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

12/11/2012Samir Arfaoui - FCAL Workshop2 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

12/11/2012Samir Arfaoui - FCAL Workshop3 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 crossing n b = number of bunch pairs colliding in ATLAS f r = 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 calibration LUCID, BCM, ZDC (van der Meer, ALFA) Calorimeters, Muon chambers, … BhaBha scattering standard candle unavailable at LHC ==> Calibration is challenging!

12/11/2012Samir Arfaoui - FCAL Workshop4 ATLAS luminosity calibration Van der Meer scan principle: measure simultaneously L = f (I 1, I 2,  x,  y ) R max = peak collision rate (arb. u.) Procedure: 25 scan steps, +-3sigma, 30s per step ATLAS-CONF  x,y x-scan y-scan 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

12/11/2012Samir Arfaoui - FCAL Workshop5 LAr Forward Calorimeter Forward Calorimeter (FCal) - Absorbers : Cu/W - Active medium : Liquid Argon - Coverage : 3.1 < |η| < EM + 2 Hadronic layers readout channels High-Voltage lines

12/11/2012Samir Arfaoui - FCAL Workshop6 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 High-voltage system (Technical cavern USA15) Feedthrough Calorimeter electrodes Room Temperature Cryostat: 88K (Liquid argon) Ground return

12/11/2012Samir Arfaoui - FCAL Workshop7 Measurement principle Total HV current Number of pairs created in the detector Total energy deposited in the detector Luminosity Pros: Trigger independent DAQ independent Linear with luminosity Cons: Low sampling rate (0.2Hz) No bunch-by-bunch capabilities f: calorimeter sampling fraction K: suppression factor for electron response wrt mip W: liquid Argon ionization potential Original study: Walter Bonivento (

12/11/2012Samir Arfaoui - FCAL Workshop8 Signal generation Charged particle traverses liquid argon gap –Liquid argon ionisation –Electrons produced drift due to electric field –Singal current i s 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

12/11/2012Samir Arfaoui - FCAL Workshop9 Linearity in test beam HiLum group quotes a non-linear fraction smaller than 0.36% for the entire equivalent LHC luminosity range. HiLum group Study LAr calorimeters upgrade for HL-LHC high luminosity environment with LAr detector prototypes Test beam 50GeV protons at ICHEP Protvino, Russia

12/11/2012Samir Arfaoui - FCAL Workshop10 Calibration Current [uA] ATLAS preferred luminosity (BCM EventOR) [10 30 cm 2 s -1 ] (Luminosity range: cm 2 s -1  cm 2 s -1 ) FCal-1-AFCal-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.

12/11/2012Samir Arfaoui - FCAL Workshop11 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 during the 2011 data-taking period.

12/11/2012Samir Arfaoui - FCAL Workshop12 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

12/11/2012Samir Arfaoui - FCAL Workshop13 Backup

12/11/2012Samir Arfaoui - FCAL Workshop14 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

12/11/2012Samir Arfaoui - FCAL Workshop15 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

Electromagnetic Calorimeter (EM) - Absorbers : Pb - Active Medium : LAr - Accordion geometry : full φ coverage - Coverage : |η| < Segmentation in η and in depth - 3 layers up to |η| = 2.5 ; 2 up to |η| = Layer 1 : Δη x Δφ = x Layer 2 : Δη x Δφ = x Presampler up to |η| = readout channels (98 % operational) - Design resolution : (from test beam) - Photon angular resolution : 12/11/2012Samir Arfaoui - FCAL Workshop16 Calorimetry: LAr CPPM LAr 2010 IEEE NSS MIC

12/11/2012Samir Arfaoui - FCAL Workshop17 ALFA & ZDC ZDC EM Module ALFA 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 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)

12/11/2012Samir Arfaoui - FCAL Workshop18 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 C 4 F 10 to enable production of Cerenkov light -Cerenkov light signal + threshold defines a LUCID “event”

12/11/2012Samir Arfaoui - FCAL Workshop19 High-voltage feedthroughs HVPS HV1 HV2 GND Return

12/11/2012Samir Arfaoui - FCAL Workshop20 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 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

12/11/2012Samir Arfaoui - FCAL Workshop21 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

12/11/2012Samir Arfaoui - FCAL Workshop22 Minimum bias events « Soft » interactions σ inel ≈ 71.5 mb ~ 23 interactions/bunch LHC lumi. (10 34 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 IP1 Low-p T particles deposit most of their energy in the EM section of the FCal

12/11/2012Samir Arfaoui - FCAL Workshop23 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