rate andtherefore also the
amount of power and servicesrequired .
For the outer layers, the requirements aremuch more relaxed in
terms of occupancy, so inorder to reduce the power, it is planed to
mul-tiplex the ADC’s to let them handle more thanone column in the
sensor. This is possible dueto the much smaller hit rate in the
outer layersand the resulting relaxed timing requirements.
An advantage of the MAPS is the eliminationof a lot of readout
electronics, because every-thing is integrated in the sensor
already whuchsimplifies the assembly significantly. Also sincewe
are using a industry CMOS processs, therea significant price
advantage compared to stan-dard HEP-style silicon and the additinal
savingsdue to the elimination of a dedicated readoutASIC.
SuperB Detector Progress Report
-
3.4 R&D Activities 17
(a) CMOS MAPS without a deep P-well implant (b) CMOS MAPS with a
deep P-well implant
Figure 12: A CMOS MAPS without a deep P-well implant (left) and
with a deep P-well implant(right).
In order to evaluate the physics potential ofMAPS based
all-pixel vertex detector we arecurrently evaluating the
performance of the Su-perB detector with different geometries of
theSVT , ranging from the SuperB baseline (Layer0+ 5 layers based
on strip detectors), through toa 4 or 6 layer all-pixel detector
with a realisticmaterial budget for the support structure for
alllayers.
3.4 R&D Activities
The technology for the Layer 0 baseline stripletdesign is
well-established but the front-end chipto be used, due to the high
background occu-pancy expected, requires some deeper
investi-gation. Performance of the FSSR2 chip, pro-posed for the
readout of the striplets and theouter layer strip sensors, are
beeing evaluated asa function of the occupancy with Verilog
simu-tion. Measurements are also possible in a test-bench in
preparation with real striplets modulesreadout with the FSSR2
chips. The redesign ofthe digital readout of the chip will be
investigateto improve its efficiency. The modification ofthe analog
part of the chip for the readout of thelong module of the external
layers are currentlyunder study. The multilayer flexible circuit,
toconnect the striplets sensor to the frontend, may
benefit from some R&D to reduce the materialbudget: either
reduce the minimum pitch onthe Upilex circuit, or adopt
kapton/aluminummicrocables and Tape Automated Bonding sol-dering
techniques with a 50µm pitch.
Although silicon striplets are a viable optionat moderate
background levels, a pixel systemwould certainly be more robust
against back-ground. Keeping the material in a pixel systemlow
enough not to deteriorate the vertexing per-formance is
challenging, and there is consider-able activity to develop thin
hybrid pixels or,even better, monolithic active pixels. These
de-vices may be part of planned upgrade path andinstalled as a
second generation layer 0.
A key issue for the readout of the pixel in theLayer0 is the
development of a fast readout ar-chitecture to cope with a pixel
rate of the orderof 100MHz/cm2. A first front-end chip for hy-brid
pixel sensor with 50 × 50 µm2 pitch anda fast readout, data driven
with timestamp forthe hits, has been realized and is currently
un-der test. A further development of the architec-ture is beiing
pursued to evolve toward a trig-gered readout architecture,
hellpful to reducethe complexity of the pixel module and possi-bly
to reduce its material budget.
SuperB Detector Progress Report
-
18 References
The CMOS MAPS technology is very promis-ing for an alternative
design of the Layer 0,but extensive R&D is still needed to meet
allthe requirements. Key aspects to be addressedare: sensor
efficiency and its radiation toler-ance, power consumption, and as
in the hybridpixel, the readout speed of the architecture
im-plemented.
After the realization of the APSEL chips withthe ST 130 nm DNW
process, with very encour-aging results, the Italians collaborators
involvedin the CMOS MAPS R&D are now evaluatingthe possibility
to improve MAPS performancewith the use of modern vertical
integration tech-nologies [9]. A first step in this direction
hasbeen the realization of a two-tier DNW MAPSby face to face
bonding of two 130 µm CMOSwafer in the Chartered/Tezzaron process.
Hav-ing the sensor and the analog part of the pixelcell in one tier
and the digital part in the sec-ond tier can improve significantly
the efficiencyof the CMOS sensor and allow a more complexin-pixel
logic. The first submission of verticallyintegrated DNW MAPS, now
in fabrication, in-cludes a 3D version of a 8x32 MAPS matrix
withthe same sparsified readout implemented in theAPSEL chips. A
new submission is foreseen inAutumn 2010 with a new generation of
the 3DMAPS implementing a faster readout architec-ture under
development, which is still data pushbut could be quite easily
evolve toward a trig-gered architecture.
