1Quark Matter 2012, Washington, August 13th-18th 2012

J/ and (2S) production in Pb-Pb collisions with the ALICE Muon

spectrometer at the LHC

Roberta ArnaldiINFN Torino (Italy)

for the ALICE Collaboration

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J/ differential RAA versus centrality of the collisionstransverse momentum(rapidity*)

J/ <pT> and <pT2>

Comparison with theoretical models

J/ and (2S) production in Pb-Pb collisions with the ALICE Muon

spectrometer at the LHC

(2S) production in Pb-Pb collisions

Results from the 2011 Pb-Pb run on:

* See I. Arsene, parallel 2D

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Quarkonia suppressionQuarkonia suppression is considered, since a long time, as one of the most striking signatures for QGP formation in AA collisions

Sequential quarkonia suppression:Information on the initial temperature of QGP …but many effects to be taken into account: cold nuclear matter, cc (re)combination

• RHIC: stronger suppression at forward rapidities

• SPS vs. RHIC: similar RAA pattern versus sLHC results can give decisive inputs, investigating at higher QGP temperature, the role of

• the large charm quark multiplicity( (

• other quarkonia states (bottomonium)

Puzzles from SPS and RHIC

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Quarkonia measurement in ALICEQuarkonium in ALICE can be measured in two ways:

Central Barrel J/ e+e-

(|y|<0.9)

Electrons tracked using ITS and TPCParticle identification: TPC (+TOF)

Forward muon arm J/ +-

(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/ production

I. Arsene, parallel 2D

µ+

µ-e+ e-

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Pb-Pb collisions at sNN=2.76 TeVResults from 2011 data set, based on dimuon triggered events

Event selection:Rejection of beam gas and electromagnetic interactions (VZER0 and ZDC)SPD used for vertex determination

Integrated luminosity ~ 70 b-1

Centrality selection:Estimate based on a Glauber model fit of the VZERO amplitude

Muon track selection:• Muon trigger matching• -4<ημ<‐2.5• 17.6<Rabs<89 cm (Rabs= track radial position at the absorber end)• 2.5<yμμ<4

VZERO

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J/ +- signalJ/ yield extracted fitting the opposite sign dimuon invariant mass spectrum

0-90% centrality, pT>0 GeV/c

Results obtained with the different techniques are combined to extract <NJ/ψ> and to evaluate systematic uncertainties

Signal: shape described by an extended Crystal Ball function

Background: several methods used (different fitting functions, event mixing)

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J/ +- signalJ/ yield extracted fitting the opposite sign dimuon invariant mass spectrum

0-90% centrality, pT bins

Thanks to the high statistics 2011 sample, J/ yields can be extracted also in narrow kinematic bins (y, pT and/or centrality)

Results obtained with the different techniques are combined to extract <NJ/ψ> and to evaluate systematic uncertainties

Signal: shape described by an extended Crystal Ball function

Background: several methods used (different fitting functions, event mixing)

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J/ acceptance x efficiencyAcceptance x efficiency computed embedding MC J/’s into real events

Weak centrality dependence of the J/ efficiency (decreasing from 14.5% to 13.3% from peripheral to central collisions)

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J/ RAA vs. centrality

RAA 0-90% (inclusive J/) = 0.497 0.006(stat) 0.078 (sys)

J/ measured in pp @s=2.76 TeV used as reference (arXiv:1203.3641)

Clear J/ suppression with almost no centrality dependence above Npart~100

Negligible effect on RAA from non-prompt J/

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J/ RAA vs. centrality

Comparison with PHENIX:

RAA 0-90% (inclusive J/) = 0.497 0.006(stat) 0.078 (sys)

J/ measured in pp @s=2.76 TeV used as reference (arXiv:1203.3641)

Clear J/ suppression with almost no centrality dependence above Npart~100

Negligible effect on RAA from non-prompt J/

weaker centrality dependence and smaller suppression for central events

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J/ RAA vs. centrality

Comparison with models:Models including a large fraction (> 50% in central collisions) of J/ produced from (re)combination or models with all J/ produced at hadronization can describe ALICE results in central collisions. Interesting, in this context, to look also at J/ v2

Comparison with PHENIX:weaker centrality dependence and smaller suppression for central events

RAA 0-90% (inclusive J/) = 0.497 0.006(stat) 0.078 (sys)

J/ measured in pp @s=2.76 TeV used as reference (arXiv:1203.3641)

Negligible effect on RAA from non-prompt J/

H. Yang, parallel 7A

Clear J/ suppression with almost no centrality dependence above Npart~100

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J/ RAA vs. centrality in pT binsJ/ production via (re)combination should be important at low pT

Different suppression for low and high pT J/

Smaller RAA for high pT J/

Comparison of the RAA centrality dependence of low (0<pT<2 GeV/c) and high (5<pT<8 GeV/c) pT J/0<pT<2 GeV/c

5<pT<8 GeV/c

Errors:• uncorrelated (box around points)• partially correlated within and

between sets ([])• 100% correlated within a set and

between sets (text)

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J/ RAA vs. centrality in pT bins

In central collisions, these models (X. Zhao et al, Y.P. Liu et al, E. Ferreiro) predict ~50% of low pT J/ to be produced via (re)combination, while at high pT the contribution is negligible

recombination

0<pT<2 GeV/c

5<pT<8 GeV/c

recombination

primordial

primordial

J/ production via (re)combination should be important at low pT

Different suppression for low and high pT J/

Smaller RAA for high pT J/

Comparison of the RAA centrality dependence of low (0<pT<2 GeV/c) and high (5<pT<8 GeV/c) pT J/

