Muon Identification in ALICE Andreas Morsch CERN PH Workshop on Muon Detection in the CBM Experiment GSI 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 ~ 1000 Members (63% from CERN MS) ~30 Countries ~90 Institutes

Muon Forward Spectrometer L3 magnet B ≤ 0.5 T Muon Forward Spectrometer 2.4 <  < 4 Total weight : 9,800 tons Overall diameter : 16 m Overall length : 26 m 130 MCHF CORE

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] Dy Pb+Pb 1 1027 5.5 Ar+Ar 6 1028 6.3 O+O 2 1029 7.0 pPb 1 1030 8.8 0.5 pp 1 1034 14 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 Spectrometer Design 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/y 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’) (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 Acceptance: 2o < Q < 9o (-4 < y < -2.5) Minimum muon momentum: 4 GeV/c Resonance pT: > 0 Mass resolution 70 MeV/c2 (100 MeV/c2) Passive Front Absorber to absorb hadrons from the interaction vertex and to reduce the K/p 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.

Absorbers - Suppress p/K®m 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 10lI are needed to suppress hadron flux. Angle measurement from hit position in first chamber and vector measured by the first station. Branson plane method DQ~30 mrad/p Angular resolution ~L Low density material close to the interaction point, high density material at the rear.

Front Absorber (FA) Steel Concrete Carbon Tungsten ~10 lI (Carbon – Concrete – Steel) FASS Steel Concrete Carbon Tungsten

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) 2° 0.8° Lead Tungsten

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 mm 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 New requirements (2000) Anode-cathode gap: 2.5 mm Pad size 5 x 7.5 mm2 Spatial resolution 43 mm 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 mm 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 mm

Stations 3-5 Rounded shape

Trigger Requirements In central Pb-Pb collisions ~8m per collision from p/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/y) > 2 GeV/c (Y family) Required resolution: 1 cm

Trigger Principle: 4 RPC planes 6x6 m2 Maximum counting rates 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 Geometrical acceptance 5% Acceptance down to pT = 0 J/ 

Low-mass Vector Mesons Gray area: geometrical acceptance Blue 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 Y’ and c at SPS and RHIC, and melting of J/Y at LHC? Magic cancellation between J/Y suppression and J/Y regeneration? LHC RHIC SPS J/y Regeneration What will happen next ? H. Satz, CERN Heavy Ion Forum, 09/06/05 J/y Melting

Quarkonia at LHC: New perspectives Y(1S) only melts at LHC. However important feed-down from higher resonances. Y ds/dy @ LHC ~20 x RHIC Y production RHIC LHC R. Vogt, hep-ph/0205330 (2S) (1S) (3S) cb(1P) cb(2P) SPS RHIC LHC PRD64,094015

Quarkonia at LHC: New challenges Important contribution to Charmonium production from B→J/y(y’) X 22% of J/y 39% of y’ 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  e+e-, m+m- (1S) & (2S) : 0-8 GeV/c Pb-Pb cent, 0 fm<b<3 fm State S[103] B[103] S/B S/(S+B)1/2 J/Y 130 680 0.20 150 Y’ 3.7 300 0.01 6.7 (1S) 1.3 0.8 1.7 29 (2S) 0.35 0.54 0.65 12 (3S) 0.42 0.48 8.1 Yields for 0.5 fm-1 (~1 month) (1S) & (2S) : 0-8 GeV/c J/Y high statistics: 0-20 GeV/c Y’ poor S/B at low pT ’’ difficult with one run

Suppression scenario Suppression-1 Suppression-2 Tc=190 MeV TD/Tc=1.7 for J/Y TD/Tc= 4.0 for . Suppression-2 Tc=190 MeV TD/Tc=1.21 for J/Y TD/Tc= 2.9 for . PRC72 034906(2005) Hep-ph/0507084(2005) Good sensitivity J/Y, (1S) & (2S)