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Charmonium in nuclear collisions

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.

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Charmonium in nuclear collisions

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  1. 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 5th CBM-India Collaboration Meeting, BHU, India

  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 baryons hadrons partons Compression heating quark-gluon matter (pion production) Neutron star Early universe In laboratory Relativistic heavy-ion collisions (RHIC) are the only tool to produce such exotic states of QCD matter

  3. Challenge: find suitable probes to indicate the formation of de-confined QCD matter vacuum The good QCD matter probes should be: Well understood in “pp collisions” hadronicmatter Slightly affected by the hadronic matter, in a well understood way, which can be accounted for QGP Strongly affected by the deconfined QCD medium... Heavyquarkonia (J/, ’, , ’, etc) are very good QCD matter probes !

  4. The story begins … First paper on the topic  1986, Matsui and Satz The most famous paper in our field (1231 citations!) Keywords 1)Hot quark-gluon plasma 2)Colour screening 3)Screening radius 4)Dilepton mass spectrum Unambiguous signature of QGP formation

  5. ...but the story is not so simple • Do we understand charmonium production in elementary • collisions ? • Are there any other effects, not related to colour screening, • that may induce a suppression of quarkonium states ? • 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? • Can the melting temperature(s) be uniquely determined ? • Do experimental observations fit in a coherent picture ? None of these questions has a trivial answer.... ... so let’s start from the beginning !

  6. threshold 3.8 GeV 3S1 y(2S) or y’ 3P2 c2 c1 3P1 Mass 3P0 c0 spin J/y orbital 3S1 total 3 GeV Charmonium states Charmonium  cc bound state If m<2mD  stable under strong decay Relative motion is non-relativistic (~0.4) non-perturbative treatment The binding of the c and cbar quarks can be expressed using the Cornell potential: Coulomb contribution, induced by gluon exchange between q and qbar Confinement term

  7. Charmonium (bottomonium) states • Various cc and bb bound states have very different • binding energy and dimensions • Strongly bound states are smaller • Ther0>rDconditioncan be met atdifferent temperaturesfor the • various resonances • Try to identify the resonances which disappear and deduce the • temperaturereached in the collision

  8. Dissociation temperatures • Quantitative predictions on dissociation temperatures come from • lattice QCD studies • potential models • effective field theories Non-perturbative domain • Results have shown significant oscillations in the recent past • Lattice results seemed to indicate high dissociation • temperatures Quarkonium dissociation temperatures – Digal, Karsch, Satz

  9. (3S) (2S) b(2P) c(1P) (2S) b(1P) J/ (1S) J/  Suppression hyerarchy • Each resonance has a typical dissociation threshold • Consider the cc (bb) resonances that decay into J/ () : Feed down Digal et al., Phys.Rev. D64(2001)094015 • The J/ () yield should exhibit a step-wise suppression when T • increases (e.g. comparing A-A data at various √s or centrality)

  10. Dynamics of charmonium dissociation Binding energy of J/y : EJ/y= 2MD – MJ/y ~ 640 MeV Size of J/y is much smaller than usual hadrons (rJ/y~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/y binding. For a pion gas this implies : 3T/5 ~ 640 MeV => T > 1 GeV

  11. Quarkonium production in elementary collisions (pp)

  12. g q Q Q g Q Q g g g g q Q Q Q Q J/ hadroproduction: pp collisions Fundamental processes for production of qurakonium pair Perturbative in nature due high mass of charm quarks Most dominating process is gg fusion.

  13. Hadronization of the QQ pair into physical bound state • No unique theoretical description : Different models : • Color Singlet Model • Color Octet Model • Color Evaporation Model Color Evaporation Model (CEM) : g c J/ • 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 g c

  14. Quarkonium production in pA collisions Cold nuclear matter effects

  15. 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 : • 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.

  16. 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.

  17. Quarkonium production in AA collisions Looking for the QGP

  18. 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).

  19. Color screening Modify quarkonium potential • Deconfined world • No confinement term • Coulomb part screened Confined world  Quarkonium states described with =0.52, k=0.926 GeV/fm (mc = 1.84 GeV) Do bound states still exist ?

