Measurement of J/ production in Pb-Pb and pp collisions at the LHC

with the ALICE experiment

M. Gagliardi (Università degli Studi e Sezione INFN, Torino)

for the ALICE Collaboration

The 11th International Conference on Nucleus-Nucleus collisionsSan Antonio, TX, USA – May 31st, 2012 1

Outline

• Motivation• J/ detection in ALICE• Results in pp collisions• Results in Pb-Pb collisions• Conclusions

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Perturbative Vacuum

cc

Color Screening

cc

- pp collisionsInsight on quarkonium production at LHC energies:hadronisation of heavy quark pairs into a bound colourless state is highly non-perturbative -> challenge for models

Reference for the study of nuclear modifications in heavy ion collisions

- Pb-Pb collisionsResonance melting by colour screening in aQuark Gluon Plasma: one of the first proposed signatures of deconfinement (T.Matsui, H. Satz, Phys. Lett. B 178 (1986) 416)

Suppression beyond cold nuclear matter effects observed at SPS and RHIC (but similar magnitude in spite of different energy densities).

J/regeneration by statistical hadronisation of ccpairs might play a role and even become dominant at LHC energies (e.g., Andronic et al, Phys. Lett. B 652 (2007) 659)

Detailed discussion by P. Zhuang and P. Braun-Munzinger in Plenary 2 and 3

Motivation

Perturbative vacuum Colour screening

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A Large Ion Collider Experiment

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Muon spectrometer -4 < < -2.5

Dipole magnet

Front absorber 10 I

10 tracking planes (Cathode Pad Chambers)Trigger system (Resistive Plate Chambers)Muon filter in front of the trigger chambers (7 I)

Central Barrel || < 0.9

Solenoidal magnet

Time Projection Chamber:-Tracking-PID via dE/dx

Inner Tracking System (Silicon Pixel, Drift, Strip Detectors)- Vertexing- Tracking- Triggering (SPD )

Time of Flight MRPCs- PID

VZERO scintillators- Triggering- Centrality in Pb-Pb- Luminosity

Only detectors used in this analysis are discussed

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J/detection in ALICE

Prompt : ~90%

From B-decay ~10%

Directly produced ~50%Feed down from ’, c ~40%Inclusive

Three sources of J/(fractions refer to the pt-integrated yield)

Central barrel: J/-> e+e- |y| < 0.9, pt > 0Data set Trigger Lint

pp √s = 7 TeV(2010)

Min bias (VZERO, SPD) 5.6 nb-1

pp √s = 2.76 TeV(2011)

Min bias (VZERO, SPD) 1.1 nb-1

Pb-Pb √sNN = 2.76 TeV(2010)

Min bias (VZERO, SPD) 1.7 b-1

Muon spectrometer: J/-> +- 2.5 < y < 4, pt > 0Data set Trigger Lint

pp √s = 7 TeV(2010)

Min bias (VZERO, SPD) AND single

15.6 nb-1

pp √s = 2.76 TeV(2011)

Min bias (VZERO, SPD) AND single

19.9 nb-1

Pb-Pb √sNN = 2.76 TeV(2011)

Min bias (VZERO)AND pair

70 b-1

Separation possibile in the electron channel in pp

pp collisions

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J/cross section measurement

- Measurement down to zero transverse momentum in a broad rapidity range- pt-differential cross section at 2.76 and 7 TeV well reproduced by Non-Relativistic QCD (Colour Singlet +Colour Octet)- 2.76 TeV results used as reference in Pb-Pb analysis

7 TeV: Phys. Lett. B 704 (2011) 442 2.76 TeV: arXiv:1203.3641v1 [hep-ex]

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J/polarisation measurement- Polarisation is a crucial observable for the comparison between data and models - Measured via polar () and azimuthal (φ) angle distributions of decay muons, analysed in two reference frames: Collins-Soper and Helicity

1 transverse polarisation 0 no polarisation-1 longitudinal polarisation

W (cos, φ) 1 + cos2 + φ sin2cos2φ + φ

sin2cosφ

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J/polarisation

Data suggest weak or no polarisationExtended pt coverage will provide a more stringent test of the models

Phys.Rev.Lett. 108 (2012) 082001

arXiv:1201.3862v1 [hep-ex]

φ

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Multiplicity dependence of J/production- Highest multiplicity in this data-sample: dNch/d~30 (i.e. 5 times the minimum bias multiplicity): comparable to semi-central Cu-Cu collisions at RHIC

- Measurement of J/production in high multiplicity collisions provides insights on the interplay between hard and soft regime in multi-partonic interactions

- A linear increase of the relative J/yield with the multiplicity is observed

- Behaviour not reproduced by Pythia (v. 6.4)

Phys. Lett. B 712 (2012) 165

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J/production from B-hadron decay

TO BE UPDATED WITH ARXIV VERSION

Good agreement among LHC (and Tevatron) experiments

- Unique measurement at mid-rapidity and low pt

- Good impact parameter resolution (σrφ < 75 μm for pt>1 GeV/c) -> contribution from B estimated via the pseudo-proper decay length

Fraction of J/ from B decay in pt> 1.3 GeV/c, |y|<0.9:fB= 0.149 ±0.037(stat)-0.027

+0.018 (syst) -0.021

+0.025 (syst polar.)arXiv:1205.5880v1 [hep-ex]

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J/production from B-hadron decay

Extrapolation to pt = 0 to get prompt J/ cross section at mid-rapidity

- Unique measurement at mid-rapidity and low pt

- Good impact parameter resolution (σrφ < 75 μm for pt>1 GeV/c) -> contribution from B estimated via the pseudo-proper decay length

