Heavy quarkonium in nuclear collisionsfrom SPS to LHC

Roma – April 21-23, 2009

E. Scomparin –INFN Torino (Italy)

• Introductionwhy heavy quarkonia are important in QGP studies ?

• Quarkonium production in elementary collisions facts and open problems

• Quarkonium interaction in cold nuclear matter Setting a reference for heavy-ion collisions

• Quarkonium interaction in hot nuclear matter Hints of deconfinement

First studies in the Old World....Florence, XII-XIII century

.. then moving to the New World

Gatlinburg, Tennessee 1891

The (real) beginning of the storyFirst paper on the topic

1986, Matsui and Satz

The most famous paper inour field (1231 citations!)

Keywords

1)Hot quark-gluon plasma

2)Colour screening

3)Screening radius

4)Dilepton mass spectrum

Unambiguous signature ofQGP formation

Everything in one slide.....

Perturbative Vacuum

cc

Color Screening

ccScreening of

strong interactionsin a QGP

• Different states, different sizes• Screening stronger at high T

• D maximum size of a bound state, decreases when T increases

Resonance melting

QGP thermometer

...but the story is not so simple

• Are there any other effects, not related to colour screening, that may induce a suppression of quarkonium states ?

... so let’s start from the beginning !

• Is it possible to define a “reference” (i.e. unsuppressed) process in order to properly define quarkonium suppression ?

• Which elements should be taken into account in the design of an experiment looking for qurkonium suppression?

None of these questions has a trivial answer....

• Do we understand charmonium production in elementary collisions ?

• Can the melting temperature(s) be uniquely determined ?

• Do experimental observations fit in a coherent picture ?

Charmonium states

The binding of the c and cbar quarks can be expressed using the Cornell potential:

krr

rV

)(

Coulomb contribution, induced by gluon exchange between q and qbar

Confinement term

3 GeV

3.8 GeV

J/

(2S) or ’

3S1

3S1

3P2

3P1

3P0

2

1

0Mas

s

thresholdDD

JS L12 spin orbital

total

Charmonium cc bound state

Relative motion is non-relativistic(~0.4) non-perturbativetreatment

If m<2mD stable under strong decay

Charmonium decay modes

• Charmonium exhibits a (nearly) infinite series of decay channels

• Decay into a pair of leptons is the only channel experimentally measured in heavy-ion collisions

Standard way of measuring muon pairs(NA50 at SPS, PHENIX at RHIC, ALICE at LHC)

beam

MuonOther

hadron absorber

and tracking

target

muon trigger

magnetic field

Iron wall

• Place a huge hadron absorber to reject hadronic background

• Implement a trigger system, based on fast detectors, to select muon candidates (1 in 10-4 interactions, in Pb-Pb collisions at SPS energy)

• Reconstruct muon tracks in a spectrometer (B + tracking detectors)

• Extrapolate muon tracks back to the target Vertex reconstruction is usually rather poor (z~10 cm)

• Correct for multiple scattering and energy loss

Second generation experiment(s):NA60 at SPS, future upgrades at RHIC,LHC

2.5 T dipole magnet

targets

beam tracker

vertex tracker

or

!

hadron absorberMuonOther

and trackingmuon trigger

magnetic field

Iron wall

Use a silicon tracker in the vertex region to track muons before they suffer multiple scattering and energy loss in the hadron absorber.

These tracks are matched in coordinate and momentum space with those of the muon chambers

Improve mass resolution

Determine origin of the muons

Quarkonium productionin elementary collisions (pp)

J/ hadroproduction: pp collisionsSimpler approach: color evaporation model (CEM)

q

q Q

Q

Q

Qg

g Q

Qg

gQ

Qg

g

• The cross section for the production of a certain charmonium state is a fixed fraction F of the production cross section for cc pairs with m<2mD

• Works rather well, but gives no detail on the “hadronization process” of the cc pair towards a bound state

Color singlet model (LO)

factor 50!