The development of a thin mechanical sup-port structure with
integrated cooling for thepixel module is continuing with promizing
re-sults. Prototypes with light carbon fiber mi-crochannels for the
coolant fluid (total materialdown to 0.15% X0) have been produced
andtested and are able to evacuate specific powerup to 1.5W/cm2
mantaining the pixel moduletemperature within the requirements.
Thesesupports could be used for hybrid pixel as forMAPS
sensors.
References
[1] The SuperB Conceptual Design Report,INFN/AE-07/02,
SLAC-R-856, LAL 07-15, Available online at:
http://www.pi.infn.it/SuperB
[2] FastSim ref, Available online at:
http://www.pi.infn.it/SuperB
[3] V. Re et al., IEEE Trans. Nucl. Sci. 53,2470 (2006).
[4] SLIM5 Collaboration - Silicon detectorswith Low Interaction
with Material, http://www.pi.infn.it/slim5/
[5] G. Rizzo for the SLIM5 Collaboration.,”Development of Deep
N-Well MAPS in a130 nm CMOS Technology and Beam TestResults on a
4k-Pixel Matrix with Digi-tal Sparsified Readout”’, 2008 IEEE
Nu-clear Science Symposium, Dresden, Ger-many, 19-25 October,
2008
[6] A. Gabrielli for the SLIM5 Collabora-tion, ”Development of a
triple well CMOSMAPS device with in-pixel signal process-ing and
sparsified readout capability” Nucl.Instrum. Meth. A 581 (2007)
303.
[7] M. Villa for the SLIM5Collaboration,”Beam-Test Results of4k
pixel CMOS MAPS and High Resistiv-ity Striplet Detectors equipped
with digitalsparsified readout in the Slim5 Low MassSilicon
Demonstrator Nucl. Instrum. Meth.A (2010)
doi:10.1016/j.nima.2009.10.035
[8] E.Paoloni for the VIPIX collaboration,Beam Test Results of
Different Configura-tions of Deep N-well MAPS Matrices Fea-turing
in Pixel Full Signal Processing, Pro-ceedings of the XII Conference
on Instru-mentation, Vienna 2010. To be publishedin Nucl. Instr.
Meth, in Phys. Res. SectionA
[9] R. Yarema, “3D circuit integration for ver-tex and other
detectors”, Proceedings 16th
SuperB Detector Progress Report
http://www.pi.infn.it/SuperBhttp://www.pi.infn.it/SuperBhttp://www.pi.infn.it/SuperBhttp://www.pi.infn.it/SuperBhttp://www.pi.infn.it/slim5/http://www.pi.infn.it/slim5/
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19
International Workshop on Vertex Detec-tors (VERTEX2007), Lake
Placid (NY,USA), September 23 - 28, 2007, Proceed-ings of Science
PoS(Vertex 2007)017.
[10] F.Bosi and M. Massa, “Development andExperimental
Characterization of Proto-types for Low Material Budget
SupportStructure and Cooling of Silicon PixelDetectors, Based on
Microchannel Tech-nology” Nucl. Instrum. Meth. A
(2010)doi:10.1016/j.nima.2009.10.138
[11] J. A. Ballin et al., “Monolithic Active PixelSensors (MAPS)
in a quadruple well tech-nology for nearly 100% fill factor and
fullCMOS pixels,”, Sensors 8 (2008) 5336.
[12] N. K. Watson et al., “A MAPS-based read-out of an
electromagnetic calorimeter forthe ILC,” J. Phys. Conf. Ser. 110
(2008)092035.
[13] J. P. Crooks et al., “A monolithic activepixel sensor for a
tera-pixel ECAL at theILC,” CERN-2008-008.
4 Drift Chamber
The SuperB drift chamber provides the chargedparticle momentum
measurements and mea-surements of ionization energy loss used for
par-ticle identification. This is the only device inSuperBto
measure velocities of particles havingmomenta below approximately
700 MeV/c. Itsdesign is based on that of BABAR, which has 40layers
of centimetre-sized cells strung approxi-mately parallel to the
beam line [1]. A subset oflayers are strung at a small stereo angle
in or-der to provide measurements along z, the beamaxis.
