Muon Identification in ALICE

Andreas MorschCERN PH

Workshop on Muon Detection in the CBM ExperimentGSI Darmstadt 16-18/10/2006

Outline

● The ALICE Experiment at the LHC● Muon Spectrometer overview● Muon Spectrometer components

– Tracking Chambers– Trigger Chambers– Absorbers– Dipole Magnet

● Expected performance

ALICE Collaboration

UKPORTUGAL

JINR

GERMANY

SWEDENCZECH REP.

HUNGARYNORWAY

SLOVAKIAPOLANDNETHERLANDS

GREECE

DENMARKFINLAND

SWITZERLAND

RUSSIA CERN

FRANCE

MEXICOCROATIA ROMANIA

CHINA

USAARMENIA

UKRAINE

INDIA

ITALYS. KOREA

~ 1000 Members

(63% from CERN MS)

~30 Countries

~90 Institutes

0

200

400

600

800

1000

1200

1990 1992 1994 1996 1998 2000 2002 2004

ALICE Collaboration statistics

LoI

MoU

TP

TRD

L3 magnet

B ≤ 0.5 T

Total weight : 9,800 tonsOverall diameter : 16 mOverall length : 26 m 130 MCHF CORE

Muon Forward Spectrometer

2.4 < < 4

Highlights

● Large Acceptance Coverage

● Large Momentum Coverage (from 100 MeV/c to > 100 GeV/c)

● High Granularity ( designed for dN/dy ~ 8000, i.e. 15 000 particles in acceptance)

– Spectroscopy and identification of hadrons and leptons

● c-, b- vertex recognition

● Excellent photon detection ( in Δφ =450 and η = 0.1)

● Large acceptance electromagnetic calorimetry added and approved recently. Jet trigger capabilities.

central Pb–Pb

pp

Nuclear collisions at the LHC

• LHC on track for start-up of pp operations in November 2007• Pb-Pb scheduled for end 2008

– Each year several weeks of HI beams (106 s effective running time)• Future includes other ion species and pA collisions.

– LHC is equipped with two separate timing systems.

System L0 [cm-2s-1]sNN max

[TeV] y

Pb+Pb 1 1027 5.5 0

Ar+Ar 6 1028 6.3 0

O+O 2 1029 7.0 0

pPb 1 1030 8.8 0.5

pp 1 1034 14 0

First 5-6 years 2-3y Pb-Pb (highest energy density)2y Ar-Ar (vary energy density)1y p-Pb (nucl. pdf, ref. data)

LHC Official Schedule

● October 2006 Last magnet delivered● March 2007 Last magnet installed● August 2007 Machine closes● November 2007 First collisions s=900 GeV● Spring 2008 Collisions at s=14 TeV

Muon SpectrometerDesign Criteria

● High multiplicity capability: The tracking detectors must be able to handle the high secondary particle multiplicity.

● Large acceptance: As the accuracy of the spectrometer is statistics limited (at least for the Y - family), the geometrical acceptance must be as large as possible.

● Low-pT acceptance: For direct J/ production it is necessary to have a large acceptance at low pT. Mapping out the charmonium suppression as a function of pT is important to distinguish between different models.

● Forward region: Muon identification in a heavy ion environment is only possible for muon momenta above 4 GeV/c because of the large amount of material required to reduce the flux of hadrons.

– Measurement of low-pT charmonia is only possible at low angles where muons are Lorentz boosted.

Design Criteria

● Invariant mass resolution: A resolution of 70 (100) MeV/c2 in the 3 (10) GeV/c2 dimuon invariant mass region is needed to resolve the J/ (’) (Y , Y and Y “) peaks. This requirement determines the – bending strength of the spectrometer magnet– the spatial resolution of the muon tracking system.– It imposes the minimization of multiple scattering and a

careful optimization of the absorber.● Trigger: The spectrometer has to be equipped with

a selective dimuon trigger system to reach the maximum trigger rate of about 1 kHz handled by the DAQ.

