1. Muon Identification in ALICE
Andreas Morsch
CERN PH
Workshop on Muon Detection in the CBM Experiment
GSI Darmstadt 16-18/10/2006

2. Outline The ALICE Experiment at the LHC Muon Spectrometer overview
Muon Spectrometer components
Tracking Chambers
Trigger Chambers
Absorbers
Dipole Magnet
Expected performance

4. ALICE Collaboration ~ 1000 Members (63% from CERN MS) ~30 Countries
~90 Institutes

5. 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

6. Highlights Large Acceptance Coverage
Large Momentum Coverage (from 100 MeV/c to > 100 GeV/c)
High Granularity ( designed for dN/dy ~ 8000, i.e 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

7. 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)

8. LHC Official Schedule October 2006 Last magnet delivered
March Last magnet installed
August Machine closes
November 2007 First collisions s=900 GeV
Spring Collisions at s=14 TeV

9. 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.

10. 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.

11. 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)

12. 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.

13. Absorbers - Suppress p/K®m decay
- Shield from secondaries in particular at small radii.

14. 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.

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

16. FA Installation

17. 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

18. Small Angle Absorber (SAA)2°
0.8°
Lead
Tungsten

19. Hit rates

20. 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

21. Dipole Installed

22. 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

23. 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

24. Occupancies
Station 1
Station 2

25. 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)

26. 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

27. Test Beam Results
Resolution 65 mm

28. Stations 3-5
Rounded shape

29. 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

30. 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: %

31. Trigger Chamber Installed

32. Expected Performance Geometrical acceptance 5%
Acceptance down to pT = 0
J/ 

33. Low-mass Vector Mesons
Gray area: geometrical acceptance
Blue area: pT > 1 GeV/c

34. 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)

35. Robustness of tracking
Hit reconstruction
Maximum Likelihood - Expectation Maximization algorithm
Tracking
Kalman filter
Reduced dependence on
background level !

36. Expected mass resolution
y=0

37. 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

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

39. 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)

40. 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) : GeV/c
J/Y high statistics: 0-20 GeV/c
Y’ poor S/B at low pT
’’ difficult with one run

41. 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 .
PRC (2005)
Hep-ph/ (2005)
Good sensitivity
J/Y, (1S) & (2S)