• First “microscopic” model for quarkonium production

Ruled out by results from theTevatron collider (circa 1995)

g

g

c

c

J/

• Requires the cc pair to be produced in a color singlet state, with the same quantum nubers of the charmonium state under study

• Gluon fusion dominant

Color octet model (NRQCD)• J/ production: perturbative vs non-perturbative aspects

• s(mc) ~ 0.25 small, perturbative treatment reasonable• Bound state dynamics is non-perturbative (v/c is small)

)/(ˆ)()( 2/1/2,

1

0 1/ Jijxfxfdxdx BjAiji

J

/)/(ˆ Jn

n

ijnQQ OCJij

Perturbative (pQCD)

Non-perturbative matrix elements(series in v/c)

• Include color-singlet and color-octet states (for octet color is neutralized via emission of soft gluon(s))

)( 13/

1 SO J For J/ (singlet)

)(),(),( 3/81

0/81

3/8 J

JJJ POSOSO (octet)

Success and open problems

Cross section valuessuccessfully reproduced!

Polarization valuescompletely missed!

Polarization measurements

pprojectile

Viewed from dimuonrest frae

ptarget

z axis

x

y

reaction plane

decay plane

+

ϕ

Collins-Soper:

Z axis is parallel to the bisector of the angle between beam and target directions in the quarkonium rest frame

Helicity:

Z axis coincides with the J/ direction in the target-projectile center of mass frame

= -1

= 0

2

θcos α1cosθ d

dσ 2

>0 transverse polarization<0 longitudinal polarization

Recent news

• NLO, NNLO* better agreement at high pT

• s-channel cut question the assumption that takes the heavy quarks forming the quarkonium as being on-shell

•Color singlet model revisited

Both cross section ANDpolarization reasonably

reproduced

Results from QM09 (PHENIX)

• Work still in progress but the situation looks promising

• High pT data from STAR also available Useful to check NNLO* calculations

Quarkonium productionin pA collisions

Cold nuclear matter effects

pA collisions: J/ is suppressed• In p-A collisions, no QGP formation is expected• A priori, no J/ suppression

• However, we observe a significant reduction of the J/ yield per nucleon-nucleon collision

J/ / DY: same behaviour

NA50, pA 450 GeV

Nuclear absorption

L• Once the J/ has been produced, it must cross a thickness L of nuclear matter, where it may interact and disappear

• If the cross section for nuclear absorption is absJ/, one expects

LJpp

JpA

JabseA

///

• It is also exepcted that weakly bound states (as ’) have a much larger nuclear absorption cross section

/' JpApA (’ is twice as large as the J/)

Nuclear absorption cross section

• As a function of L, the pA cross section can be described

LJpp

JpA

JabseA

///

• From the set of data taken by NA50 at 450 GeV, one extracts the nuclear absorption cross section

mb 0.54.5σJ/ψabs

• L can be calculated in the frame of the Glauber model (geometrical quantity)

’ vs J/

• As expected, the nuclear absorption cross section is larger for the ’

mb 0.98.3σψ'abs

• It is important to note that the charmonium production process happens on a rather long timescale

p

c

cg

J/• The nucleus “sees” the cc in a (mainly) color octet state• Hadronization can take place outside the nucleus

Nuclear effects vs xF

I. Abt et al., arXiv:0812.0734

ApppA

The J/ absorption is parameterized through

< 1 suppression > 1 enhancement

• Nuclear effects show a strong variation vs kinematic variables

• Final state nuclear absorption is only one of the relevant effects to be taken into account

• In particular shadowing (modifications of parton distribution functions in the nucleus) plays an important role

Shadowing parameterizations

• The dominant J/ production process around midrapidity is gluon fusion• Unfortunately the gluon pdfs are less known than the quark ones

• Various parameterizations (EKS, EPS, ...) give significantly different values

• In particular the low-x region (RHIC,LHC) is poorly known

Influence of shadowing

SPSTevatron (FT) RHIC

Increasing √s From anti-shadowing to shadowing

At SPS, the “true” nuclear absorption cross section is larger than the “effective” one

Why absJ/ is so relevant ?

• The cold nuclear matter effects present in pA collisions are of course present also in AA and can mask genuine QGP effects

L

J//N

coll

L

J//N

coll/

nu

cl.

Ab

s.

