charmonium production in pb-pb collisions atj/ψ elliptic flow (v 2) • charm quarks, if...

Mohamad Tarhini, IPN-Orsay

Charmonium production in Pb-Pb collisions at √sNN = 5.02 TeV measured with ALICE

INTERNATIONAL SCHOOL OF SUBNUCLEAR PHYSICS, Erice 2017

Mohamad Tarhini, IPN-Orsay 2

Introduction and motivation

Original proposal:• Measurement of charmonium production in heavy-ion collisions is a probe for the de-

confinement aspect of the QGP

c ̅

c c

c ̅

Charmonium in the vacuum Charmonium in the QGP:Colour screening

• Sequential dissociation of different charmonium states can serve as a QGP thermometer


Introduction and motivation

But:• If there are enough cc ̅pairs, charmonium can be (re)generated

c

c ̅

c

c ̅This process would enhance the charmonium production and compensate the dissociation via colour screening

J/ψ

pro

duct

ion

prob

abili

ty

1

energy density

J/ψ

pro

duct

ion

prob

abili

ty

1

energy density

𝜎cc ̅


ALICE detector and datasets

• Inclusive charmonia are reconstructed via their dimuon decay down to zero pT

muon spectrometer-4 < ƞ < -2.5

• The QGP is expected to be created in heavy-ion collisions• The LHC collide Pb ions and ALICE is the detector dedicated to study these collisions


Charmonium reconstruction

• The signal is extracted by fitting the dimuon invariant mass distribution with (background+signal) functions• Different signal and background shapes are considered

• Number of charmonium candidates are further corrected by the detector acceptance times efficiency obtained via MC simulations

• Muon pair selections:• 2.5 < y < 4• Opposite sign charges

• Main single muon selections:• -4 < ƞµ < -2.5• Matching between trigger and tracker


Nuclear modification factor

• How to quantify the suppression ?

• The nuclear modification factor:

• Ncol is a normalisation factor that take into account the number of binary collisions in A-A collisions. Obtained using MC based on Glauber model.

• NAA is the number of measured J/psi in A-A collision• Npp is the number of j/psi measured in proton-proton collisions where no QGP effects are

expected

{=1: The A-A collisions are only a superposition of many proton-proton collisions

< 1: There is a J/Ѱ suppression in A-A collisions

> 1: There is an enhancement of J/Ѱ in A-A collisionsRAA

RAA

=NJ/

AA

Ncol

⇥NJ/ pp


[1] arXiv:1606.08197[2] PLB 734 (2014) 314-327J/ψ RAA vs Centrality

⟩part

N⟨0 50 100 150 200 250 300 350 400

AA

R

0

0.2

0.4

0.6

0.8

1

1.2

1.4-µ+µ → ψALICE, inclusive J/

c < 8 GeV/T

p < 4, y2.5 <

= 5.02 TeVNN

sPb −Pb

= 2.76 TeVNN

sPb −Pb

ALI-PUB-110153

• Clear J/ψ suppression and almost no centrality dependence for Npart > 100 (centrality < 50 %)• A systematic difference of ~15% between the RAA at two energies. The increase is within the

uncertainties

[1]

[2]


• Results are compared with calculations based on different models:• Two transport models (TM1 [1] and TM2 [2]): continuous interplay between dissociation

and (re)generation• Statistical hadronisation model [3]: all J/ψ are dissociated in the plasma.

(Re)generation occurs at the phase boundary• Comover model (CIM) [4]: J/ψ are suppressed via interaction with a parton/hadron co-

moving medium. (Re)generation added as a gain term• The large uncertainties on the models is mainly due to the input cc cross section

[1] Nucl. Phys. A 859 (2011) 114–125 [2] Phys. Rev. C 89 no. 5, 459 (2014) 054911[3] Nucl. Phys. A 904-905 (2013) 535c [4] Phys. LeX. B 731 (2014) 57–63[5] ALICE Coll. PRL116 (2016) 222301

J/ψ RAA vs centrality (comparison with models)

⟩part

N⟨0 50 100 150 200 250 300 350 400 450

AA

R

0

0.2

0.4

0.6

0.8

1

1.2

1.4 = 5.02 TeVNNsPb −ALICE, Pb-µ+µ → ψInclusive J/

c < 8 GeV/T

p < 4, 0.3 < y2.5 <

(TM1, Du and Rapp)c > 0.3 GeV/T

pTransport, Transport (TM2, Zhou et al.)Statistical hadronization (Andronic et al.)Co-movers (Ferreiro)

