charmonium in nuclear collisions

Charmonium in nuclear collisions

5th CBM-India Collaboration Meeting, BHU, India

Partha Pratim Bhaduri VECC, Kolkata

• Introduction• Quarkonium production in elementary collisions • Quarkonium interaction in cold nuclear matter• Quarkonium interaction in hot nuclear matter

2

Introduction• States of matter, their defining features and transition between them always been

one of the fundamental issues of physics. Strongly interacting matter opens up a new chapter for such studies.

• Statistical QCD predicts at high temperature and/or densities, strongly interacting matter will undergo a transition from color neutral hadronic phase to a state of de-confined color charged quarks & gluons- QGP

Early universe

Compression heating quark-gluon matter (pion production)

baryons hadrons partons

In laboratory Relativistic heavy-ion collisions (RHIC) are the only tool to produce such exotic states of QCD matter

Neutron star

Challenge: find suitable probes to indicate the formation of de-confined QCD matter

vacuum

QGP

hadronicmatter

The good QCD matter probes should be:

Heavy quarkonia (J/, ’, , ’, etc) are very good QCD matter probes !

Well understood in “pp collisions”

Slightly affected by the hadronic matter, in a well understood way, which can be accounted for

Strongly affected by the deconfined QCD medium...

The story begins …First 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

...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 (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

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

• Lattice results seemed to indicate high dissociation temperatures

Quarkonium dissociation temperatures – Digal, Karsch, Satz

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/ () : Feed down

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

Dynamics of charmonium dissociation

Binding energy of J/ : EJ/ = 2MD – MJ/ ~ 640 MeV

Size of J/ is much smaller than usual hadrons (rJ/ ~0.25 fm. << 1fm.)

By what kind of dynamical interaction such a state can be dissociated?

Small spatial size : sufficiently hard probe to resolve the structure

Confined medium :-

pion gas f(p) ~ exp(-p/T) : <p> = 3T

Gluon distribution inside hadron g(x) ~ (1-x)3 ; x = k/p ; k & p being gluon & pion momentum respectively : <k> = 3T/5

Deconfined medium :-

Free gluons f(k) ~exp(-k/T) : <k> = 3T

For T>= 1.2Tc <k> ~ 640 MeV free gluons are hard enough to overcome J/ binding.

For a pion gas this implies : 3T/5 ~ 640 MeV => T > 1 GeV

Quarkonium productionin elementary collisions (pp)

J/ hadroproduction: pp collisions

q

q Q

Q

Q

Qg

g Q

Qg

gQ

Qg

g

Fundamental processes for production of qurakonium pair

Perturbative in nature due high mass of charm quarks

Most dominating process is gg fusion.

Hadronization of the QQ pair into physical bound state

g

g

c

c

J/

Color Evaporation Model (CEM) :

• 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

No unique theoretical description : Different models :1.Color Singlet Model2.Color Octet Model3.Color Evaporation Model

Quarkonium productionin pA collisions

Cold nuclear matter effects

• In p-A collisions presence of normal nuclear matter can affect charmonium production.

• No formation time for the medium : provide a tool to probe charmonium production, evolution & absorption in nuclear matter.

• Nuclear effects can arise in all the evolution stages of charmonium production :

a) Modification of initial state pdf’s due to presence of other nucleons inside the nucleus : enter in the perturbative cc production cross section :

=> decrease ( shadowing) or increase (anti-shadowing) in production rate

b) Once produced cc pair can suffer absorption in the pre-resonance or resonance stage : successive interactions with target nucleons : Normal nuclear suppression

Since we eventually want to probe the effect which the “secondary” medium produced by nucleus-nucleus collision has on charmonium production, it is of course essential to account correctly for any effects of nuclear medium initially present.

At fixed collision energy, quarkonium production rates per target nucleon decrease with increasing A.

The production rates decrease for increasing J/y momentum as measured in the nuclear target rest frame

The nuclear reduction appears to become weaker with the increasing collision energy ( SPS QM’09 results)

For fixed collision energy, mass number and J/y rapidity, the reduction appears to increase with the centrality of the collision.

At sufficiently high momentum in the target rest frame , the different charmonium states appear to suffer the same amount of reduction while at lower energy, the y’ is affected more than J/y.

At present, there does not exist a theoretical scenario able to account qualitatively for all these observations.

Putting everything together....

Quarkonium productionin AA collisions

Looking for the QGP

Several possible and quite different effects have been considered as consequences of the “produced medium” on quarkonium production:

Suppression by co-mover collision :

A charmonium state produced in a primary NN collision can be dissociated through interactions with the constituents of any medium subsequently formed in the collision. Such dissociation can occur in a confined as well as in a de-confined medium.

Suppression by color screening :

If the produced medium is a hot QGP, it will dissociate by color screening the charmonium states produced in primary NN collisions. Due to rareness of thermal charm quarks in the medium , the separated c & c-bar combine at hadronization with light quarks to form open charm mesons.

