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Quarkonia in deconfined matterAcitrezza, September 28th-30th 2011
Roberta ArnaldiINFN, Torino
for the ALICE Collaboration
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• J/ψ production in ultra-peripheral collisions• J/ψ flow
Physics motivations
J/ψ measurement in PbPb collisions @ 2.76 TeV with ALICE
RAA and RCP results
Comparison with results from other experiments
Comparison with models
Prospects:
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Physics motivations Quarkonium suppression is considered since a long time as one ofthe most striking signatures for QGP formation in AA collisions
…but many effects complicate the picture:•cold nuclear matter (cc break-up,shadowing)•cc recombination
Sequential suppression of quarkonium states
thermometer for the temperature reached in heavy ion collisions
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Towards LHC results…
• RHIC: stronger suppression at forward
rapidities• SPS vs. RHIC:not clear trend of the suppression
pattern versus energy
LHC results can shed some light, investigating the role of
• the large charm quark multiplicity
• other quarkonia states (bottomonium)(almost unexplored in HI collisions: STAR ~200ϒ(1S+2S+3S) in AuAu@200GeV)
After SPS and RHIC J/ψ results many questions are still open…
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First LHC PbPb run The first LHC heavy ion run took place in November 2010
Peak luminosity increasing from 3x1023 to 3x1025 Hz/cm2
Similar integrated luminosity for ALICE, ATLAS, CMS ~9 μb-1
PbPb collisions at √s=2.76 TeV
Quarkonium measurement in ALICE
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µ+
µ-
Quarkonium in ALICE can be measured in two ways:
e e
Central Barrel J/ψ e+e-
(|y|<0.9)
Electrons tracked using ITS and TPCParticle identification: TPC (+TOF)
Forward muon arm J/ψμ+μ-
(2.5<y<4)
Muons identified and tracked in the muon spectrometer
Acceptance coverage in both y regions down to zero pT
ALICE results refer to inclusive J/ψ production
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Pb-Pb collisions at √s=2.76TeV
Trigger:Minimum bias trigger (V0A and V0C and SPD)~17 106 MB events collected
Event selection:Rejection of beam gas events (V0 timing)
Negligible EM background in the centrality range (0-80%)
Integrated luminosity ~ 2.7 μb-1
Centrality:Estimate based on a Glauber model fit of the V0 amplitude
Statistics imposes rather wide centrality bins for J/ψ analysis:
J/ψ μ+μ-:0-10,10-20,20-40,40-80%J/ψ e+e-: 0-40, 40-80%
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J/ψ e+e- at mid-rapidity
Visible J/ψ signal, in spite of the low S/B (~0.04-0.3)
Electrons tracking: ITS and TPC
Particle identification: TPC (on-going PID with TRD and TOF)
Cuts:|ηe+e-|<0.8
0-40% 40-80%
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J/ψ μ+μ- at forward rapidity
• to improve the purity of the muon sample, both muons reconstructed in the tracking and trigger chambers should satisfy the trigger algorithm
reduction of hadronic background and fake tracks
• cut on the track position at the end of the front absorber
• 2.5 < yJ/ψ < 4, 2.5<ημ<4, pT J/ψ>0 GeV/c
Event/Track selection:
No identification of J/ψ from B decays scattering in the absorber spoils the muon vertex information
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Signal extraction
• Crystal Ball shape for the signal (tails fixed to pp data, pure MC, MC signal embedded into real events)
• sum of two exponentials for the background• ψ(2S) negligible with this statistics
S/B centrality dependent (~0.1-0.7), but good significance also in most central bin
J/ψ yield extracted from a fit (2-5 GeV/c2) to the invariant mass spectrum assuming
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Alternative signal extraction
Results obtained with different techniques combined to extract <NJ/ψ> and to evaluate systematic uncertainties
Mixed event technique also used to estimate the background•mixed pair invariant mass distribution normalized to data in
(1.5, 2.5) GeV/c2
•fit the background subtracted mass distribution with a Crystal Ball function and a straight line (or exponential) to account for a small residual background
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Centrality dependence of J/ψ parameters
The increase of detectors occupancy with centrality may worsen the reconstruction resolution, affecting the J/ψ shape
centrality dependence of J/ψ parameters is studied embedding a MC J/ψ into a real event
No centrality dependence of J/ψ width and Crystal Ball parameters
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Acceptance x efficiency Two approaches:
Standard Monte-Carlo:
J/ψ y and pT distributions interpolated from Phenix, CDF and LHC data
A x eff = 19.44 ± 0.04 (stat)%
small decreasing (~5%) of the reconstruction efficiency when increasing centrality
MC J/ψ embedded in real event
allows the study of the A x eff centrality dependence
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Nuclear modification factor
in each centrality bin ( )TeVppT
YR
inclJ
iAA
iJi
AA 76.2@ψ
ψ
σ×=
( ) ( ) iMB
iJi
J NAllJBRN
Y×××→
= −+ εψψ
ψwith:
pp reference: measured J/ψ production cross section in pp@√s=2.76 TeV (3 days data taking in March 2011 see talk G. Bruno)
( ) blumisystsyststatTeVppinclJ μσ ψ ).(28.0)(32.0)(13.046.376.2@ ±±±=
%8040%8040 −−=
AAJ
iAA
iJi
CP TY
TYR
ψ
ψ
RAA
RCP
peripheral reference:Warning: 40-80% centrality bin…not really peripheral…
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Systematic uncertainties
sources centrality
0-10% 10-20% 20-40% 40-80% correlated
NJ/ψ 19 % 14 % 17% 14% -
NJ/ψ/NJ/ψ40-80% 12% 8% 7% - -
Acc. inputs - - - - 3%
Trigger eff. - - - - 4%
Tracking eff. 4% 2% 1% 0% 5%
Reco eff. - - - - 2%
Branching ratio - - - - 1%
Cross Section - - - - 13%
<TAA> 4% 4% 4% 6% -
<TAA>i/<TAA>40-80% 6% 5% 4% - -
Total for RAA 20% 15% 17% 15% 15%
Total for RCP 14% 10% 8% - -
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Inclusive J/ψ RAA0-80% = 0.49 ± 0.03 (stat.) ± 0.11 (syst.)
