Physics with muons in the ALICE experiment at the LHC
Z Buthelezi
for the ALICE Collaboration
iThemba LABS, Cape Town, South Africa
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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Outline
•Introduction – Heavy Quarks in QGP medium
• ALICE status and performance
• Results in the forward rapidity region of ALICE
•proton-proton collisions
•PbPb collisions
•Outlook on new study : W  production in the forward
rapidity region of ALICE
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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Heavy quarks in QGP medium
Diagnostic tool for the formation of the QGP
• Heavy quarks are produced at the
beginning of the collisions (high Q2)
• Pass through the medium and interact
with it, losing energy by gluon radiation
and elastic collisions
- energy loss mechanisms
– parton mass (Dead cone effect)
– colour charge (Casimir factor)
-
ΔEg > ΔEu,d ,s > ΔE c > ΔE b
Compare heavy / light flavour observables
1 d 2 N AA / dpt d
RAA ( pt , ) 
2
N coll d N pp / dpt d
- Probe medium properties
Medium density & size
RAA() < RAAD < RAAB
Yu. Dokshitzer and D.E. Kharzeev, Phys.Lett. B 519, 199‐206 (2001).
Armesto, Salgado, Wiedemann, PRD 69 (2004) 114003.
Djordjevic, Gyulassy, Horowitz, Wicks, NPA 783 (2007) 493.
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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Quarkonia, heavy ions and the QGP?
History:
– J/ψ suppression as a QGP signature (1986,
Matsui and Satz- Phys. Lett. B178 (4): 416)
machine
sN (GeV)
SPS RHIC
17 200
– NA38, NA50, NA60 @ SPS
– PHENIX, STAR @ RHIC
Left open questions:
–J/ψ suppression “anomalously” similar @ RHIC
& SPS
– Sequential suppression (’, c) only
– cold nuclear matter effect - still weakly
constrained
– statistical hadronization, recombination?
that could be answered by the LHC
enhanced J/ production yields @ LHC Thews et al. Phy.
Rev. C 63(5):054905 (2001)
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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HF muon production @ the LHC?
machine
sNN (GeV)


SPS
17
RHIC
200
LHC
2760
sLHC → best conditions to study QGP(S. Abreu, et al. arXiv:0711.0974)
Access hard probe production cross sections
Muon sources @ LHC
- Light mesons decays: , K,
- Charm decays: D, cc mesons (J/ +-) (~ 20 x RHIC)
- Beauty decays: B, bb mesons (+-) (~100 x RHIC)
- W / Z decays