Challenge

● High particle multiplicity per event, needs– Optimized absorber design– Optimizes chamber design (segmentation)– Careful simulation

● Different transport code Geant3, C95+G3, FLUKA● Conservative primary particle multiplicity (2x HIJING

multiplicity)

Detector Layout

• Passive Front Absorber to absorb hadrons from the interaction vertex and to reduce the K/ decay background.

• High granularity Tracking System with 10 detection planes (CPCs)• Dipole Magnet, largest warm dipole ever built• Passive Muon Filter wall followed by 4 planes of Trigger Chambers (RPCs)_• Inner Beam Shield to protect the chambers from secondaries produced at large

rapidities.

Acceptance: 2o < < 9o (-4 < y < -2.5)Minimum muon momentum: 4 GeV/cResonance pT: > 0Mass resolution 70 MeV/c2 (100 MeV/c2)

Absorbers

- Suppress /K decay- Shield from secondaries in particular at small radii.

Front absorber design

● Equalize mass resolution contributions from– Multiple scattering – Energy Straggling– Tracking (chamber resolution + bending strength)

● At least 10I are needed to suppress hadron flux.– Angle measurement from hit position in first chamber and

vector measured by the first station.● Branson plane method● ~30 mrad/p

– Angular resolution ~L● Low density material close to the interaction point, high density

material at the rear.

Front Absorber (FA)

Concrete

Steel

Carbon

Tungsten

● ~10 I (Carbon – Concrete – Steel)

FASS

FA Installation

Small Angle Absorber Design

● Complex integration issues:– Inner interface

● Vacuum system, bake-out, bellows, flanges– Outer interface

● Tracking chambers, recesses● Complex cost optimisation

– W ideal but expensive– Optimise W-Pb distribution

Small Angle Absorber (SAA)

Tungsten

Lead

2°

0.8°

Hit rates

Dipole Magnet

• 3 Tm, resistive coil, 4 MW• Distance to IP 7 m• Bnom = 0.7 T• Gap l x h x w = 5 m x 5.1m x (2.5 – 4.1) m • Weight: 850 t

Dipole Installed

Tracking Chambers

● All stations with cathode segmentation varying with distance to beam axis – Higher hit density close to the beam-pipe, keep occupancy

– Both cathodes segmented

– Bending plane resolution <100 m

– Transparent: X/X0 ~ 3%

– Total area 100 m2

● 5 Tracking stations each made of 2 chambers

– 2 before dipole, 1 inside, and 2 behind dipole

– 1st station as close as possible to front absorber

– Robust combined angle-angle and sagita measurement

– Total number of channels ~106

● Muon stations 1-2

– Quadrants

– “Frameless” chambers ● Muon stations 3-5

– Slat design similar for all stations

– Production shared between several labs

Chamber segmentation

Stations 1st zone 2nd zone 3rd zone Max. hit

density

1 4x6 mm2 4x12 mm2 4x24 mm2 0.07 cm-2

2 5x7.5 mm2 5x15 mm2 5x30 mm2 0.03 cm-2

3,4,5 5x25 mm2 5x50 mm2 5x100 mm2 0.007 cm-2

Occupancies

Station 1 Station 2

Station 1

• 1999 Prototype– Anode-cathode gap: 2.5 mm

– Pad size 5 x 7.5 mm2

– Spatial resolution 43 m

– Efficiency 95%

– Gain homogeneity ± 12%

● New requirements (2000)– Suppression of the Al frames of Stations 1, 2 (+7% acceptance)– Decrease of the occupancy of Station 1

● Decrease of the pad sizes ( 4.2 x 6.3 mm2)● Decrease of anode-cathode gap (2.1 mm)

Station 1

● Mechanical prototype (fall 2001)

– Max. deformation 80 m

● Full quadrant (June 2002)

– 0.7 m2 frameless structure– 14000 channels per cathode – Gas : 80% Ar + 20 % CO2

– 3 zones with different pad sizes

Test Beam Results

Resolution 65 m

Stations 3-5

Rounded shape

Trigger Requirements

● In central Pb-Pb collisions ~8 per collision from /K decays.