Contribution from B feed-down:~ 10% from pp measurement (LHCb Coll., arXiv:1103.0423)Rough estimation assuming simple Ncoll scaling : ~11% reduction of RAA
0-80%
Error bars:Statistical uncertainties
Empty boxes:Centrality-dependent systematic uncertainties
Blue box:common systematic uncertainties
RAA vs. centrality
Suppression with no strong centrality dependence
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Comparison with PHENIX
*>part<N0 50 100 150 200 250 300 350 400
AA
R
0
0.2
0.4
0.6
0.8
1
1.2
1.4 >0 (preliminary)T
= 2.76 TeV), 2.5<y<4, pNNsALICE (Pb-Pb
>0 (arXiv:1103.6269)T
= 0.2 TeV), 1.2<|y|<2.2, pNNsPHENIX (Au-Au
coll> is weighted by N
part(*) ALICE <N
Large ALICE centrality bins the <Npart> values have been weighted by Ncoll
J/ψ RAA is larger with respect to PHENIX results at forward y Difference increases with centrality
*>part<N0 50 100 150 200 250 300 350 400
AA
R
0
0.2
0.4
0.6
0.8
1
1.2
1.4 >0 (preliminary)T
= 2.76 TeV), 2.5<y<4, pNNsALICE (Pb-Pb
>0 (arXiv:1103.6269)T
= 0.2 TeV), 1.2<|y|<2.2, pNNsPHENIX (Au-Au
>0 (nucl-ex/0611020)T
= 0.2 TeV), |y|<0.35, pNNsPHENIX (Au-Au
coll> is weighted by N
part(*) ALICE <N
J/ψ RAA is compatible with RHIC mid-rapidity result, except for the most central collisions
…but CNM effects may be different at the two energies
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Comparison with CMS results
ALICE RAA is significantly higher than the CMS result, but the covered kinematical region (y and pT) is different
CMS PAS HIN-10-006
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Nuclear shadowing
centrality40-80% 20-40% 10-20% 0-10%
AA
R
0
0.2
0.4
0.6
0.8
1
1.2
1.4>0 (preliminary)
T = 2.76 TeV), 2.5<y<4, pNNsALICE (Pb-Pb
EPS09 (R. Vogt , priv. comm.)
Let’s consider shadowing as the only CNM effect (cc break-up cross section expected to be negligible at LHC energies)
R. Vogt, Phys.Rev.C81:044903, 2010JP. Lansberg, QM2011
Different approaches (J/ψ production through 2 1 or 2 2 partonic processes) and several nPDF adopted
Important shadowing contribution, but nPDF are affected by large uncertainties pA @ LHC (end 2012?)