Muon pt distribution in HI is sensitive to b-quark energy loss
effects  RAA reduction due to nuclear shadowing of PDF
W / Z yields expected to scale with <Ncoll> in parton-parton
scattering  calibration for HF RAA & test of the binary
scaling assumptions
Z.Conesa del Valle et al., arXiv:0712.0051v1
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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The ALICE Detector
Size: 16 x 26 meters
Weight: 10,000 tons
Detectors: 18
- HI collisions: measure all known observables to characterise the medium formed in the collisions
- pp collisions: baseline for A-A and intrinsic interest, test pQCD models J. Instrum. 3, S08002 (2008)
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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ALICE Acceptance
• Central barrel: -0.9 <  < 0.9
2  tracking, PID (ITS, TPC, TRD, TOF)
– single arm RICH (HMPID)
– single arm em. calo (EMCAL ) Reduced
– PHOS
acceptance
Quarkonia: e+e- channel
Heavy flavour: hadronic channel, semi-leptonic decays
(e+/-)
• Forward muon arm: 2.5<y<4  Probe small
Bjorken-x (10-5 – 10-3)
Resolution (ΔP/P) goal: 1% @ 20 GeV/c, 4% @ 100 GeV/c
absorber, 3 Tm dipole magnet
10 tracking + 4 trigger chambers
Quarkonia: +- channel
Heavy flavour: semi-leptonic decays (+/-)
•Backward + Forward small acceptance detectors:
multiplicity, centrality, luminosity & triggering
(charged particles)
µ arm
J. Instrum. 3, S08002 (2008)
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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Results from proton-proton collisions @ 7 TeV in 2.5<y<4.0
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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cross section vs pt and y
Invariant mass distribution
Inclusive J/
measurements
Yield vs multiplicity
•Highest charge multiplicity (dNch/d  30)
reached.
• linear increase of the yield vs multiplicity  test
interplay between hard & soft interactions in
multi-partonic interactions and/or underlying
event.
Polarization vs pt
Comparison with other LHC exp:
•ALICE covers large rapidity range.
•Good agreement with LHCb data.
•Data well described by the NRQCD
calculations
Phys. Let. B 704 (2011) 442455
First J/Ψ polarization measurement @ LHC.
- θ in HF - longitudinal polarization @ low pt
which tends to vanish with increasing pt.
CSF it is constant with polarisation
-λφ is constant with 0.
-Results can be used as stringent constraint to
tune QCD models (NRQCD) for heavy quark
production
ArXiv:1111.1630, Phys. Rev Lett., in press
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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Low-mass dimuons
arXiv:1112.2222v1 [nucl-ex]
•Dimuon mass spectrum given as resonance (, ) decays into
muons and semi-leptonic decays of charmed mesons
•Tune models to data especially for hadrons containing u, d, s
quarks
•Give insight soft QCD processes (multi-parton interactions, pile
up), which affects the high pt studies.
Upsilon () production
•Member of B-quark family
•Preliminary plot of the
invariant mass distribution
Analysis is ongoing…
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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Production of HF single muons
•
•
•
Background (,K) subtracted using MC after
normalization to data @ low pt.
Differential cross-sections are well described
according to FONLL( pQCD).
Muon from beauty decay dominate for pt >~6
GeV/c.
arXiv:1201.3791, accepted by Phys. Lett. B
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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Results from Pb-Pb collisions @ 2.76 TeV in 2.5<y<4.0
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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Inclusive J/ RAA
1 d 2 N PbPb / dpt d
RAA ( pt , ) 
2
N coll d N pp / dpt d
-Little to no dependence on centrality.
-Less suppression observed at LHC than
@ RHIC in the forward rapidity region.
 more shadowing @ LHC but nuclear
absorption is less to none.
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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Exclusive J/ production - ultra peripheral collisions
- Two ions (or protons) pass each other with impact parameters, b > 2R
-Electromagnetic interactions are possible
-Number of photons scale as Z2 for single source
⇒ exclusive particle production in HI dominated by electromagnetic interaction
- In Pb+Pb collisions at 5.5 TeV: (hadronic) ~ 8b
R. Bruce et al., Phys.Rev. 12 071002 (2009) (e+e-) ~ 281 b
(EMD) ~ 226 b
- Allow access to information on the gluon density
- Nuclear gluon shadowing
- Coherent (J/ + A production in UPC)
- MUON in coincidence with V0C, but V0A vetoed.
- Veto activity on V0C outside muon acceptance.
- 2 good tracks in the muon acceptance, both tracks
match the trigger.
- At least one track has a pt > 1 GeV/c.
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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J/ elliptic flow
azimuthal distribution of particles in the plane  to the beam axis, z

d 3N
1 d 2N 

E 3 
1   2 n cos n  R 
d p 2 pt dpt d y  n 1

Y. Liu, N. Xu, P. Zhuang, Nucl.Phys.A834 (2010) 317c
E - energy of particle,
P - momentum,
pt - transverse momentum,
y – rapidity,
- azimuthal angle,
R - reaction plane angle
n- Fourier coefficient, lowest
order terms:
1- direct flow
2- elliptic flow
•Regeneration / combination of c quarks for J/
production will dominate the J/ flow @ LHC
compared to those at low energies
•Various methods are being investigated for v2J/
elliptic flow.
•Analysis is in progress . High statistics sample from
2011 PbPb runs will allow to measure J/ flow from
pt = 0 GeV/c.
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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Production of HF single muons
RCP ( pt ) 
1 / T
1 / T
AA
AA
 dN  / dpt

 dN / dpt


central
peripheral
TAA – nuclear overlap function
Grey – normalization error: ratio <TAA>peripheral /<TAA>central
Red box: systematic errors dN/dpt
Bars: statistical errors
-Background from , K decays were not subtracted.
-Muons from HF decays dominate for pt > 4 GeV/c.
-Suppression of a factor ~2.5 in the most central collision.
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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Outlook on new study:
W± production via single muon channel in the forward rapidity
region of ALICE: 2.5 < y < 4.0
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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Motivation
Initial study “W production in pp, PbPb & pPb collisions @ LHC
energies: W± detection in the ALICE Muon Spectrometer”
Z.Conesa del Valle et al., Eur. Phys. J. C61 (2009) 729-733
Scientific motivation:
- W muons have a high pt ~ MW /2  W± as reference for observing
QGP induced effects on QCD probes, e.g. suppression of high pt
heavy quarks
-
Probe PDF in the Bjorken-x:
x (10-4–10-3)  2.5 < y < 4.0 @ Q2 ~ M2W±
MW = 80.398 ± 0.25 GeV/c2,
electric charge: ±1 e,
spin = 1
weak interactions  change
generation of a particle
(quark flavour change)
- study nuclear modification of quark distribution, validate binary
scaling
Measurements in the forward rapidity region of ALICE: 2.5 < y < 4.0 , pt>1 GeV/c, P > 4 GeV/c
will be complimentary to those done by ATLAS & CMS @ central rapidity || 2.5,
with large pt > 3-4 GeV/c
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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Predictions with PYTHIA
Process: 2  1, initial and final state radiation ff W 
Decay channels : W →  +  (10.57 ± 0.15%),
W  cX ... Y (33.6 ± 2.6 %)
PbPb collisions @ s = 5.5 TeV: pp, nn & np combinations.
PDF  EKS98 shadowing parameterization K.J Eskola et al
Euro. Phys. J. C, 9:61, 1999
Lowest & 2nd order diagrams for W production in
hadron-hadron collisions
Differential cross section - Frixione and Mangano, Hep-ph/0405130