Enhancement by recombination: In the hadronization of QGP, charmonium formation can occur by binding of a c with a c-bar from different NN collisions (exogamous production) as well as from the same (endogamous production).

Color screening

Modify quarkonium potential

Perturbative Vacuum

cc

Color Screening

cc

krr

rV

)(Dre

rrV /)(

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 ?

AA results – SPS energy - QM09• Recent results on pA at 158 GeV 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!

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

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

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

• 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

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

No chrmonium data below 158 AGeV available.

cc production :

• at near-threshold for CBM energies!

• cc produced in first inelastic interactions

• pA: Cold Nuclear Matter effects

• AA: dissolved in medium?

... difference for J/ and '?

(sequential melting /co-mover absorption?)

No regeneration : clear suppression signature

Effect of high baryon density ?

measure energy and system-size dependence!

Charmonium at FAIR

rescaled to 158 GeV

The Compressed Baryonic Matter (CBM) experiment will measure charmonia through its decay into de-leptons in the energy regime 10 - 40 AGeV.

Charmonia at FAIR : some thoughts ….In CBM experiment at FAIR we are expecting a moderate temperature but a very dense baryonic medium to be created.

Experimental observables are expected to be sensitive to density as well as temperature.

What is the effect of net baryon density on charmonium ? Can we define dissociation densities for different charmonium states like dissociation temperatures (potential model study ……)?

How does charmonium production is modified in a baryon rich medium?

Charm propagation in cold nuclear matter (pA). Can we isolate different CNM effects (nuclear absorption, shadowing, anti-shadowing) ?

Will charm quarks thermalize with the dense medium ? We can look at the charmonium flow (if at all it exists) for example elliptic flow (v2) and if it exhibits an NCQ scaling .

Can we do something on this ?

Thank You All

Back Ups

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

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

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

• 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

Putting everything together....

At fixed collision energy, quarkonium production rates per target nucleon decrease with increasing A.

The production rates decrease for increasing J/y momentum as measured in the nuclear target rest frame

The nuclear reduction appears to become weaker with the increasing collision energy ( SPS QM’09 results)

For fixed collision energy, mass number and J/y rapidity, the reduction appears to increase with the centrality of the collision.

At sufficiently high momentum in the target rest frame , the different charmonium states appear to suffer the same amount of reduction while at lower energy, the y’ is affected more than J/y.

At present, there does not exist a theoretical scenario able to account qualitatively for all these observations.

Putting everything together....

Quarkonium productionin AA collisions

Looking for the QGP

Several possible and quite different effects have been considered as consequences of the “produced medium” on quarkonium production:

Suppression by co-mover collision :

A charmonium state produced in a primary NN collision can be dissociated through interactions with the constituents of any medium subsequently formed in the collision. Such dissociation can occur in a confined as well as in a de-confined medium.

Suppression by color screening :

If the produced medium is a hot QGP, it will dissociate by color screening the charmonium states produced in primary NN collisions. Due to rareness of thermal charm quarks in the medium , the separated c & c-bar combine at hadronization with light quarks to form open charm mesons.

Enhancement by recombination: In the hadronization of QGP, charmonium formation can occur by binding of a c with a c-bar from different NN collisions (exogamous production) as well as from the same (endogamous production).

Color screening

Modify quarkonium potential

Perturbative Vacuum

cc

Color Screening

cc

krr

rV

)(Dre

rrV /)(

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

AA results – SPS energy - QM09• Recent results on pA at 158 GeV 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!

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

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

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

• 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

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

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!)

No chrmonium data below 158 AGeV available.

cc production :

• at near-threshold for CBM energies!

• cc produced in first inelastic interactions

• pA: Cold Nuclear Matter effects

• AA: dissolved in medium?

... difference for J/ and '?

(sequential melting /co-mover absorption?)

No regeneration : clear suppression signature

Effect of high baryon density ?

measure energy and system-size dependence!

Charmonium at FAIR

rescaled to 158 GeV

The Compressed Baryonic Matter (CBM) experiment will measure charmonia through its decay into de-leptons in the energy regime 10 - 40 AGeV.

Charmonia at FAIR : some thoughts ….In CBM experiment at FAIR we are expecting a moderate temperature but a very dense baryonic medium to be created.

Experimental observables are expected to be sensitive to density as well as temperature.

What is the effect of net baryon density on charmonium ? Can we define dissociation densities for different charmonium states like dissociation temperatures (potential model study ……)?

How does charmonium production is modified in a baryon rich medium?

Charm propagation in cold nuclear matter (pA). Can we isolate different CNM effects (nuclear absorption, shadowing, anti-shadowing) ?

Will charm quarks thermalize with the dense medium ? We can look at the charmonium flow (if at all it exists) for example elliptic flow (v2) and if it exhibits an NCQ scaling .

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

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

Fate of a cc bound state in a de-confined medium

Modify quarkonium potential

Perturbative Vacuum

cc

Color Screening

cc

krr

rV

)(Dre

rrV /)(

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 ?

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

Role of shadowingIn AA collisions the initial state effects (shadowing) affect not only the target, but also the projectile 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?

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

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