The drift chamber is required to provide mo-mentum measurements
with the same preci-sion as the BABAR drift chamber
(approximately0.4% for tracks with a transverse momentum of1
GeV/c), and like BABAR uses a helium-basedgas mixture in order to
minimize measurementdegradation from multiple scattering. The
chal-lenge is to achieve comparable or better perfor-mance than
BABAR but in a high luminosity en-vironment. Both physics and
background rateswill be significantly higher than in BABAR and asa
consequence the system is required to accom-modate the 100-fold
increase in trigger rate andluminosity-related backgrounds
primarily com-posed of radiative Bhabhas and
electron-pairbackgrounds from two-photon processes. How-ever, the
beam current related backgrounds willonly be modestly higher than
in BABAR. Thenature and spatial distributions of these back-grounds
dictate the overall geometry of the driftchamber.
The ionization loss measurement is requiredto be at least as
sensitive to particle discrimi-nation as BABAR which has a dE/dx
resolutionof 7.5%. In BABAR, conventional dE/dx driftchamber
methods were used in which the to-tal charge deposited on each
sense wire was av-eraged after removing the highest 20% of
themeasurements as a means of controlling Landaufluctations. In
addition to this conventional ap-proach, the SuperB drift chamber
group is ex-ploring a cluster counting option which, in prin-
SuperB Detector Progress Report
-
20 4 Drift Chamber
ciple, can improve the dE/dx resolution by ap-proximately a
factor of two. This technique in-volves counting individual
clusters of electronsreleased in the gas ionization process. In
sodoing, we remove the sensitivity of the specificenergy loss
measurement to fluctuations in thenumbers of electrons produced in
each cluster,fluctuations which significantly limit the intrin-sic
resolution of conventional dE/dx measure-ments. As no experiment
has employed clustercounting , this is very much a detector
researchand development project but one which poten-tially yields
significant physics payoff at SuperB.
4.1 Backgrounds
The dominant source of background in the Su-perBDCH is expected
to be radiative Bhabhascattering. Photons radiated collinearly
tothe initial e− or e+ direction can bring thebeams off-orbit and
ultimately produce show-ers on the machine optic elements. This
processcan happen meters away from the interactionpoint and the
hits are in general uniformly dis-tributed over the drift chamber
volume. Large-angle e+e− → e+e−(γ) scattering has the well-known
1/ϑ4 cross section; simulation studies arepresently underway to
evaluate the need to de-sign tapered endcaps (either conical or a
withstepped shape) at small radii to keep under con-trol the
occupancy in the very forward regionof the detector. The actual
occupancy and itsgeometrical distribution in the detector dependon
the details of the machine elements, on theamount and placement of
shields, on the driftchamber geometry, and on the time needed
tocollect the signal in the detector. Preliminaryresults obtained
with GEANT4 simulations in-dicate that in a 1µs time window at
nominalluminosity (1036 cm−2s−1) the occupancy aver-aged over the
whole drift chamber volume is3.5 %, and slightly larger (about 5 %)
in the in-ner layers. Intense work is presently underwayto validate
these results and study their depen-dence on relevant
parameters.
4.2 Mechanical Structure
The drift chamber mechanical structure mustsustain the wire load
– about 3 tons for 10 000cells – with small deformations, while at
thesame time offer minimum material to the sur-rounding detectors.
Carbon Fiber-resin com-posites have high elastic modulus and low
den-sity, thus offering performances superior toAluminum-alloys
based structures. Endplateswith curved geometry can further reduce
mate-rial thickness with respect to flat endplates fora given
deformation under load. For example,the KLOE drift chamber [2]
features 8 mm thickCarbon Fiber spherical endplates of 4 m
diam-eter. Preliminary design of Carbon Fiber end-plates for SuperB
indicate that adequate stiff-ness (≤ 1 mm maximum deformation) can
beobtained with 5 mm thick spherical endplates,corresponding to
0.02X0 (compare 0.13X0 forthe BABAR Aluminum DCH endplates).
Figure 13 shows two possible endcap layouts,respectively with
spherical (a) or stepped (b)endplates. We are also considering a
con-vex spherical endplate, which provides a bet-ter match to the
geometry of the forward PIDand calorimeter systems, and would
reduce theimpact of the endplate material on the per-formance of
these detectors, at the cost ofgreater sensitivity to the
wide-angle Bhabhabackground.
4.3 Drift Chamber Geometry
The SuperB drift chamber will have a cylin-drical geometry. The
dimensions are being re-optimized through detailed simulation
studiesrespect to BABAR since:a) in SuperB there will be no support
tube;b) the possibility is being considered to add aPID device
between the drift chamber and theforward calorimeter, and an EMC in
the back-ward direction.
