Luminosity Measurement with the ATLAS Forward Calorimeter Samir Arfaoui [email protected] CERN/PH-LCD
Jan 04, 2016
Luminosity Measurement with the ATLAS Forward Calorimeter
Samir Arfaoui
CERN/PH-LCD
12/11/2012 Samir Arfaoui - FCAL Workshop 2
Outline
• The ATLAS Forward Region• Luminosity determinations• Luminosity calibration: beam separation scans• The Liquid Argon Forward Calorimeter (FCal)
– Geometrical layout & High-voltage system– Measurement principle & Linearity– Calibration & Results
12/11/2012 Samir Arfaoui - FCAL Workshop 3
ATLAS Forward Detectors• LUCID : Cerenkov
detectors
• BCM : diamond-based Beam Conditions Monitors
• ZDC : Zero-Degree Calorimeters
• MBTS : Minimum Bias Trigger Scintillators
• ALFA : Absolute Luminosity For ATLAS
• FCal : Liquid Argon Forward CalorimetersAll these detectors are sensitive to luminosity
12/11/2012 Samir Arfaoui - FCAL Workshop 4
ATLAS luminosity determination• Three handles on the luminosity
– Event counting : number of events passing a specific selection per bunch crossing• OR algorithms : signal at least on one side (A or C)• AND algorithms : coincidence signal on both sides (A and C)
– Hit counting : number of hits per bunch crossing• OR algorithms : hit at least on one side (A or C)• AND algorithms : coincidence hits on both sides (A and C)
– Particle counting : number of particles per beam crossing• Number of charged particles• Particle flux going through a detector
• For event-counting algorithms :
= number of inelastic pp collisions per bunch crossingnb = number of bunch pairs colliding in ATLASfr = LHC revolution frequency (11245.5 Hz)inel = total inelastic pp cross-section (71.5 mb)vis= number of detected events per bunch crossing = acceptance x efficiency of luminosity detectorvis = visible cross-section = luminosity calibration constant
calibration
LUCID, BCM, ZDC
(van der Meer, ALFA)
Calorimeters, Muon chambers, …
Bhabha scattering standard candle unavailable at LHC
==> Calibration is challenging
Tracker
12/11/2012 Samir Arfaoui - FCAL Workshop 5
ATLAS luminosity calibrationVan der Meer scan principle:
measure simultaneouslyL = f (I1 , I2 , Sx , Sy )
Rmax = peak collision rate (arb. u.)
Procedure:25 scan steps, +-3sigma, 30s per step
ATLAS-CONF-2011-011
Sx,y
x-scan
y-scan Bunch-sensitive detectors can be absolutely calibrated using
this method (LUCID, BCM)
==> For the forward calorimeter, use the resulting luminosity for
calibration
Samir Arfaoui - FCAL Workshop 612/11/2012
LAr Forward CalorimeterForward Calorimeter (FCal)- Absorbers : Cu/W- Active medium : Liquid Argon- Coverage : 3.1 < |η| < 4.9- 1 EM + 2 Hadronic layers- 3524 readout channels - 112 High-Voltage lines
12/11/2012 Samir Arfaoui - FCAL Workshop 7
High-voltage system• Goal: Provide electric field E ≈ 1 kV/mm in
each liquid argon gap• Adjustable voltage up to 3kV / HV line• Slow control infrastructure for operation and
monitoring → V, I, …• ~4500 HV lines ↔ ~182000 calorimeter cells• Power supplies ↔ Detector: ~110m cables
High-voltage system(Technical cavern USA15)
FeedthroughCalorimeter electrodesRoom
TemperatureCryostat: 88K (Liquid argon)
Ground return
12/11/2012 Samir Arfaoui - FCAL Workshop 8
Measurement principle
Total HV current
Number of pairs created
in the detector
Total energy deposited in the detector
Luminosity
eWK
fEσLeWK
fPeNI eventRpairs
Pros:• Trigger independent• DAQ independent• Linear with luminosityCons:• Low sampling rate (0.2Hz)• No bunch-by-bunch capabilities
f: calorimeter sampling fractionK: suppression factor for electron response wrt mipW: liquid Argon ionization potential
Original study: Walter Bonivento (http://cdsweb.cern.ch/record/684140)
12/11/2012 Samir Arfaoui - FCAL Workshop 9
Signal generation• Charged particle traverses liquid argon gap
– Liquid argon ionisation– Electrons produced drift due to electric field
– Singal current is • produced by capacitive coupling in the LAr gap • proportional to energy deposited
– To maintain electric field constant• HV system injects iHV to compensate
12/11/2012 Samir Arfaoui - FCAL Workshop 10
Linearity in test beamhttp://iopscience.iop.org/1748-0221/5/05/P05005/
HiLum group quotes a non-linear fraction smaller than 0.36% for the entire equivalent LHC luminosity range.
HiLum groupStudy LAr calorimeters upgrade for HL-LHC high luminosity environment with LAr detector prototypesTest beam50GeV protons at ICHEP Protvino, Russia
12/11/2012 Samir Arfaoui - FCAL Workshop 11
CalibrationC
urre
nt [
uA]
ATLAS preferred luminosity (BCM EventOR) [1030 cm2s-1](Luminosity range: 1033 cm2s-1 4 1033 cm2s-1)
FCal-1-A FCal-1-C
Method: Select a single ATLAS run in and fit the FCal HV lines currents to extract calibration. Then apply calibration to the rest of the data.
