B-physics with the initial ATLAS detector Aleandro Nisati for the ATLAS Collaboration INFN Commissione Scientifica I February 3rd, 4th 2003.

B-physics with the initial ATLAS detector

Aleandro Nisati for the ATLAS Collaboration

INFN Commissione Scientifica I

February 3rd, 4th 2003

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outline

• The initial experiment conditions• The ATLAS Physics Programme• The ATLAS detector & trigger• B-physics potential with the nominal detector @

L=1033 cm-2 s-1;• Preliminary estimate of the B-physics potential

with the initial detector and luminosity;• Conclusions

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The initial experiment conditions

Many uncertainties:

1. The LHC luminosity: the target initial luminosity was doubled to L= 2 x 1033 cm-2 s-1;

2. The detector configuration; in particular the initial HLT/DAQ system bandwidth and processing power (resources limitations);

3. The physics rates (uncertainties on ,K and heavy flavour production cross-sections);

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The initial experiment conditions

• Results presented here refer to the nominal detector configuration and L= 1x1033 cm-2 s-1

[Yellow Report CERN 2000-004] ;• The analysis with the initial detector layout

(including the change of the B-layer radial position, re-evaluation of the material distribution in the ID) as a function of luminosity and the trigger conditions is on-going (within the Data Challenge project); however some preliminary indications on the degradation of the physics performances will be provided.

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The ATLAS Physics Programme1. The most prominent issues for the LHC are the

quest for the origin of the spontaneous symmetry-breaking mechanism (SM and MSSM) and the search for new physics: SuSy, Heavy Bosons, etc…

2. ATLAS (and CMS) is a general-purpose experiment optimized to maximize the potential discovery new physics: Higgs boson(s) , SuSy particles, W’ and Z’, etc…

3. However we have to consider that:– The LHC is a beauty factory dedicated B-experiment

(LHCb);– The ATLAS detector allows also a wide programme of

B-physics studies, competitive with LHCb in some channels, “for free”…

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Cross-sections and rates• huge range of cross-section

values and rates– listed for 1034 cm-2 s-1

– total 100 mb (109 Hz)

– b production 0.7 mb (7106 Hz)

– W/Z production 200/60 nb (2/0.6 kHz)

– Top production 0.8 nb (80 Hz)

– SM Higgs (mH = 150 GeV)

30 pb (3 Hz)

• With branching ratios included– W e 150 Hz

– Z ee 15 Hz

– H 0.003 Hz

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B-simulation

• Monte Carlo generator: PYTHIA 5.7/JETSET 7.4;– Flavour creation, flavour excitation and gluon splitting

included;

– CTEQ2L parton distribution;

– Peterson function b=0.007

• Full GEANT3 simulation of the detector response; in some case integrated with fast simulation;

• Total inelastic cross-section: 80 mb; bb cross section: 500 b;

8 Tile Calorimeter Module(s)

RPC chambers

Muon Trigger Elx and Algor.

LAr e.m. endcap module

Pixel module

MDT chamber assembly

The ATLAS experiment

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Detectors

Front-endPipelines

ReadoutBuffers

EventBuilder

Buffers &ProcessingFarms

DataStorage

Readout Drivers

1 GHz interaction rate /

<75 (100) kHz

O(1) kHz output rate

O(100) Hz output rate

~100 GB/s output data flow

O(100) MB/s output data flow

O(1) GB/s output data flow

2 s latency

O(10) ms latency

~ seconds latency

40 MHz bunch-crossing rate

Level 1

Level 2

Event Filter

LVL2– Region-of-

Interest (RoIRoI)

– Specialized algorithms– Fast selection with early

rejection EF

– Full event available– Offline derived algorithms– Seeding by LVL2– Best calibration / alignment– Latency less demanding

LVL1– Hardware based

(FPGA and ASIC)– Coarse calorimeter

granularity– Trigger muon

detectors: RPCs and TGCs

The ATLAS Trigger/DAQ System

RoI Pointers

HLTHLT

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The Atlas B-Physics Programme• The main physics processes that can be studied:

– CP violation:• Asymmetry in B0

d J/ K0s measurement of sin2;

• Asymmetry in B0s J/ test of the SM;

• Asymmetry in B0d,s hh measurement of ;

– B0s - B0

s oscillations;– Rare B-decays with dimuons: B0

d,s+ , B0d*0+

, B0s0+ , …

• Also: – B-production cross-section measurement;– b polarisation measurement;– Related to B-physics: direct J/, production

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The B-trigger -1• L=1 x 1033 cm-2 s-1; • Level-1: single muon trigger pT > 6 GeV/c, ||

<2.5;– Rate is expected to be about 23 kHz;– dominated by in-flight decays of ,K and heavy flavour

muon production;– Dimuon trigger possibly with lower thresholds; – Raise thresholds for higher luminosities;

• Level-2, step 1: confirm level-1 muon trigger in RoI;– Use precision muon system together with ID for

momentum measurement important rejection of in-flight decays;

– Rate: about 5 kHz;

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The B-trigger -2• Level-2, step 2:

– Specific selections are applied for different channels; in all cases we perform a track reconstruction in the Inner Detector with:

1. Either an ID full-scan;

2. Or RoI-based ID track reconstruction.

– ID full scan: unguided search for tracks in all Pixel system; track extrapolation to the SCT+TRT (electrons down to 1 GeV);

– RoI approach: consider only regions with calorimeter activity tagged by level-1 system: example: em cluster ET>2 GeV; hadronic cluster: ET >5 GeV; it requires less processing power resources (but less efficient)

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The B-trigger -3

• J/Psi : two opposite muons pT1>6

GeV and pT2 >3 GeV ( in TileCal); mass

cuts;• J/Psiee : two opposite-charge electrons

with both pT1 >1GeV; mass cuts; rate @

lvl2: 40 Hz (lvl1 mu8, L=1 x 1033 cm-2 s-1);• B hadrons: example: B hh: two

opposite tracks with pT>4 GeV; mass cuts;

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The B-trigger -4

• Event Filter: track refit, including a vertex fit; decay length and fit quality cuts are applied;

• about a factor of 10 wrt LVL2 can be achieved by exclusive selections.

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B0d J/ K0

s

• J/ ll reconstruction; mass resolution: 40 MeV (muons) and 60 MeV (electrons);

• K0s: 4.5 – 7.0 MeV

mass resolution;• B0

d: 3D kinematic fit applying vertex and mass constraint; B0

d mass resolution: 19 (26) MeV;

• Background mainly from B decays with a J/ in the finals state; small contribution from false J/ ;

B0d reconstruction; CDF has shown

a similar signal; b/tot and prod. rate improved at LHC

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B0d J/ K0

s

• Flavour tagging:– Opposite-side tagging:

muon (trigger) or electron (pt>5 GeV); Dtag = 0.5;

– Same-side tagging: B- algorithm (charged meson associated with the B-hadron); Dtag = 0.16; • Event yeld for 30 fb @

L=1x1033cm s

J/(ee)K0s

signal back.

J/()K0s

signal back.

e tags - - 5800 500

tags 14400 900 11900 1100

B- tags

- - 376100 13700

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B0d J/ K0

s

J/(ee)K0s J/()K0

s

lepton tags 0.018 0.023

B- tags - 0.015

Estimate of the statistical error of sin2 using a time-dependentanalysis with an integrated luminosity of 30 fb.

Overall statistical error:• 10 fb-1: 0.018• 30 fb-1: 0.010Competitive with LHCb and B-factories

Statistical Error:

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B0d J/ K0

s

Systematic Error:

• analysis of control samples:– B+ J/()K+

– B0dJ/()K*0

– Provide measurements of Dtag, and Ap. Invariant mass distribution for

B0d J/ K+ with superimposed the

Estimated background.

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B0d J/ K0

s

• Systematic errors– Dtag/Dtag: 0.003

– Dback/Dback: 0.006

– AP: 0.0005;

Global systematic error: < 0.01

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B0d,s hh

• Expected to provide measurements of the CP asymmetry related to the angle .