1

Anomalous suppression!

pA

AA

• It is very important to measure cold nuclear matter effects before any claim of an “anomalous” suppression in AA collisions

pA collisions – SPS energies• Particularly relevant for the interpretation of heavy-ion data at SPS

absJ/ = 4.2±0.5 mb,

(J//DY)pp =57.5±0.8

• extrapolated to AA assuming

• Onset of the suppression at Npart 80• Good overlap between Pb-Pb and In-In

pA collisions

Reference for the J/ suppression in AA(cold nuclear matter effects aka nuclear abs.)

• tuned using pA at 400/450 GeV (NA50)

(Glauber analysis)

In-InPb-Pb AA collisions

absJ/ (158 GeV) = abs

J/ (400/450 GeV)

Observed suppression in AA exceeds nuclear absorption

E=158 GeV/nucleon

pA collisions – SPS energies QM09 news

• For the first time pA data have been taken at 158 GeV, i.e. the same energy of nucleus-nucleus data

158 GeV 400 GeV

abs J/ (158 GeV) = 7.6 ± 0.7 ± 0.6 mbabs J/ (400 GeV) = 4.3 ± 0.8 ± 0.6 mb

• “Surprising” result: cold nuclear matter effects stronger at lower energy!

Expect consequences for anomalous suppression

pA collisions at fixed target – after QM09

Two effects may help toexplain the 158 GeV result

1) Stronger nuclear effects when decreasing √s

2) Stronger nuclear effects when moving towards higher xF

• Coherent and satisfactory theoretical description still missing

• Other effects may play a role (initial state energy loss, intrinsic charm)

What happens at higher energy ?• d-Au collisions have been studied at RHIC• Statistics rather poor up to now

ppJ

dAuJ

dAucoll

dAu NR

/

/1 (and similarly for AA) is the quantity usually studied

at RHIC to quantify nuclear effects

• Shadowing plays an important role• Nuclear absorption (break-up) smaller than at SPS

Influence of shadowing at RHIC

Forward Mid Backwardd Au

d Au

• RHIC data sit in the Shadowing region (forward and midrapidity) Anti-shadowing region (backward rapidity)

Shape of RdAu vs rapidity largely determined by shadowing

d-Au collisions – news from QM09• Considering various production processes, one gets different results for cold nuclear matter effects

“Intrinsic” production

gg J/(following emission of soft gluon(s)

does not modify kinematics)

“Extrinsic” productiongg J/ + g

(emission of a hard gluon)

Differentx2 range

d-Au collisions – news from QM09• Large statistics sample (run-8)

• First preliminary results (RCP)

EKS shadowing

EPS08

σ = 0 mb

σ = 4 mb

EKS

σ = 0 mb

σ = 4 mb

Putting everything together....

• Global interpretation of cold nuclear matter effects not easy• √s-dependence clearly visible in the data

• Collect pA data in the same kinematic domain of AA data

Quarkonium productionin AA collisions

Looking for the QGP

cc pair in a deconfined medium

Modify quarkonium potential

Perturbative Vacuum

cc

Color Screening

cc

krr

rV

)( Drer

rV /)(

Confined world Quarkonium states described with =0.52, k=0.926 GeV/fm (mc = 1.84 GeV)

Deconfined worldNo confinement term Coulomb part screened

Do bound states still exist ?

Conditions for melting

Drer

pH

/

2

2

“Screened Hamiltonian”

22 1 rp

Drerr

rE

/22

1)( with

• The condition 0r

Ehas NO solutions for D

84.0

1

fm41.01

We have

fmTg

PQCDD 36.01

3

2)(

2while, for a 3-flavor QGP

with T=200 MeV one has

The condition D

84.0

1is verified

No bound statein a T = 200 MeV

QGP

Charmonium (bottomonium) states• Various cc and bb bound states have very different binding energy and dimensions

• Strongly bound states are smaller

• The r0>rD condition can be met at different temperatures for the various resonances

• Try to identify the resonances which disappear and deduce the temperature reached in the collision

Suppression hyerarchy

J/

(3S) b(2P)(2S)

b(1P)

(1S)

(2S)c(1P)

J/

Digal et al., Phys.Rev. D64(2001)094015

• Each resonance has a typical dissociation threshold• Consider the cc (bb) resonances that decay into J/()

• The J/ () yield should exhibit a step-wise suppression when T increases (e.g. comparing A-A data at various √s or centrality)