12/11/2012 Samir Arfaoui - FCAL Workshop 12
Results
Average number of interactions per bunch crossing ratio of various luminosity algorithms and BCM as a function of time during the 2011
data-taking period.
Average number of interactions per bunch crossing ratio of
various luminosity algorithms and BCM as a function of <μ> during
the 2011 data-taking period.
12/11/2012 Samir Arfaoui - FCAL Workshop 13
Summary• Due to the nature of pp collisions, luminosity calibration the LHC
is a big challenge– need for as many handles as possible– event, hit, or particle counting methods
• Main luminosity detectors: LUCID & BCM– absolutely calibrated using the van der Meer scan method
• The Liquid Argon Forward Calorimeter provides an additional measurement using the currents drawn from its High-Voltage system– linear up to the LHC design luminosity– independent from Trigger/DAQ– however, bunch-by-bunch blind (Slow Control)
• Calibration has proven robust and reliable– time and interaction rate dependence under control
• Measurement is now fully integrated in the ATLAS luminosity infrastrcuture and continuously monitored
12/11/2012 Samir Arfaoui - FCAL Workshop 14
Backup
12/11/2012 Samir Arfaoui - FCAL Workshop 15
Calorimetry: Overview
Goals:- Trigger on electrons, photons, jets and missing transverse energy- Electron, photon, jet energy and time measurements - Missing transverse energy measurements- LAr EM: electron and photon identification
Samir Arfaoui - FCAL Workshop 16
Electromagnetic Calorimeter (EM)- Absorbers : Pb - Active Medium : LAr- Accordion geometry : full φ coverage- Coverage : |η| < 3.2- Segmentation in η and in depth - 3 layers up to |η| = 2.5 ; 2 up to |η| = 3.2
- Layer 1 : Δη x Δφ = 0.0031 x 0.1- Layer 2 : Δη x Δφ = 0.025 x 0.025
- Presampler up to |η| = 1.8- 173312 readout channels
(98 % operational)
- Design resolution : (from test beam)
- Photon angular resolution :
12/11/2012
Calorimetry: LAr
CPPM
LAr status @ 2010 IEEE NSS MIC
12/11/2012 Samir Arfaoui - FCAL Workshop 17
ALFA & ZDC
ZDC EM ModuleALFA detector and electronics
ALFA:- Elastic scattering at small angles + total elastic pp cross-section- Absolute luminosity calibration (1%)- Scintillating fibre trackers close to the beams- All 8 roman pot stations installed and ready since winter 2010ZDC:- Neutrons for Heavy Ions centrality measurements- Trigger for pp runs- Luminosity capabilites (similar to LUCID)
12/11/2012 Samir Arfaoui - FCAL Workshop 18
LUCID & BCMBCM:- Diamond based detectors- 4x2 detectors located in the Tracker,
close to the beam pipe- Primary purpose: provide beam abort
signal to LHC to protect tracker- Can measure collision rate
handle on luminosity
LUCID:- Goal is to provide relative luminosity
determination to ATLAS- Aluminium tubes placed around beam pipe- Filled with C4F10 to enable production of
Cerenkov light- Cerenkov light signal + threshold defines a
LUCID “event”
12/11/2012 Samir Arfaoui - FCAL Workshop 19
High-voltage feedthroughsHVPS
HV1
HV1HV2
GND
Return
12/11/2012 Samir Arfaoui - FCAL Workshop 20
High-voltage power supplies
1. High-voltage generator from 24V main supply: 1/board or 1/line (top picture)2. Analog-to-Digital Converter for voltage and current measurements3. One high-voltage line: has its own voltage regulation circuit4. Micro-controller chip: contains EEPROM + firmware5. CAN controller: enables communication with the power supply unit
1
2
345
12/11/2012 Samir Arfaoui - FCAL Workshop 21
Return current measurement
Primary purposeMonitor grounding of the high-voltage systemApparatusIntegrated current transformers placed around ground returns
- Possible to monitor luminosity using return currents
- Less sensitive than HVPS current measurements
- Still very useful to perform ground diagnostics of the LAr systems
12/11/2012 Samir Arfaoui - FCAL Workshop 22
Minimum bias events• « Soft » interactions
• σinel ≈ 71.5 mb
• ~ 23 interactions/bunch crossing @ LHC lumi. (1034 cm-2 s-1)
• Products: mostly low pT neutral pions (=> photon pairs)
• Flux increases with η => Most of the energy is deposited in the forward region
η=0 η=3.2
η=4.9
IP1
Low-pT particles deposit most of their
energy in the EM section of the FCal
12/11/2012 Samir Arfaoui - FCAL Workshop 23
FCal high-voltage distribution
One FCal readout cell
One HV sector#HV lines = 4
FCal 1 (EM)
FCal 1 module• 1008 readout cells• 16 HV sectors• 64 HV lines
• Each sector is fed by 4 separate HV lines• Each HV line feeds ¼ of a readout cell (for redundancy)
• Innermost (and edge) cells are fed by only one HV line
===> Current measured in one HV line corresponds roughly to ¼ of the current induced in the HV sector by minimum bias events