th
W   
exp
1
1 N Obs
W  
BR W     Ldt AW
<L>PbPb~5·1032 cm-2s-1 for t = 107 s (~ 0.5 nb-1) G. Martinez, hepex/0505021v1 (2005), F. Carminati et al. J. Phys G30, 1517, 2004
(W)PYTHIA x BRW→ = 17.3 nb , spectra normalised to NLO
calculations: th(W)NLO x BRW→ = 20.9 nb Lai et al, arXiv:hepph/9060399v2,10 Aug 996
W muons have a high pt  MW /2  30 – 50 GeV/c. It is estimated that @ pt =(30, 50) GeV/c the
reconstructed yield, NW = 6.9x103 the forward rapidity range: 2.5 < y < 4.0 . Z Conesa del Valle et al.,
arXiv:0712.0051v1
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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Recent PbPb runs @ the LHC
http://lpcc.web.cern.ch/LPCC/index.php?page=luminosity_charts
Increase in luminosity  statistics increase by ~ 18 times more than in 2010.
High pt reach for HF muon production, e.g. W and Z decay muons ?
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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Strategy for W±± production in PbPb collisions with ALICE
–
Measure differential cross sections as
function of pt and y in forward region:
2 <  < 9   2.5 < y < 4, pt >1 GeV/c, P >
4 GeV/c
–
–
–
Z Conesa del Valle et al., arXiv:0712.0051v1
Subtract background in the pt region
below pt  MW / 2  30–50 GeV/c.
Use high level trigger to reject
background. J. Phys: Conf. Series 219 (2010) 02244
Compare single muons ratios: + / -
 charge asymmetry.
–
Extract RAA, RCP.
–
Compare with theoretical predictions.
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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Summary
•ALICE has measured inclusive J/, low mass dimuons and HF single muon cross
sections in pp collisions at 7 TeV. Good progress also on analysis at 2.76 TeV.
•Upsilon analysis at 7 TeV is in progress.
•Data show a suppression of HF decays in PbPb collisions is seen in RCP.
•It is observed that suppression increases with increasing centrality.
•For the J/ case, the observed suppression is almost constant wrt centrality.
•Exclusive J/ measurements in ultra-peripheral PbPb collisions in the forward rapidity
region of ALICE are also underway.
•Statistics from the 2011 PbPb runs should allow to study W production in the forward
rapidity region of ALICE.
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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Back up slides
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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Cross Section evaluation
The same procedure adopted for the 7TeV paper has been followed
Where:
NMB ~ 22.5 106
NMB ~ 1.4 104
N-MB ~ 4.2 105
number of CINT1B triggers. After physics selection, a small decrease
is expected (was ~1-2% in LHC10e)
Number of single  with pT>1GeV/c, surviving standard
cuts, in CINT1B (NMB) or CMUS1B (N-MB ) trigger
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch,
South Africa
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Centrality measurements in PbPb collisions @ 2.76 TeV
Events: - MB trigger events
Centrality class: 0-80% determined by V0
Glauber model analysis of large-η V0
scintillator amplitudes :V0A : 2.8 < η < 5.1,
V0C : -3.7 < η < -1.7
Phys. Rev. Lett. 106, 032301 (2011)
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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HF muon cross sections in different pt bins, pp collisions @ 7TeV
arXiv:1201.3791, accepted by Phys. Lett. B
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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Comparison of theoretical prediction for W production at the LHC (Anastasiou et al, hepph/0312266v2)
PbPb collisions @ 2.76 TeV: J/Psi RCP measurements in 2.5<y<4.0
RCP ( pt ) 
1 / T
1 / T
AA
AA
 dN  / dpt
 dN  / dpt


central
peripheral
ALICE data exhibits less suppression @
forward rapidity and low pt compared to
ATLAS
TAA – nuclear normalisation factor
Exploring QCD Frontiers: from RHIC and LHC to EIC, 29 Jan - 3 Feb 2012, Stellenbosch, South Africa
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