Luminosity Measurement with the ATLAS Forward Calorimeter Samir
- Slides: 23
Luminosity Measurement with the ATLAS Forward Calorimeter Samir Arfaoui samir. arfaoui@cern. ch CERN/PH-LCD
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 All these detectors are sensitive to luminosity 12/11/2012 Samir Arfaoui - FCAL Workshop • FCal : Liquid Argon Forward Calorimeters 2
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 LUCID, BCM, ZDC • 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 • Calorimeters, Muon chambers, … For event-counting algorithms : 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 (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 detector vis = visible cross-section = luminosity calibration constant 12/11/2012 Bha scattering standard candle unavailable at LHC ==> Calibration is challenging! Samir Arfaoui - FCAL Workshop 3
ATLAS luminosity calibration Van der Meer scan principle: measure simultaneously L = f (I 1 , I 2 , Sx , Sy ) Rmax = peak collision rate (arb. u. ) ATLAS-CONF-2011 -011 Sx, y Procedure: 25 scan steps, +-3 sigma, 30 s per step y-scan x-scan 12/11/2012 Problem for FCal: - Instantaneous luminosity too low for FCal - Scan steps is too short - Cannot increase step duration: - Too costly in terms of beam time - If scan is too long, emittance growth can become an issue Use calibrated LUCID/BCM to fit FCal Samir Arfaoui - FCAL Workshop 4
LAr 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 5
High-voltage system • • • Goal: Provide electric field E ≈ 1 k. V/mm in each liquid argon gap Adjustable voltage up to 3 k. V / HV line Slow control infrastructure for operation and monitoring → V, I, … ~4500 HV lines ↔ ~182000 calorimeter cells Power supplies ↔ Detector: ~110 m cables Ground return Feedthrough High-voltage system (Technical cavern USA 15) 12/11/2012 Room Temperature Cryostat: 88 K (Liquid argon) Samir Arfaoui - FCAL Workshop Calorimeter electrodes 6
Measurement principle Original study: Walter Bonivento (http: //cdsweb. cern. ch/record/684140) Total HV current Number of pairs created in the detector f: calorimeter sampling fraction K: suppression factor for electron response wrt mip W: liquid Argon ionization potential 12/11/2012 Total energy deposited in the detector Luminosity Pros: • Trigger independent • DAQ independent • Linear with luminosity Cons: • Low sampling rate (0. 2 Hz) • No bunch-by-bunch capabilities Samir Arfaoui - FCAL Workshop 7
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 i. HV to compensate Voltage 12/11/2012 Samir Arfaoui - FCAL Workshop 8
Linearity in test beam http: //iopscience. iop. org/1748 -0221/5/05/P 05005/ Hi. Lum group Study LAr calorimeters upgrade for HL-LHC high luminosity environment with LAr detector prototypes Test beam 50 Ge. V protons at ICHEP Protvino, Russia Hi. Lum group quotes a non-linear fraction smaller than 0. 36% for the entire equivalent LHC luminosity range. 12/11/2012 Samir Arfaoui - FCAL Workshop 9
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 -C Current [u. A] FCal-1 -A ATLAS preferred luminosity (BCM Event. OR) [1030 cm 2 s-1] (Luminosity range: 1033 cm 2 s-1 4 1033 cm 2 s-1) 12/11/2012 Samir Arfaoui - FCAL Workshop 10
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 11
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 12
Backup 12/11/2012 Samir Arfaoui - FCAL Workshop 13
Luminosity - Main feature that characterises a particle collider - Expressed in cm-2 s-1 - Crucial for cross-section measurements, exclusions, and discoveries For a given process, To produce rare events (with small cross-sections): Need for high luminosity To reduce the uncertainty on the cross-section measurement: Need for a precise luminosity determination 12/11/2012 Samir Arfaoui - FCAL Workshop 14
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 15
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 = 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) CPPM - Design resolution : (from test beam) - Photon angular resolution : LAr status @ 2010 IEEE NSS MIC 12/11/2012 Samir Arfaoui - FCAL Workshop 16
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 - September + October 2011: dedicated ALFA runs with special LHC beam optics ZDC: - Neutrons for Heavy Ions centrality measurements - Trigger for pp runs - Luminosity capabilites (similar to LUCID) 12/11/2012 Samir Arfaoui - FCAL Workshop 17
LUCID & BCM: - Diamond based detectors - 4 x 2 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 C 4 F 10 to enable production of Cerenkov light - Cerenkov light signal + threshold defines a LUCID “event” 12/11/2012 Samir Arfaoui - FCAL Workshop 18
High-voltage feedthroughs HVPS GND HV 1 HV 2 12/11/2012 Samir Arfaoui - FCAL Workshop Return 19
High-voltage power supplies 5 4 3 2 1 1. High-voltage generator from 24 V 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 12/11/2012 Samir Arfaoui - FCAL Workshop 20
Return current measurement Primary purpose Monitor grounding of the highvoltage 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 12/11/2012 Samir Arfaoui - FCAL Workshop 21
Minimum bias events • • • « Soft » interactions σinel ≈ 71. 5 mb ~ 23 interactions/bunch crossing @ LHC lumi. (1034 cm-2 s-1) Products: mostly low p. T 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 IP 1 12/11/2012 Samir Arfaoui - FCAL Workshop Low-p. T particles deposit most of their energy in the EM section of the FCal 22
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 23
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