ALI-PUB-109779


J/ψ RAA vs centrality (in pT ranges)New

• For central events, the suppression is smaller at lower pT which can be explained by the domination of regeneration at lower pT

• RAA shows a stronger centrality dependence at high pT

⟩ part

N ⟨0 50 100 150 200 250 300 350 400

AA

R

0.2

0.4

0.6

0.8

1

1.2

1.4 -µ+µ → ψALICE Preliminary, inclusive J/

< 4y = 5.02 TeV, 2.5 < NN

sPb-Pb

c < 2 GeV/T

p0.3 <

c < 5 GeV/T

p2 <

c < 8 GeV/T

p5 <

c < 12 GeV/T

p8 <

ALI-PREL-117114


• A negligible rapidity dependence of the RAA

• Without regeneration, the suppression is expected to be larger at mid-rapidity

J/ψ RAA vs RapidityNew

y0 0.5 1 1.5 2 2.5 3 3.5 4

AA

R

0.2

0.4

0.6

0.8

1

1.2

1.4

= 5.02 TeV, Centrality 0-90%NN

sALICE Preliminary, Pb-Pb

c > 0 GeV/T

p, -

e+ e→ ψInclusive J/

2.3%±, global syst = c < 12 GeV/T

p, 0 < -µ+µ → ψInclusive J/

ALI−PREL−121651

mid-rapidity

forward-rapidity


• For centrality intervals with too small ψ(2S) significance, an upper limit with 95% CL is calculated

• The ψ(2S) is more suppressed than the J/ψ in semi-central and central collisions

ψ(2S) RAA

• The ratios between different charmonium states are essential to discriminate between different models

⟩part

N⟨0 50 100 150 200 250 300 350 400

AA

R

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

(2S) (Preliminary)ψ

(arXiv:1606.08197)ψJ/

Upper limits include global uncertainties

c<8 GeV/T

p<4, 0<y = 5.02 TeV, 2.5<NN

s(2S), Pb-Pb ψ, ψALICE inclusive J/

ALI−PREL−120671


Summary

• The J/ψ RAA in Pb-Pb collisions at √sNN = 5.02 TeV is measured as a function of centrality, pt and rapidity

• A clear suppression but with no centrality dependence in semi-central and central collisions at low pt

• Results compared to models calculations and to results at √sNN = 2.76 TeV • High precision measurement can constrain the models

• The ψ(2s) is more suppressed than the J/ψ in central and semi-central collisions

at √sNN = 5.02 TeV

• Measurement of charmonium production in heavy-ion collisions is important to study the de-confinement of the QGP

• ALICE at the LHC is dedicated to study heavy-ion collisions

Extra Slides

J/Ψ RAA Vs pT

ALI-PREL-126572

State J/Ψ 𝛘c Ψ′ 𝜰 𝛘

b 𝜰′ 𝛘b′ 𝜰′′

mass [GeV] 3.10 3.53 3.68 9.46 9.99 10.02 10.26 10.36

ΔE[GeV] 0.64 0.20 0.05 1.10 0.67 0.54 0.31 0.20

ΔM[GeV] 0.02 -0.03 0.03 0.06 -0.06 -0.06 -0.08 -0.07

radius [fm] 0.25 0.36 0.45 0.14 0.22 0.28 0.34 0.39

H. Satz, J. Phys. G32 (2006) R25

Charmonia decay schemes:

ALICE Vs PHENIXALICE Coll. PLB 734 (2014) 314

Excess at very low pT

Signal extraction

2c

Co

un

ts p

er

50

Me

V/

50

100

150

200

250

300

310×

Centrality 0-10 %

= 0.10σ3

/ndf = 1.10, S/B2χ

= 5.02 TeVNNsALICE Pb-Pb, -1bµ 225 ≈ intL

)2c (GeV/µµm2.5 3 3.5 4 4.5

2c

Counts

per

50 M

eV

/

0

5000

10000

15000

20000

25000

Centrality 0-10 %

/ndf = 0.932χ

Mixed-event background

subtracted

100

200

300

400

500

Centrality 70-80 %

= 3.52σ3

/ndf = 0.82, S/B2χ

c < 12 GeV/T

p < 4, y2.5 <

)2c (GeV/µµm2.5 3 3.5 4 4.5

0

100

200

300

400

500

Centrality 70-80 %

/ndf = 0.852χ

Mixed-event background

subtracted

ALI-PUB-109775

J/Ψ elliptic flow (v2) extraction

• Ψ and ɸµµ are the azimuthal angle of the event plane and the muon pairs

• Ψ is determined using two different detectors (V0A and SPD) that cover two different pseudo-rapidity ranges and do not overlap with the muon spectrometer acceptance

v2 (mµµ) = vJ/ 2 ↵(mµµ) + vbkgd

2 (mµµ)[1 � ↵(mµµ)]

v2 = hcos2 (�µµ � )i• For each muons pair, calculatein bins of invariant mass (B)