Simulation studies performed on several sig-nal samples with
both high (e.g. B → π+π−),and medium-low (e.g.B → D∗K)
momentumtracks indicate that:a) due to the increased lever arm,
momentum
SuperB Detector Progress Report
-
4.4 Gas Mixture 21
(a) Spherical endplates design. (b) Stepped endplates
design.
Figure 13: Two possible SuperB DCH layouts.
resolution improves as the minimum drift cham-ber radius Rmin
decreases, see Fig. 14; Rmin isactually limited by mechanical
integration con-straints with the cooling system and the SVT.b) The
momentum and especially the dE/dxresolution for tracks going in the
forward orbackward directions are clearly affected by thechange in
number of measuring samples whenthe chamber length is varied of
10−30 cm. How-ever the fraction of such tracks is so small thatthe
overall effect is negligible.
Figure 14: Track momentum resolution for dif-ferent values of
the drift chamber in-ner radius.
The drift chamber outer radius is constrainedto 809 mm by the
DIRC quartz bars. As dis-
cussed before, the DCH inner radius will be assmall as possible:
since conclusive designs ofthe final focus cooling system are not
availableyet, in Fig.13 the the nominal BABAR DCH in-ner radius of
236 mm has been used. Similarly,a nominal chamber length of 2764 mm
at theouter endplate radius is used in Fig.13: as men-tioned above,
this dimension has not been fixedyet, since it depends on the
presence and thedetails of forward PID and backward EMC sys-tems,
still being discussed. Finally, as the restof the detector, the
drift chamber is shifted bythe nominal BABAR offset (367 mm) with
respectto the interaction point.
4.4 Gas Mixture
The gas mixture for SuperBshould satisfy therequirements which
already concurred to thedefinition of the BABAR drift chamber gas
mix-ture (80%He − 20%iC4H10), i.e. low density,small diffusion
coefficient and Lorentz angle, lowsensitivity to photons with E ∼
10 keV. Tomatch the more stringent requirements on oc-cupancy rates
of SuperB, it could be useful toselect a gas mixture with a larger
drift veloc-ity in order to reduce ion collection times andso the
probability of hits overlapping from unre-lated events. The cluster
counting option wouldinstead call for a gas with low drift velocity
andprimary ionization.
SuperB Detector Progress Report
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22 4 Drift Chamber
4.5 Cell Design and Layout
The baseline design for the drift chamber em-ploys small
rectangular cells arranged in concen-tric layers about the axis of
the chamber whichis approximately aligned with the beam direc-tion.
The precise cell dimensions and numberof layers are to be determine
for the TDR phasebut the expectation is that they will be between10
and 20 mm on a side and that there will beapproximately the same
number of layers as inBABAR (40) if the inner radius is not
decreased.The cells are grouped radially into superlayerswith the
inner and outer superlayers parallel tothe chamber axis (axial). In
BABAR the chamberalso had stereo layers in which the superlayersare
oriented at a small “stereo” angle relative tothe axis in order to
provide the z-coordinates ofthe track hits. The stereo layer layout
in SuperBis to be determined for the TDR and dependson the cell
occupancy associated with machinebackgrounds.
Each cell has one 20µm diameter gold coatedsense wire surrounded
by a rectangular grid ofeight field wires. The sense wires will be
ten-sioned with a value consistent with electrostaticstability and
with the yield strength of the wire.The baseline calls for a gas
gain of approxi-mately 5 × 104 which requires a voltage of
ap-proximately +2 kV to be applied to the sensewires with the field
wires held at ground.
The field wires are aluminum with a diameterwhich will be chosen
to keep the electric field onthe wire surface below 20 kV/cm as a
means ofsuppressing the Malter effect. These wires willbe tensioned
in order to provide a gravitationalsag that matches that of the
sense wires.
At a radius inside the inner most superlayerthe chamber has an
additional layer of axiallystrung guard wires which serve to
electrostati-cally contain very low momentum electrons pro-duced
from background particles showering inthe DCH inner cylinder and
SVT. A similarlymotivated layer will be considered at the outermost
radius to contain machine background re-lated backsplash from
detector material just be-yond the outer superlayer.
