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

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

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

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 (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 Bhabha scattering standard candle unavailable at LHC ==> Calibration is challenging 12/11/2012 Samir Arfaoui - FCAL Workshop

ATLAS luminosity calibration ATLAS-CONF-2011-011 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 12/11/2012 Samir Arfaoui - FCAL Workshop

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

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

Measurement principle Total HV current Number of pairs created in the detector Total energy deposited in the detector Luminosity Original study: Walter Bonivento (http://cdsweb.cern.ch/record/684140) 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 12/11/2012 Samir Arfaoui - FCAL Workshop

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 12/11/2012 Samir Arfaoui - FCAL Workshop

Samir Arfaoui - FCAL Workshop Linearity in test beam http://iopscience.iop.org/1748-0221/5/05/P05005/ 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

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  4 1033 cm2s-1) 12/11/2012 Samir Arfaoui - FCAL Workshop

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

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 12/11/2012 Samir Arfaoui - FCAL Workshop

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

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

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) Design resolution : (from test beam) Photon angular resolution : CPPM LAr status @ 2010 IEEE NSS MIC 12/11/2012 Samir Arfaoui - FCAL Workshop

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) 12/11/2012 Samir Arfaoui - FCAL Workshop

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” 12/11/2012 Samir Arfaoui - FCAL Workshop

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

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 12/11/2012 Samir Arfaoui - FCAL Workshop

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 12/11/2012 Samir Arfaoui - FCAL Workshop

Samir Arfaoui - FCAL Workshop 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

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