Fraction of J/ from B decay in pt> 1.3 GeV/c, |y|<0.9:fB= 0.149 ±0.037(stat)-0.027

+0.018 (syst) -0.021

+0.025 (syst polar.)arXiv:1205.5880v1 [hep-ex]

Pb-Pb collisions

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J/in Pb-Pbtriggercentrality, track selection

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TriggerElectrons (min bias)VZERO-A AND VZERO-C AND SPD Muons VZERO-A AND VZERO-C AND di-muon trigger (pt > 1 GeV/c)

Centrality selectionGlauber model fit of the VZERO amplitude

Track selectionElectrons: - identified via dE/dx in TPC: |ne|<3, |nσp,π|>3.5- pairs with|yee| < 0.9

Muons- muon tracks are requested to have hits in the trigger chambers -> efficient hadron rejection due to the iron wall before the trigger system- pairs with 2.5<y<4

PRL 106, (2011) 032301

J/in Pb-Pbsignal extraction

Muons- Several shapes for background- Double Crystal Ball function for signal- Fit of signal + background 15

Electrons- Background from event-mixing- Scaling to same-event spectrum and subtraction- Signal from bin counting in 2.92 < M < 3.16 GeV/c2

~40000 J/

~2000 J/

J/in Pb-PbAx correction

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Weak centrality dependence for both electrons (7.8% -8.9%) and muons (13.3%-14.5):

Less than 10% loss of efficiency from peripheral to central events

Computed from MC simulations with realistic detector configuration

Electrons:HIJING events enriched with J/ -> ee

Muons:J/->embedded in real Pb-Pb events

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J/in Pb-Pbnuclear modification factor RAA

-YJ/ PbPb = NJ/

PbPb / (A BR NPbPb)

- RAAi = YJ/

PbPb,i / (TAAi J/

pp) (ith centrality bin)

J/pp: pp data at √s = 2.76 TeV

TAAi : Glauber MC

Integrated RAA

Forward rapidity:RAA

0-90% = 0.497 ± 0.006 (stat.) ± 0.078 (syst.)(uncertainty is dominated by pp reference)

Mid-rapidity:RAA

0-80% = 0.66 ± 0.10 (stat.) ± 0.24 (syst.)(uncertainty is dominated by signal extraction and pp reference)

Clear J/suppression at forward rapidityData show weak centrality dependence for Npart > 100Similar pattern at mid-rapidity, but larger uncertainties

Red: 2.5<y<4Blue: |y|<0.9

J/in Pb-Pbnuclear modification factor RAA

ALICE 2.5 < y < 4

Black: PHENIX - pt > 0, 1.2 < |y| <2.2

Blue: CMS - pt > 6.5 GeV/c, |y| < 2.4

PHENIX and CMS (at high pt): RAA decreasing with centrality

Transport models: 50% of regenerated J/ in the most central collisions

Statistical hadronisation:all J/ produced at hadronisation

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Red: ALICE - pt>0, 2.5<y<4

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J/RAA vs pt at forward rapidity

RAA decreases with pt

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J/RAA vs pt at forward rapidity

Red: ALICE 2.5 < y < 4Blue: CMS |y| < 2.4Black: PHENIX 1.2 < |y| < 2.2

New behaviour at low pt?

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J/RAA vs pt at forward rapidity

Reproduced by models with regenerated J/ component

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J/RAA vs rapidity

RAA decreases with rapidity (by 40% from y = 2.5 to y =4)

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J/elliptic flow: v2 Pressure gradients in a thermalised medium convert initial spatial anisotropy in momentum anisotropy

dN/dφ 1+v2 cos (φ)φφ-RP , RP = reaction plane

The anisotropy is quantified by the v2 coefficient

J/ flow at LHC energies may be driven by regeneration or recombination of charm quarks; low and mid pt ranges are the most interesting

J/signal extracted in 6 φφ-RPbins for 4 pt bins in 20%-60% centrality

Event plane determined using three different subsets of detectors with large gaps

v2 extracted from φ distributions

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J/v2 vs pt

Hint for non-zero v2 (2.2 significance) for 2 GeV/c < pt < 4 GeV/c

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J/v2 vs pt

Hint for non-zero v2 (2.2 significance) for 2 GeV/c < pt < 4 GeV/c

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J/v2 vs pt

Hint for non-zero v2 (2.2 significance) for 2 GeV/c < pt < 4 GeV/cCompatible with transport model predictions

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Conclusions and outlookALICE results on J/ production:

in pp collisions

- cross section down to pt =0 - beauty feed-down contribution- weak or null polarisation- linear increase of the yield with multiplicity

Looking forward to p-Pb collisions (winter 2012), to constrain shadowing of nuclear structure functions at LHC energies

in Pb-Pb collisions

- weak or no centrality dependence of RAA at large Npart

- RAA is larger at low pt

- significant decrease of RAA at large rapidities- hint for non-zero elliptic flow at low pt

Backup

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Non-prompt J/ and bb cross section in pp collisions

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J/-> e+e- RAA: systematic uncertanties

Uncorrelated in centrality:

Correlated in centrality:

- pp reference 26%

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J/-> +- RAA: systematic uncertantiesUncorrelated in centrality:

Correlated in centrality:- Trigger efficiency 6.4%- Tracking efficiency 6%- MC J/distribution 5%- Trigger matching efficiency and relative trigger normalisation: 2.8%- pp reference 8.2%

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J/ RAA vs Npart at high pt

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J/RAA : more model comparisons

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J/RAA : more model comparisons

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J/RAA : more model comparisons

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Prompt J/ RAA

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J/ v2 vs centrality