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Comparison to models
Statistical hadronization:
• Screening by QGP of all direct J/ψ
• CNM (shadowing) on open charm
• Charmonium production at phase boundary by statistical combination of uncorrelated charm quarks
P. Braun-Muzinger, J. Stachel, PLB490 (2000) 196A.Andronic@QM2011, Andronic et al.,PLB571(2003)36
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Comparison to models
• Prompt J/ψ dissociation in QGP• J/ψ regeneration by charm quark
pair recombination• Feed-down from B decays• Shadowing
Parton transport models:R.Rapp, X. Zhao, NPA 859, 114(2011)R.Rapp, X. Zhao, PRC82 064905(2010)
Prediction including:
X.Zhu, P.Zhuang,N.Xu,PLB607(2005)107L.Yan,P.Zhuang,N.Xu,PRL97(2006)232301Y.Liu,Z.Qu,N.Xu,P.Zhuang,PLB678(2009)72P.Zhuang, WE-Heraeus Seminar 2011
• Shadowing and Cronin effect• Prompt J/ψ dissociation in QGP• J/ψ regeneration by charm
quark pair recombination • Feed-down contribution from B
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RCP
Error bars:Statistical uncertainties
Empty boxes:Centrality-dependent systematic uncertainties
RCP is normalized to the centrality bin 40-80%
Statistical uncertainty of the reference bin propagated to the ratioNon centrality-dependent uncertainties cancel out
Values close to 1 reflect the RAA flatness vs. centrality
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Comparison with ATLAS result
centrality40-80% 20-40% 10-20% 0-10%
nor
mal
ized
to 4
0-80
%C
PR
0
0.2
0.4
0.6
0.8
1
1.2
= 2.76 TeVNNsPb-Pb >0 (preliminary)
TALICE, 2.5<y<4, p
>6.5 GeV/c (arXiv:1012.5419)T
ATLAS, |y|<2.5, p
J/ψ RCP larger for ALICE than for ATLAS… but different rapidity and pT coverage can play a role
ALICE:•2.5<y<4•pT > 0
ATLAS:•|y|<2.5•80% of J/ψ with pT>6.5 GeV/c
centrality40-80% 20-40% 10-20% 0-10%
nor
mal
ized
to 4
0-80
%C
PR
0
0.2
0.4
0.6
0.8
1
1.2
= 2.76 TeVNNsPb-Pb >0 (preliminary)
TALICE, 2.5<y<4, p
>0 (preliminary)T
ALICE, |y|<0.8, p>6.5 GeV/c (arXiv:1012.5419)
TATLAS, |y|<2.5, p
Very challenging analysis… error bars are still large. Wait for 2011 high luminosity run!
Inclusive J/ψ RCP can be also measured in ALICE at mid-rapidity in the dielectron channel
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J/ψ in ultra-peripheral collisions Probe gluon distribution in the nuclei (down to x~10-4)
Search for tracks in the central barrel (ITS+TOF) or muon spectrometer, with a veto on V0 detectors
J/ψ e+e- J/ψ μ+μ-
Visible J/ψ signals in ultra peripheral PbPb collisions
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Prospects: J/ψ flow
At LHC, J/ψ regeneration could play an important role and will dominate the J/ψ flow
Several methods explored: here event plane method, using TPC tracks
Study is feasible, now waiting for larger data sample in 2011
Y. Liu, N. Xu, P. Zhuang, Nucl.Phys.A834(2010) 317c
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Summary and outlook
The ALICE experiment has measured the inclusive J/ψ production in PbPb collisions at √sNN = 2.76 TeV at both mid-rapidity and forward rapidity
A deeper understanding requires a precise knowledge of cold nuclear matter effects: waiting for pA at LHC in 2012!
• normalized to J/ψ cross-section in pp at the same energy• shows a clear suppression of the J/ψ yield with respect to
Ncoll scaling• forward values are larger than forward RHIC results• RAA (RCP) values are larger than those measured by CMS
(ATLAS), but the kinematical coverage is different
Next 2011 PbPb run with >5 times Lint (2010) should improve the quality of the measurements, allowing the study of J/ψ flow and production in ultra-peripheral collisions
RAA
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backup
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Comparison RHIC/SPSRAA comparison between SPS and RHIC
…picture not yet clear!
…but recently the pp reference was obtained directly from NA60 pA @ 158 GeV
the comparison looks different!
The initial estimate of the pp reference was obtained from pA data at higher energy, 450 GeV, (and rescaled to 158GeV)
All RAA looked similar!
pp reference is crucial to correctly interpret the results!
CAVEAT: at SPS no pp data taking @ 158GeV need to build the reference extrapolating pA data to A=1
Comparison SPS and RHIC
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front absorber (10λI)
tracking system (10 detection sub-planes, 70μm resolution in the bending plane)
dipole magnet (3Tm)
muon-filter wall (7λI)
trigger chambers(4 planes, programmable pT cut, with pT
μ>0.5GeV/c)
It is designed in order to have:
• large geometrical acceptance to increase dimuon statistics• acceptance down to pT~0 where direct J/ψ production dominates• good mass resolution to separate the ϒ family• tracking/trigger high granularity read-out to cope with the high multiplicity
The muon spectrometer
LHC
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√s G
eV/c
1986 1990 ~2000 2010
2760
200
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RHIC
SPS
Year
Physics motivations Quarkonium suppression is considered since a long time as one ofthe most striking signatures for the QGP formation in AA collisions
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Quarkonium measurement in ALICE
3 sources of J/ψ
• Direct production• Feed down from heavier
charmonium states
• J/ψ from b-hadron decay
Prompt J/ψ
J/ψ from B
feasible in the central barrel, thanks to the good impactparameter resolution (σrφ < 75 μm for pT>1 GeV/c)
forward detection more difficult3-muon eventsB cross section from single muon measurement
ALICE results refer to inclusive J/ψ production
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