• ATLAS does not have an event-by-eveny particle identification, but can separate on statistical basis; the signals from all significant two-body decays of b-hadrons will overlap:

B0dB0

dB0

sB0sb

pbp

ACP: about 0.1 .Can provide cross-check of resultsfrom dedicate B-experiments

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B0s - B0

s oscillations

• Processes considered:– B0

s Ds+ and B0

s Dsa1

+ ; (Ds -;

• Event reconstruction includes vertices and masses reconstruction;

• Proper time resolution: rms=0.06 ps.• Event yield in 30 fb:

– 7100 B0s Ds

+ and 2600 B0s Ds

a1+ ;

– Background: mainly from B0dDs

a1+, B0

dDs+ (2200

events) and from the combinatorial background (11300 events)

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B0s - B0

s oscillations

• ms reach evaluated with the amplitude-fit technique; it is measurable with more than 5if:

– ms < 22.5 ps-1; L=10 fb1;

– ms < 29.5 ps-1; L=30 fb1;

The measurement significance as a function of ms for L=30 fb1;

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Rare decays with dimuons

• The decays B0s+ and B0

d + have very small BR but they can be selected by the atlas trigger even at the nominal LHC luminosity. With 130 fb of data the reaction B0

s+ can be seen with 4.7 assuming the S.M. BR of 4.9 109.

• Another interesting class of reactions are exclusive decays such as B0

s0+ , B0d0+ ,

B0d*0+ , …

• Detailed measurements of the decays can test the SM search for new physics (eg AFB in decay)

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Initial ATLAS configuration• New radial position of the B-layer since the Yellow report

(CERN 2000-004)• Limited resources and technical/schedule constraints

– effect: detector staging and TDAQ staging.

• Stage the following components (defer for 1-2 years)– The middle pixel layer (not the B-layer)– Outermost TRT wheels, half of the CSC layers– MDT chambers in transition region (EES, EEL)– Cryostat gap scintillators, part of high luminosity shielding– Reduction of Read-Out Drivers for LAr calorimeter

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Initial detector configuration

• Main effect to the B-physics performance due to the detector layout wrt to the Yellow Book results comes from the change of the B-layer radial position (from 4.30 cm to 5.05 cm) and from the material in and before that layer (increased thickness of the beam pipe and pixel services); preliminary estimations with DC1 data analysis:– Impact parameter resolution and proper time resolution

degraded by about 30%;– Mass resolutions degraded by about 15%;– Reconstruction efficiencies: no important degradation

found;

• Effects of the missing pixel layer under study.

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T/DAQ Deferrals• Temporary re-allocation of TDAQ sub-system resources

will be used to fund overcosts in common projects– Would lead to drastic reduction in initial HLT/DAQ system

if additional funds not obtained (only about 1/2 of the HLT/DAQ CORE budget remaining!)

• Impact of deferrals on rate capability is difficult to estimate.– Evaluation of rate capability versus cost requires understanding

behavior of HLT/DAQ (whose design is not yet complete) as a function of many parameters

– At this time, we use a simplified cost model with significant uncertainties

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T/DAQ Deferrals & LHC lum.• The target initial luminosity was doubled to L= 2 x

1033 cm-2 s-1; increase the low-pT inclusive muon trigger threshold; – include the low pt inclusive muon trigger with a low-pT

dimuon trigger;

• Consequences:– CP violation: sin2: 0.010 0.015 (dimuon trigger

only); – Mixing cannot be studied with dimuon trigger only;– Rare B decays: unaffected;

• Restore the low lumi trigger menu as soon as L approaches values close to 1x1033 cm-2 s-1;

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Summary

Although ATLAS is designed to probe the O(1TeV) energy scale, this experiment can make several useful measurements in the B-physics sector:– Sensitivity to sin2 comparable to that of LHCb;

– Measurement of the B0s- B0

s oscillations;

– Unique opportunity to search for rare B0s decays:

potential indirect evidence of new physics.