1

J/ψ production in p+p and Pb+Pb with

ALICE at LHC

Hugo Pereira Da Costa, CEA Saclay,

for the ALICE collaboration

PANIC 2011, Boston, USA

July 25, 2011

Outline

2

• Introduction

motivations and apparatus

• p+p collisions @ 7 TeV and 2.76 TeV

J/ψ cross section vs pT and rapidity

J/ψ production vs event multiplicity

• Pb+Pb collisions @ 2.76 TeV

J/ψ nuclear modification factor (RAA) vs centrality

comparisons to models and other experiments

Introduction

3

Motivations

4

Puzzles:

• suppression smaller at RHIC than first expected when extrapolating from SPS

• more suppression at forward rapidity than at mid rapidity

• less suppression for high pT J/ψ

J/ψ production is considered a probe of deconfinement and Quark Gluon Plasma

formation in Heavy Ion collisions: suppression in QGP via Debye like color screening

Extensively studied at SPS and at RHIC

Complications:

• J/ψ production mechanism in p+p not understood (CEM, COM, CSM)

• Strong (and poorly understood) Cold Nuclear Matter effects (CNM)

at SPS and RHIC (shadowing, nuclear absorption, etc.)

• Feed-down contributions

• Possible other effects in QGP (e.g. recombination of uncorrelated charm quark pairs)

Need (amongst other things) measurements at mid and forward

rapidity and higher energy

Matsui and Satz, PLB 178, 416 (1986)

Forward Spectrometer:

J/ψ → + - @ 2.5<y<4.0

Front absorber removes hadrons

Muons identified in trigger chambers

Tracked in 5x2 planes of Pad Chambers

+ dipole magnet (3 T.m)

5

J/ψ measurements in ALICE

Central Barrel:

J/ψ → e+e- @ |y|<0.9

Electrons tracked using ITS and TPC in

0.5 T solenoid field.

Particle id using TPC and TOF (Pb+Pb)

at both mid and forward rapidity, measurements start at pT = 0 GeV/c

Vertex is measured with ITS

Collision centrality (Pb+Pb) is

measured with V0

I. p+p collisions @ 7 TeV and 2.76 TeV

6

acc x eff vs pT at forward rapidity

Analysis details

7

s J/ψ→ + - J/ψ→e+e-

7 TeV 16 nb-1 3.9 nb-1

2.76 TeV 20 nb-1 1.1 nb-1

Integrated luminosity:

Systematic uncertainties include:

• signal extraction

• acc x eff corrections

(input distributions, trigger and reconstruction efficiencies, etc.)

• integrated luminosity

• J/ψ polarization (quoted separately)

Acc x eff corrections are evaluated

using simulations and confirmed

with calculations based on detector

efficiencies evaluated with real data

Signal extraction:

• at mid-rapidity, using bin counting

• at forward rapidity, using Crystal Ball fit

for the signal and exponentials for the

background

di-lepton invariant mass distributions in p+p

8

Mid rapidity: J/ψ → e+e-

J/ψ mass resolution ~ 50 MeV

Forward rapidity: J/ψ → + -

J/ψ mass resolution ~ 90 MeV

pp, s=2.76 TeV

ALICE Performance

15/05/2011

J/ψ production cross-section vs pT (1)

9

Forward rapidity data only (2.5 < y < 4)

measured at both 7 TeV and 2.76 TeV (reference for Pb-Pb) and compared to

NRQCD calculation

J/ψ production cross-section vs pT (2)

10

Cross-section measured at both mid and forward-rapidity, at

s = 7 TeV, compared to measurements from other LHC experiments

ALICE arXiv:1105.0380

ATLAS arXiv:1104.3038

CMS arXiv:1011.4193

LHCb arXiv:1103.0423

J/ψ production cross-section vs rapidity

11

Rapidity dependence used to calculate total

inclusive J/ψ cross-section

The curve is a MNR NLO

calculation for the cc production

cross section, scaled to match the

CDF point

J/ψ production vs multiplicity (1)

12

Linear increase with multiplicity is observed.

Should help understand the interplay between hard and soft

interactions in the context of multi-partonic interactions (MPI), and/or

underlying event, in p+p collisions

J/ψ production vs multiplicity (2)

13

Here, J/ψ relative yield is divided by high pT muons (mostly from heavy flavors)

relative yield, as suggested in ArXiV:1012.0719 (S. Porteboeuf & R. Granier de Cassagnac)

Hint for a decrease is observed with no clear explanation so far

II. Pb+Pb collisions @ 2.76 TeV

14

Analysis details

15

acc x eff vs centrality at forward rapidity

J/ψ signal at forward rapidityIntegrated luminosity: 2.7 b-1

Signal extraction:

• Combinatorial background from event mixing

• Residual background fitted using exponentials

• Signal evaluated using several techniques,

(here crystal-ball function)

Centrality dependence of Acc x eff corrections is

evaluated with embedding

Centrality evaluated using Glauber model fit of

the V0 amplitudes

Systematic uncertainties include:

• signal extraction

• acc x eff corrections

• integrated luminosity

• p+p reference (for RAA)

• Ncoll

J/ψ RAA vs centrality

16

Vertical bars: statistical uncertainties

Boxes: systematic uncertainties

Blue box: scaling uncertainty

Contribution from B feed-down: ~ 10% from p-p measurement [LHCb arXiv:1103.0423]

Rough estimation assuming simple Ncoll scaling: ~ 11% reduction of RAA0-80%

RAA ~ 0.5, with little to no dependence on centrality

yield in AA

TAA J/ψpp

RAA =

Large uncertainties on the calculation, due (mostly) to EPS09 uncertainties

centrality

40-80% 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, pNN

sALICE (Pb-Pb

EPS09 (R. Vogt , priv. comm.)