Dissociation temperatures• Quantitative predictions on dissociation temperatures come from

• lattice QCD studies• potential models• effective field theories

• Results have shown significant oscillations in the recent past

Non-perturbative domain

• Calculate spectral functions for the various states

• Lattice spectral functions seemed to indicate high dissociation temperatures

These conclusions are now regarded as premature

Recent results on Tdiss

Ebin Tweak binding

Ebin Tstrong binding

• Binding energies for the various states from potential models• Assume a state “melts” when Ebind < T • Result: J/ dissociated at RHIC

• Recent development: include viscosity effects

Smaller screening massStronger binding

AA results – SPS energy - QM09• Recent results on pA at 158 GeV (see previous slides) imply a modification in the interpretation of AA data

abs J/ (158 GeV) > abs J/ (400 GeV)

smaller anomalous suppression with respect to previous estimates

Published results QM09 new reference

B. Alessandro et al., EPJC39 (2005) 335R. Arnaldi et al., PRL99 (2007) 132302

In-In 158 GeV (NA60)Pb-Pb 158 GeV (NA50)

Still a ~30% effect incentral Pb-Pb!

Role of shadowingIn AA collisions the initial state effects (shadowing) affect not only the target, but also the projectile (poster by R. Arnaldi et al.) to be included in the extrapolation of the reference from pA to AA

Even in absence of anomalous suppression, the use of the standard reference (no shadowing) induces a 5-10% suppression signal sizeable effect

Reference curves for InIn and PbPb,including shadowing

Using the new reference (shadowing in the projectile and target)• Central Pb-Pb: still anomalously suppressed• In-In: almost no anomalous suppression?

AA results - RHIC• Cold nuclear matter effects poorly known Results shown as RAA

• Systems studied: AuAu, CuCu

Main observationsStrong suppression in Au-AuForward rapidity J/ are more suppressed

AA results – RHICAnomalous suppression

Compare CuCu and AuAuwith expected nuclearabsorption

1) CuCu compatible with nuclear absorption

AuAu2) Midrapidity: compatible with nuclear absorption3) Forward rapidity Anomalous suppression at Npart > 100200

Cold matter effects still based on low-statistics d-Au data

SPS vs RHIC• Try to plot together SPS and midrapidity RHIC results (in terms of RAA)

The agreement between SPS/NA38+NA50+NA60and RHIC/PHENIX is morethan remarkable.......

...but difficult to understand!

• Different s• Different shadowing• Different nuclear absorption

RHIC AA results – news from QM09

• Push coverage up to high pT

• (Maybe) small disagreement STAR vs PHENIX• Rule out class of models based on AdS/CFT (+hydro)• Increase at high pT already seen at SPS

Pb-Pb NA50

What do these results mean?

• 3 main results• Cold nuclear matter effects cannot explain J/ suppression• Similar suppression at SPS and RHIC energies• Forward y suppression larger (at RHIC)

SPS RHIC LHC

s (GeV) 17.2 200 5500

Ncc ≈ 0.2 ≈10 ≈100-200

x (at y=0) ≈ 10-1 ≈ 10-2 ≈ 10-4

• 2 classes of models• Only J/ from ’ and c decays are suppressed at SPS and RHIC

Expect same suppression at SPS and RHIC Reasonable if Tdiss

J/~ 2Tc

• Also direct J/ are suppressed at RHIC but cc multiplicity high

cc pairs can recombine in the later stages of the collision The 2 effects may balance: suppression similar to SPS

Sequential suppression

0 = 1 fm/cused here

SPS overall syst (guess) ~17%

PHENIX overall syst ~12% & ~7%

• Quantitative comparison of energy densities not easy (different formation times RHIC vs SPS)

• Nuclear absorption taken (approx) into account

• Can higher large-y suppression be explained in this scenario?• Note: suppression larger than total and ’ fraction...

• Possible mechanism gluon saturation at forward y (CGC)

=0

=2

This calc. is for open charm, butJ/ similar

hep-ph/0402298

Recombination?

• Most direct way for a quantitative estimateMeasure open charm cross section with good accuracy

Still not the case at RHIC....