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J/ RAA vs. pT

As an alternative view, RAA is shown as a function of the J/ pT for various centrality bins

Splitting in centrality bins we observe that the difference low vs high pT suppression is more important for central collisions

Suppression is stronger for high pT J/ (RAA~0.6 at low pT and ~0.35 at high pT)

0-90% 0-20% vs 40-90%

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J/ RAA vs. pT

As an alternative view, RAA is shown as a function of the J/ pT for various centrality bins

Splitting in centrality bins we observe that the difference low vs high pT suppression is more important for central collisions

Suppression is stronger for high pT J/ (RAA~0.6 at low pT and ~0.35 at high pT)

Fair agreement between data and models including a large contribution from (re)combination (~50% in central and ~30% in peripheral collisions)

0-20% vs 40-90%0-90%

recombination

total

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J/ pT spectra

J/ <pT> and <pT2> values are extracted

from fits to d2N/dydpT

x

T

T

T

pp

pdydpNd

2

0

2

1

Comparison with lower energy results can be carried out bycomparing <pT> and <pT

2> vs centrality

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J/ <pT> and <pT2>

The J/ <pT> and <pT2> show a decreasing trend as a function of

centrality, confirming the observation that low pT J/ are less suppressed in central collisionsThe trend is different wrt the one measured at lower energies, where an increase of the <pT> and <pT

2> with centrality was observed

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(2S) in pp and Pb-Pb collisionsThe (2S) yield is compared to the J/ one in Pb-Pb and in ppCharmonia yields are extracted fitting the invariant mass spectrum in two pT bins: 0<pT<3 and 3<pT<8 GeV/c and, for Pb-Pb, also as a function of centrality

Pb-Pb: S/B (at 3 around the (2S)) varies between 0.01 and 0.3 from central to peripheral collisions

pp@s=7TeV Pb-Pb@sNN=2.76TeV

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(2S)/J/ double ratio

Large statistics and systematic errors prevent a firm conclusion on the (2S) enhancement or suppression versus centrality

ALICE result, in central collisions, does not show a large (2S) enhancement

Main systematic uncertainties (some sources cancel out in the double ratio) are the signal extraction and the choice of the MC inputs for acc. calculation

Reference: pp data @s=7TeV (small s- and y-dependence from [(2S)/J/]pp results by CDF, LHCb and CMS taken into account in the syst. uncertainty). (Error on the reference is shown as dashed line)

pp

PbPb

JS

JS

2

2

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Conclusions

A still deeper understanding requires a precise knowledge of cold nuclear matter effects: waiting for pA at LHC in 2013!

The J/ nuclear modification factor has been measured in Pb-Pb collisions at forward rapidity, down to pT=0

RAA centrality and pT dependence have been presented• RAA vs centrality saturates (RAA~0.4) beyond Npart =100• Less suppression at low with respect to high pT, with stronger pT dependence for central events • Lower energy experiments show an opposite behaviour (see pT vs Npart )Models including J/ production via (re)combination succeed in describing ALICE results in central collisions

First ALICE results on (2S) : no firm conclusions on enhancement or suppression with respect to J/, but strong enhancement for central Pb-Pb collisions is unlikely

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Backup

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Systematic uncertainties on differential RAA

Sourcepp reference 9% (for y,pt

integrated)

MC inputs 5%

Tracking 6%

Trigger 6.4%

Matching 2%

TAA 3.8% (for 0-90%)

Normalization 2%

• Type A: uncorrelated (shown as filled box around points)

• Type B: partially correlated within and between sets (shown as [] around points)

• Type C: 100% correlated within a set and between sets (global quantity for all sets)

• Type C: normalization, corr. syst on pp

• Type B: uncorr. syst on pp, MC inputs, trigger, tracking, matching, TAA

• Type A: signal extraction

For the RAA versus centrality:

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Signal extractionSignal extraction: evaluated separately for each centrality/y/pt bin

• Fit with a Double CB + variable width Gaussian (case 1) • Fit with a Double CB + pol0 (case 2)

(other background functions, different CB2 tails values, different assumptions for J/ width tested and included in the systematic uncertainty extraction)

J/ and (2S) yields obtained as weighted average of results from several approaches based on

1) Fit to the invariant mass spectrum2) Fit to the signal after event mixing background subtraction

Systematic uncertainties on the signal extraction are given by the RMS of the distributions

Fitting procedure:

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J/ RAA vs. centrality in pT bins

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J/ RAA vs. pT in centrality bins

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J/ RAA vs. centrality in y bins

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J/ RAA vs. rapidity

Inclusive J/ measured also as a function of rapidity: RAA decreases by 40% from y=2.5 to y=4

Suppression increases with centrality and it is stronger in the more forward region

2.5<y<33.5<y<4

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J/ RAA vs. rapidity

Suppression beyond the current shadowing estimates. Important to measure cold nuclear matter effects (incoming pA data taking)

Inclusive J/ measured also as a function of rapidity: RAA decreases by 40% from y=2.5 to y=4

Suppression increases with centrality and it is stronger in the more forward region

Comover+regeneration modelseems to predict a weaker rapidity dependence

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RAA versus centrality

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Inclusive J/ measured in ALICE

Estimate of prompt J/ RAA using:• b-fraction measured by CDF, CMS

and LHCb• Interpolation at s= 2.76 TeV• Different b-quenching hypothesis

from RAA(B)=0.2 to RAA(B)=1

Influence on RAA of J/ from B decay

J/ from b-hadrons decays have a negligible influence on our measurements

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Comparison ALICE vs CMS and PHENIX

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Theoretical model: contributions

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J/ <pT2> vs centrality

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(2S)/ J/ in pp collisions