1. Luminosity Measurement with the ATLAS Forward Calorimeter
Samir Arfaoui
CERN/PH-LCD

2. Samir Arfaoui - FCAL Workshop
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 Calorimeters
All 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 :
LUCID, BCM, ZDC
Tracker
Calorimeters, Muon chambers, …
calibration
(van der Meer, ALFA)
 = number of inelastic pp collisions per bunch crossing
nb = number of bunch pairs colliding in ATLAS
fr = LHC revolution frequency ( Hz)
inel = total inelastic pp cross-section (71.5 mb)
vis= number of detected events per bunch crossing
 = acceptance x efficiency of luminosity detector
vis = visible cross-section = luminosity calibration constant
Bhabha scattering standard candle unavailable at LHC
==> Calibration is challenging
12/11/2012
Samir Arfaoui - FCAL Workshop

5. ATLAS luminosity calibration
ATLAS-CONF
Van der Meer scan principle:
measure simultaneously
L = f (I1 , I2 , Sx , Sy )
Rmax = peak collision rate (arb. u.)
Procedure:
25 scan steps, +-3sigma, 30s per step
Sx,y
y-scan
Bunch-sensitive detectors can be absolutely calibrated using this method (LUCID, BCM)
==> For the forward calorimeter, use the resulting luminosity for calibration
x-scan
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Samir Arfaoui - FCAL Workshop

6. Samir Arfaoui - FCAL Workshop
LAr Forward Calorimeter
Forward 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 ↔ ~ calorimeter cells
Power supplies ↔ Detector: ~110m cables
Ground return
Feedthrough
Calorimeter electrodes
High-voltage system
(Technical cavern USA15)
Room
Temperature
Cryostat: 88K (Liquid argon)
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
Original study: Walter Bonivento (
f: calorimeter sampling fraction
K: suppression factor for electron response wrt mip
W: liquid Argon ionization potential
Pros:
Trigger independent
DAQ independent
Linear with luminosity
Cons:
Low sampling rate (0.2Hz)
No bunch-by-bunch capabilities
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Samir Arfaoui - FCAL Workshop

9. Samir Arfaoui - FCAL Workshop
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
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Samir Arfaoui - FCAL Workshop

10. Samir Arfaoui - FCAL Workshop
Linearity in test beam
HiLum group
Study LAr calorimeters upgrade for HL-LHC high luminosity environment with LAr detector prototypes
Test beam
50GeV protons at ICHEP Protvino, Russia
HiLum group quotes a non-linear fraction smaller than 0.36% for the entire equivalent LHC luminosity range.
12/11/2012
Samir Arfaoui - FCAL Workshop

11. Calibration
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.
FCal-1-A
FCal-1-C
Current [uA]
ATLAS preferred luminosity (BCM EventOR) [1030 cm2s-1]
(Luminosity range: 1033 cm2s-1  cm2s-1)
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Samir Arfaoui - FCAL Workshop

12. Samir Arfaoui - FCAL Workshop
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. Samir Arfaoui - FCAL Workshop
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
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Samir Arfaoui - FCAL Workshop

14. Samir Arfaoui - FCAL Workshop
Backup
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Samir Arfaoui - FCAL Workshop

15. Samir Arfaoui - FCAL Workshop
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
12/11/2012
Samir Arfaoui - FCAL Workshop

16. Calorimetry: LAr 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 = x 0.1
Layer 2 : Δη x Δφ = x 0.025
Presampler up to |η| = 1.8
readout channels
(98 % operational)
Design resolution :
(from test beam)
Photon angular resolution :
CPPM
LAr 2010 IEEE NSS MIC
12/11/2012
Samir Arfaoui - FCAL Workshop

17. ALFA & ZDC ALFA detector and electronics ZDC EM Module 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 2010
ZDC:
Neutrons for Heavy Ions centrality measurements
Trigger for pp runs
Luminosity capabilites (similar to LUCID)
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Samir Arfaoui - FCAL Workshop

18. Samir Arfaoui - FCAL Workshop
LUCID & BCM
BCM:
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”
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Samir Arfaoui - FCAL Workshop

19. Samir Arfaoui - FCAL Workshop
High-voltage feedthroughs
HVPS
GND
HV1
HV1
HV2
Return
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Samir Arfaoui - FCAL Workshop

20. Samir Arfaoui - FCAL Workshop
High-voltage power supplies 3 5 4 2 1
1. High-voltage generator from 24V main supply: 1/board or 1/line (top picture)
2. Analog-to-Digital Converter for voltage and current measurements
3. One high-voltage line: has its own voltage regulation circuit
4. Micro-controller chip: contains EEPROM + firmware
5. CAN controller: enables communication with the power supply unit
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Samir Arfaoui - FCAL Workshop

21. Samir Arfaoui - FCAL Workshop
Return current measurement
Primary purpose
Monitor grounding of the high-voltage system
Apparatus
Integrated 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
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Samir Arfaoui - FCAL Workshop

22. Samir Arfaoui - FCAL Workshop
Minimum bias events
« Soft » interactions
σinel ≈ 71.5 mb
~ 23 interactions/bunch 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
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Samir Arfaoui - FCAL Workshop

23. Samir Arfaoui - FCAL Workshop
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
12/11/2012
Samir Arfaoui - FCAL Workshop