Luminosity Measurement with the ATLAS Forward Calorimeter

Samir Arfaoui

samir.arfaoui@cern.ch

CERN/PH-LCD

12/11/2012 Samir Arfaoui - FCAL Workshop 2

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 CalorimetersAll these detectors are sensitive to luminosity

12/11/2012 Samir Arfaoui - FCAL Workshop 3

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 crossingnb = number of bunch pairs colliding in ATLASfr = 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 detectorvis = 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!

12/11/2012 Samir Arfaoui - FCAL Workshop 4

ATLAS luminosity calibrationVan der Meer scan principle:

measure simultaneouslyL = f (I1 , I2 , Sx , Sy )

Rmax = peak collision rate (arb. u.)

Procedure:25 scan steps, +-3sigma, 30s per step

ATLAS-CONF-2011-011

Sx,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

Samir Arfaoui - FCAL Workshop 512/11/2012

LAr Forward CalorimeterForward 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 6

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

High-voltage system(Technical cavern USA15)

FeedthroughCalorimeter electrodesRoom

TemperatureCryostat: 88K (Liquid argon)

Ground return

12/11/2012 Samir Arfaoui - FCAL Workshop 7

Measurement principle

Total HV current

Number of pairs created

in the detector

Total energy deposited in the detector

Luminosity

eWK

fEσLeWK

fPeNI eventRpairs

Pros:• Trigger independent• DAQ independent• Linear with luminosityCons:• Low sampling rate (0.2Hz)• No bunch-by-bunch capabilities

f: calorimeter sampling fractionK: suppression factor for electron response wrt mipW: liquid Argon ionization potential

Original study: Walter Bonivento (http://cdsweb.cern.ch/record/684140)

12/11/2012 Samir Arfaoui - FCAL Workshop 8

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

Voltage

12/11/2012 Samir Arfaoui - FCAL Workshop 9

Linearity in test beamhttp://iopscience.iop.org/1748-0221/5/05/P05005/

HiLum group quotes a non-linear fraction smaller than 0.36% for the entire equivalent LHC luminosity range.

HiLum groupStudy LAr calorimeters upgrade for HL-LHC high luminosity environment with LAr detector prototypesTest beam50GeV protons at ICHEP Protvino, Russia

12/11/2012 Samir Arfaoui - FCAL Workshop 10

CalibrationC

urre

nt [

uA]

ATLAS preferred luminosity (BCM EventOR) [1030 cm2s-1](Luminosity range: 1033 cm2s-1 4 1033 cm2s-1)

FCal-1-A FCal-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.

12/11/2012 Samir Arfaoui - FCAL Workshop 11

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 12

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 13

Backup

12/11/2012 Samir Arfaoui - FCAL Workshop 14

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 15

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

Samir Arfaoui - FCAL Workshop 16

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 :

12/11/2012

Calorimetry: LAr

CPPM

LAr status @ 2010 IEEE NSS MIC

12/11/2012 Samir Arfaoui - FCAL Workshop 17

ALFA & ZDC

ZDC EM ModuleALFA 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 2010- September + October 2011: dedicated ALFA runs with special

LHC beam opticsZDC:- Neutrons for Heavy Ions centrality measurements- Trigger for pp runs- Luminosity capabilites (similar to LUCID)

12/11/2012 Samir Arfaoui - FCAL Workshop 18

LUCID & BCMBCM:- 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 19

High-voltage feedthroughsHVPS

HV1

HV1HV2

GND

Return

12/11/2012 Samir Arfaoui - FCAL Workshop 20

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 measurements3. One high-voltage line: has its own voltage regulation circuit4. Micro-controller chip: contains EEPROM + firmware5. CAN controller: enables communication with the power supply unit

1

2

345

12/11/2012 Samir Arfaoui - FCAL Workshop 21

Return current measurement

Primary purposeMonitor grounding of the high-voltage systemApparatusIntegrated 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 22

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 23

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