• Using S/B parameters from the J/Ψ signal extraction (A), fit the distribution with:

• Extract the J/psi signal in bins of Δ𝜑 = Ψ-ɸµµ (C)

• Fit the distribution with:N0[ (1+2v2)*cos(2 Δ𝜑) ]

First method Second method

)2c (GeV/µµM2.5 3 3.5 4 4.5

2c

cou

nts

/ 2

5 M

eV

/

2000

4000

6000

8000

10000ALICE Preliminary

= 5.02 TeV, 20-40%NNsPb-Pb

c < 4 GeV/T

p < 4, 2 < y, 2.5 < -µ+µ → ψInclusive J/

Opposite sign pairs

Fit total

Fit signal

Fit background

ALI−PREL−119045

)2c (GeV/µµM2.5 3 3.5 4 4.5

⟩)

EP

,2Ψ

-

µµ

ϕ c

os

2(

⟨

0.05−

0

0.05

0.1

0.15 ALICE Preliminary

= 5.02 TeV, 20-40%NNsPb-Pb

c < 4 GeV/T

p < 4, 2 < y, 2.5 < -µ+µ → ψInclusive J/

= 1.1η∆

Opposite sign pairs

)µµM(2v Fit total

)µµM(2bkgv Fit background

ALI−PREL−119025

(rad)EP,2Ψ - µµ

ϕ = ϕ∆0 0.5 1 1.5 2 2.5 3

ϕ∆/

ψJ/

dN

3000

3200

3400

3600

3800

4000

4200

4400

4600ALICE Preliminary

= 5.02 TeV, 20-40%NN

sPb-Pb

c < 4 GeV/T

p < 4, 2 < y, 2.5 < −µ+µ → ψInclusive J/

= 5.3η∆

Data

Fit

ALI-PREL-119389

CBA

J/ψ elliptic flow (v2)

• Charm quarks, if thermalised in the QGP, should exhibit the elliptic flow generated in this phase➡ Non zero v2 for re-generated J/Ψ

• A non zero J/ψ v2 seen in semi-central collisions (20-40%) [7.6 𝜎 significance in 4< pT < 6 GeV/c] • Precision is significantly increased in run-2 measurement with respect to run-1• Similar measured v2 values for hidden and open charm• Models have difficulties to reproduce the measured J/ψ v2 in the measured pT interval

ALI-PREL-119009 ALI-PREL-118891

v2 @ 2.76 TeV

significance 2.2 sigma

pp cross section at 5.02 TeV

ALI-PUB-122010 ALI-PUB-122074

rAA at 2.76 TeV

Lower energies


ψ(2S)/J/ψ


ALICE:Inclusive

3 < pT < 8 GeV/c2.5 < y < 4

• The results of the double ratio at √sNN = 5.02 TeV are compatible within uncertainties with the ones from CMS

CMS:Prompt

3 < pT < 30 GeV/c1.6 < y < 2.4

⟩part

N⟨0 50 100 150 200 250 300 350 400

)ψ

(J/

AA

R(2

S))

/ψ(

AA

R

0

0.5

1

1.5

2

2.5

<4.0, Inclusive (Preliminary)y, 2.5<c<8 GeV/T

pALICE, 3<

<2.4, Prompt only (arXiv:1611.01438)y, 1.6<c<30 GeV/T

pCMS, 3<

Upper limits include global uncertainties

= 5.02 TeVNNs(2S), Pb-Pb ψ, ψJ/

Cent.0-90%

ALI−PREL−120726

• The J/ψ <pT> is smaller in central events than in peripheral ones→ (re)generation

• The results of rAA at √sNN = 5.02 TeV and √sNN = 2.76 TeV [1] are compatible within uncertainties

• Discrepancies are seen in some centralities (e.g > 3 𝜎 for 30-40 %) between the measurements and calculations based on TM1[2] model

J/ψ average pT (<pT>)

• The J/ψ <pT> and the rAA are complementary observables to the RAA and the v2

rAA =hp2

T iAA

hp2T ipp

charmonium production in pb-pb collisions atj/ψ elliptic flow (v 2) • charm quarks, if...

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