● To reduce rate of muons not accompanied by high pT muon, pT cut is needed on the trigger level

– > 1 GeV/c (J/)

– > 2 GeV/c (Y family)

● Required resolution: 1 cm

Trigger

● Principle:– Transverse momentum cut using correlation of position and angle

● Deflection in dipole + vertex constraint● 4 RPC planes 6x6 m2

● Maximum counting rates – 3 Hz/cm2 in Pb-Pb– 40 Hz/cm2 in Ar-Ar– 10 Hz/cm2 in pp

● important contribution from beam gas● The chambers

– Single gap RPC, low resistivity bakelite (3 109 cm), streamer mode

– Electrode surface smoothened with linseed-oil– Gas mixture: Ar-C2H2F4-C4H10-SF6 @ 50.5-41.3-7.2-1%

Trigger Chamber Installed

Expected Performance

J/

Acceptance down to pT = 0Geometrical acceptance 5%

Low-mass Vector Mesons

Gray area: geometrical acceptanceBlue area: pT > 1 GeV/c

Mass Resolution

Design values

Contribution from front absorber higher- Non-Gaussian straggling- Electrons produced close to muons

Current value after full simulation and reconstruction:90 MeV (goal < 100 MeV)

Robustness of tracking

● Hit reconstruction– Maximum Likelihood - Expectation Maximization algorithm

● Tracking– Kalman filter

Reduced dependence on background level !

Expected mass resolution

dNch

/dy=2x6000 @ y=0

Quarkonia in HI Collisions

• Still more questions than answers– Melting of ’ and at SPS and RHIC, and melting of J/ at LHC?

– Magic cancellation between J/ suppression and J/ regeneration?

SPS

H. Satz, CERN Heavy Ion Forum, 09/06/05

RHIC

LHC

J/ Regeneration

J/ Melting

What will happen next ?

Y productionY production

RHICRHIC LHCLHC

R. Vogt, hep-ph/0205330

Quarkonia at LHC: New perspectives

(2S) (1S)

(3S)b(1P)b(2P)

SPS RHIC LHC

PRD64,094015

Y(1S) only melts at LHC.However important feed-down fromhigher resonances.

Y d/dy @ LHC ~20 x RHIC

Quarkonia at LHC: New challenges

● Important contribution to Charmonium production from B→J/(’) X – 22% of J/– 39% of ’

● Normalisation – Correlated continuum dominated by semileptonic heavy flavor decays.

● Drell-Yan not available for normalisation● Probes qq instead of gg

– W,Z ?● Different Q2, x

– Heavy Flavor● Energy loss ?● Thermal charm production ?

– Alternatives● MB method (NA50)● RAA (PHENIX)

Quarkonia ee

• (1S) & (2S) : 0-8 GeV/c• J/ high statistics: 0-20 GeV/c• ’ poor S/B at low pT

• ’’ difficult with one run

Yields for 0.5 fm-1 (~1 month)

State S[103] B[103] S/B S/(S+B)1/2

J/ 130 680 0.20 150

’ 3.7 300 0.01 6.7

(1S) 1.3 0.8 1.7 29

(2S) 0.35 0.54 0.65 12

(3S) 0.20 0.42 0.48 8.1

Pb-Pb cent, 0 fm<b<3 fm

Suppression scenario

• Suppression-1 Tc =270 MeV D/Tc=1.7 for J/ D/Tc= 4.0 for .

• Suppression-2 Tc=190 MeV D/Tc=1.21 for J/ D/Tc= 2.9 for .PRC72 034906(2005)

Hep-ph/0507084(2005)

Good sensitivity J/, (1S) & (2S)