charmonium in nuclear collisions
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Charmonium in nuclear collisions. Partha Pratim Bhaduri VECC, Kolkata. Introduction Quarkonium production in elementary collisions Quarkonium interaction in cold nuclear matter Quarkonium interaction in hot nuclear matter. - PowerPoint PPT PresentationTRANSCRIPT
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