4.6 R&D work
Various R&D programs are underway towardsthe definition of
an optimal drift chamber forSuperB, in particular: make precision
measure-ments of fundamental parameters (drift veloc-ity, diffusion
coefficient, Lorentz angle) of po-tentially useful gas mixtures;
study with smalldrift chamber prototypes and simulations
theproperties of different gas mixtures and cell lay-outs; verify
the potential and feasibility of thecluster counting option.
A precision tracker made of 3 cm diameterAluminum tubes
operating in limited streamermode with a single tube spatial
resolution ofaround 100µm has been set up.
A small prototype with a cell structure re-sembling the one used
in the BABAR DCH hasbeen also built and commissioned. Tracker
andprototype have been collecting cosmic ray datasince October
2009. Tracks can be extrapolatedin the DCH prototype with a
precision of 80µmor better. Different gas mixtures have beentried
in the prototype: starting with the originalBABAR mixture (80%He−
20%iC4H10) used asa calibration point, both different quencher
pro-portions and different quenchers (e.g methane)have been tested
in order to explore the phasespace leading to lighter and possibly
faster oper-ating gas. Fig. 15a shows the space-time correla-tion
for one prototype cell: as mentioned before,the cell structure is
such as to mimic the overallstructure of the BABAR DCH. Preliminary
anal-ysis shows that the spatial resolution are con-sistent with
what has been obtained with theoriginal BABAR DCH. A space to time
relation isdepicted in Fig. 15b with a 52%He− 48%iCH4gas mixture.
This gas is roughly a factor twofaster and 50% lighter than the
original BABARmix: preliminary analysis shows space resolu-tion
performances comparable to the originalmix, however detailed
studies of the Lorentz an-gle have to be carried out in order to
considerthis mixture as a viable alternative.
To improve performances of the gas trackera possible road could
be the use of the Clus-ter Counting method. If the individual
ion-ization cluster can be detected with high ef-
SuperB Detector Progress Report
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References 23
[cm]-1.5 -1 -0.5 0 0.5 1 1.5
[ns]
0
100
200
300
400
500
600
10H4Space-time relation - 80%He20%C 10H4Space-time relation -
80%He20%C
(a) 80%He-20%iC4H10 gas mixture.
[cm]-1.5 -1 -0.5 0 0.5 1 1.5
[ns]
0
100
200
300
400
500
600
4Space-time relation - 52%He48%CH
4Space-time relation - 52%He48%CH
(b) 52%He-48%iCH4 gas mixture.
Figure 15: Examples of measured space-time relation in different
He-based gas mixtures.
ficiency, it could in principle be possible tomeasure the track
specific ionization by count-ing the clusters themselves, providing
a two-fold improvement in the resolution compared tothe traditional
truncated mean method. Hav-ing many independent time measurements
ina single cell, the spatial accuracy could alsoin principle be
improved substantially. Thesepromises of exceptional energy and
spatial reso-lution must however fit with the available
datatransfer bandwidth, require a gas mixture withwell-separated
clusters and high detection effi-ciency. The preamplifier rise time
and noise arealso issues.
Comparisons of the traditional methods toextract spatial
position and energy losses andthe cluster counting method are being
setup atthe moment of writing the present report.
References
[1] The BABAR Collaboration, The BABAR De-tector, Nucl. Instr.
Meth, in Phys Res.A479 (2002) 1.
[2] M.Adinolfi et al., The KLOE Collabora-tion, The tracking
detector of the KLOE
experiment, Nucl. Instr. Meth, in Phys Res.b A488 (2002) 51.
SuperB Detector Progress Report
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24 5 Particle Identification
5 Particle Identification
5.1 Detector concept
The DIRC (Detector of Internally ReflectedCherenkov light) [1]
is an example of innova-tive detector technology that has been
crucialto the performance of the BABAR first-class sci-ence
program. Excellent flavor tagging will con-tinue to be essential
for the program of physicsanticipated at SuperB, and the gold
standardof particle identification in this energy region isagreed
to be that provided by internally reflect-ing ring-imaging devices
(the DIRC class of ringimaging detectors). The challenge for SuperB
isto retain (or even improve) the outstanding per-formance attained
by the BABAR DIRC [2], whilealso gaining an essential factor of 100
in back-ground rejection to deal with the much
higherluminosity.
We are planning to build a new Cherenkovring imaging detector
for the SuperB barrel,called the Focusing DIRC, or FDIRC. It
willuse the existing BABAR bar boxes and mechan-ical support
structure. We will attach to thisstructure a new photon “camera”,
which will beoptically