Luminosity Measurement with the ATLAS Forward Calorimeter Samir

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Luminosity Measurement with the ATLAS Forward Calorimeter Samir Arfaoui samir. arfaoui@cern. ch CERN/PH-LCD

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

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

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

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

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

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

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 –

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

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

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

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

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

Backup 12/11/2012 Samir Arfaoui - FCAL Workshop 13

Luminosity - Main feature that characterises a particle collider - Expressed in cm-2 s-1

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 -

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

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

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

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

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

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

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

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 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