• Indirect way• Look at the y and pT distributions in AA vs pp pA• If recombination is a sizeable effect

• Rapidity spectra narrower in AuAu than in pp• pT spectra of recombined pairs should not increase

• Provides a natural explanation for larger suppression at forward y

pT distributions, v2

Statistics is not so good, but pT behaviour looks rather flat

Mid rapidity Forward rapidity

• Smoking gun for regeneration: J/ flow

If regeneration important, J/ should inherit c quark flow

Rappc & bc v2

J/ v2

Some examples of regeneration models

Yan, Zhuang, Xunucl-th/0608010

Thews Eur.Phys.J C43, 97 (2005)

Grandchamp, Rapp, BrownPRL 92, 212301 (2004)

• Features of RHIC results qualitatively reproduced

If regeneration important J/ enhancement at LHC

Statistical hadronization• J/ production by statistical hadronization of charm quarks (Andronic, BraunMunzinger, Redlich and Stachel, PLB 659 (2008) 149)

• All charm quarks produced in primary hard collisions• Survive and thermalize in QGP • Charmed hadrons formed at chemical freeze-out (statistical laws)• No J/ survival in QGP

Reproduces RHIC data very well Decisive test at LHC

Gluon saturation effects on J/ suppression

• Factorization badly broken in pA and AA collisions in pQCD

Conclusion of the authors:

Heavy quarkonium at ALICE• Can be measured at both

• Midrapidity (central barrel, via electron tagging in the TRD)• Forward rapidity (2.5<y<4, in the muon arm)

• Many questions still to be answered at LHC energy

• Role of the large charm quark multiplicity• Will J/ regeneration dominate the picture for charmonium ? (RHIC results still not conclusive, at this stage)

• Bottomonium physics• Still completely unexplored in HI collisions• Will the tightly bound (1S) be melted at the LHC ?

(...estimates subject to a non-negligible time evolution!)

A look at the expected mass spectra

• No suppression/enhancement assumed• Comb. background to be estimated via event mixing

Expected statistics (central PbPb)

State S[103] B[103] S/B S/(S+B)1/2

J/ 130 680 0.20 150

’ 3.7 300 0.01 6.7

(1S) 1.3 0.8 1.7 29

(2S) 0.35 0.54 0.65 12

(3S) 0.20 0.42 0.48 8.1

Numbers refer to

L = 51026 cm-2s-1

106 s running time

• Significances not dramatically different between J/ and smaller statistics compensated by drastic background reduction

• Worst situation for the ’ : statistics , but much larger background

• Situation improves for the J/ when moving towards peripheral (background essentially combinatorial)• For the , no significant centrality dependence (background dominated by correlated open beauty)

A suppression scenario

• Suppose absJ/=0 (reasonable, extrapolating from RHIC ?)

• No b quark energy loss • Take into account feed-down from higher resonances

Use openbeauty asreference

• Suppression-1• Tc=270 MeV• TD/TC=1.7 (4.0) for J/ ()

• Suppression-2• Tc=190 MeV• TD/TC=1.21 (2.9) for J/ ()

Good sensitivity to inputparameters, for variousscenarioes

Quarkonia in the dielectron channel

• Complementary measurement wrt the dimuon channel• Allows evaluation of the fraction of J/ from B decays (thanks to ITS)

• TRD• Electron ID for p>1 GeV• Electron trigger for p>3 GeV (not for central PbPb)

• Efficiency between 0.8-0.9 for reconstruction of single e-

• Good momentum resolution with the typical bremsstrahlung tail

Invariant mass spectra

L = 51026 cm-2s-1

106 s running time

10% centralevents

Background frommisidentified

likely to be suppressed(quenching)

dNch/d = 3000

pT spectra

• For J/ significance is still reasonable up to pT= 10 GeV

Conclusions

• J/ suppression considered for a long time as the “golden” signature for QGP formation, but:

• A very careful study (and a corresponding theoretical effort) is necessary to understand cold nuclear matter effects

• Even elementary production processes are not so “elementary” (interplay perturbative vs non-perturbative)

• A clear signal of anomalous suppression has been seen at both SPS and RHIC

• RHIC interpretation more difficult (recombination effects)

• LHC: can J/ still be considered as a hard probe ? Suppression of bottomonium states new frontier