J/ψ RAA and extrapolated CNM (1)

17

Calculation from R. Vogt, using EPS09 npdf prescription

No absorption cross-section added

J/ψ RAA and extrapolated CNM (2)

18

Two production models tested, with (left) and without (right) kT smearing

Two npdf parametrizations: EKS98 (blue), nDSG (red)

No absorption cross-section added

J.P. Lansberg – Quark Matter 2011

Large uncertainties on the calculation

Stronger centrality dependence for CNM than for the data

Difference between CNM and data is smaller for more central collisions

Comparison to models (1)

19

R. Rapp – 486. WE Heraeus Seminar (2011)

• Shadowing

• prompt J/ψ dissociation in QGP

• J/ψ regeneration by charm quark

pair recombination

• Feed-down contributions from B

Ingredients to Zhao and Rapp calculation:

Comparison to models (2)

20

P. Zhuang – 486. WE Heraeus Seminar (2011)Ingredients to Yan et. al. calculation:

• Shadowing and Cronin effect

• prompt J/ψ dissociation in QGP

• J/ψ regeneration by charm quark

pair recombination (detailed

balance)

• Feed-down contributions from B

Comparison to models (3)

21

P. Braun Munzinger – 486. WE Heraeus Seminar (2011)

Ingredients to Andronic et. al. calculation:

• Screening by QGP of all direct J/ψ’s

• CNM (shadowing) on open charm

• Charmonium production at phase

boundary by statistical combination of

uncorrelated charm quarks

*>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, pNN

sALICE (Pb-Pb

>0 (arXiv:1103.6269)T

= 0.2 TeV), 1.2<|y|<2.2, pNN

sPHENIX (Au-Au

coll> is weighted by N

part(*) ALICE <N

Comparison to RHIC data

22

Less suppression observed at LHC than at RHIC (forward rapidity),

but cold nuclear matter effects could be quite different

(more shadowing at LHC, but less to no nuclear absorption)

Besides, x axis (Npart) might not be the relevant one

Conclusions

23

p+p collisions

J/ψ production measured at s = 7 TeV and 2.76 TeV.

Establishes a solid reference for Pb-Pb collisions.

Total cross-section scales nicely with measurements performed at other energies

Intriguing features measured as a function of event multiplicity. Could give insight

on multi-parton interactions and/or underlying event in p+p collisions.

(Note: would be nice to see the same at RHIC and other LHC experiments)

Pb+Pb collisions

J/ψ RAA measured at forward rapidity

It is ~0.5 and shows little to no dependence on centrality

To-do list includes (amongst other things):

• x10 statistics using 2011 data

• more peripheral (and more central) bins

• measuring rapidity and pT dependence

• more precise measurement at mid-rapidity

• J/ψ elliptic flow

• p-A collisions to measure otherwise largely unconstrained CNM

Additional material

24

J/ψ pT and pT2 vs collision energy

25

Approximate logarithmic increase observed over the full energy range

J/ψ signal in ultra peripheral Pb+Pb

collisions

26

centrality

40-80% 20-40% 10-20% 0-10%

no

rma

lize

d t

o 4

0-8

0%

CP

R

0

0.2

0.4

0.6

0.8

1

1.2

= 2.76 TeVNN

sPb-Pb

>0 (preliminary)T

ALICE, 2.5<y<4, p

>0 (preliminary)T

ALICE, |y|<0.8, p

>6.5 GeV/c (arXiv:1012.5419)T

ATLAS, |y|<2.5, p

Comparison to ATLAS and to ALICE mid-

rapidity

27

Also shown:

• ALICE measurement at mid-rapidity

• ATLAS measurement for pT > 6.5 GeV/c

%8040%8040

%100%100

%100

AAinv

AAinv

CPTN

TNR

Comparison to CMS and RHIC data

28

*>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, pNN

sALICE (Pb-Pb

>0 (arXiv:1103.6269)T

= 0.2 TeV), 1.2<|y|<2.2, pNN

sPHENIX (Au-Au

>0 (nucl-ex/0611020)T

= 0.2 TeV), |y|<0.35, pNN

sPHENIX (Au-Au

coll> is weighted by N

part(*) ALICE <N

Left:

• STAR (pT>5 GeV)

• CMS (6.5<pT<30 GeV)

Right:

• PHENIX (pT>0 GeV)

• ALICE (pT>0 GeV)