  20. AA results – SPS energy - QM09 • Recent results on pA at 158 GeV imply a modification in the interpretation of AA data absJ/ (158 GeV) > absJ/ (400 GeV)  smaller anomalous suppression with respect to previous estimates In-In 158 GeV (NA60) Pb-Pb 158 GeV (NA50) QM09 new reference Published results B. Alessandro et al., EPJC39 (2005) 335 Still a ~30% effect in central Pb-Pb! R. Arnaldi et al., PRL99 (2007) 132302

  21. AA results - RHIC • Cold nuclear matter effects poorly known  Results shown as RAA • Systems studied: AuAu, CuCu Strong suppression in Au-Au Forward rapidity J/ are more suppressed Main observations

  22. SPS vs RHIC • Try to plot together SPS and midrapidity RHIC results • (in terms of RAA) The agreement between SPS/NA38+NA50+NA60 and RHIC/PHENIX is more than remarkable....... ...but difficult to understand! • Different s • Different shadowing • Different nuclear absorption

  23. 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) • 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 TdissJ/~ 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

  24. 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

  25. rescaled to 158 GeV Charmonium at FAIR The Compressed Baryonic Matter (CBM) experiment will measure charmonia through its decay into de-leptons in the energy regime 10 - 40 AGeV. • 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/y and y'? • (sequential melting /co-mover absorption?) • No regeneration : clear suppression signature • Effect of high baryon density ? measure energy and system-size dependence!

  26. 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 ?

  27. Thank You All

  28. Back Ups

  29. L Nuclear absorption • 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 • It is also exepcted that weakly bound states (as ’) have a • much larger nuclear absorption cross section (’ is twice as large as the J/)

  30. Nuclear absorption cross section • As a function of L, the pA cross • section can be described • From the set of data taken by • NA50 at 450 GeV, one extracts the • nuclear absorption cross section • L can be calculated in the frame of the Glauber model • (geometrical quantity)

  31. J/ c p c g ’ vs J/ • As expected, the nuclear absorption • cross section is larger for the ’ • It is important to note that the • charmonium production process • happens on a rather long timescale • The nucleus “sees” the cc in • a (mainly) color octet state • Hadronization can take place • outside the nucleus

  32. AA 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 pA J//Ncoll 1 J//Ncoll/nucl. Abs. Anomalous suppression! L L • It is very important to measure cold nuclear matter effects before • any claim of an “anomalous” suppression in AA collisions

  33. In-In Pb-Pb pA collisions – SPS energies • Particularly relevant for the interpretation of heavy-ion data at SPS pA collisions Reference for the J/ suppression in AA (cold nuclear matter effects aka nuclear abs.) • tuned using pA at 400/450 GeV (NA50) absJ/ = 4.2±0.5mb, (J//DY)pp =57.5±0.8 (Glauber analysis) • extrapolated to AA assuming absJ/ (158 GeV) = absJ/ (400/450 GeV) AA collisions Observed suppression in AA exceeds nuclear absorption • Onset of the suppression at Npart 80 • Good overlap between Pb-Pb and In-In E=158 GeV/nucleon

  34. 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 absJ/ (158 GeV) = 7.6 ± 0.7 ± 0.6 mb absJ/ (400 GeV) = 4.3 ± 0.8 ± 0.6 mb • “Surprising” result: cold nuclear matter effects stronger at lower energy! Expect consequences for anomalous suppression

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

  36. 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

  37. 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.

  38. Quarkonium production in AA collisions Looking for the QGP

  39. 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).

  40. Color screening Modify quarkonium potential • Deconfined world • No confinement term • Coulomb part screened Confined world  Quarkonium states described with =0.52, k=0.926 GeV/fm (mc = 1.84 GeV) Do bound states still exist ?

  41. Conditions for melting “Screened Hamiltonian” with has NO solutions for • The condition We have while, for a 3-flavor QGP with T=200 MeV one has No bound state in a T = 200 MeV QGP is verified The condition

  42. AA results – SPS energy - QM09 • Recent results on pA at 158 GeV imply a modification in the interpretation of AA data absJ/ (158 GeV) > absJ/ (400 GeV)  smaller anomalous suppression with respect to previous estimates In-In 158 GeV (NA60) Pb-Pb 158 GeV (NA50) QM09 new reference Published results B. Alessandro et al., EPJC39 (2005) 335 Still a ~30% effect in central Pb-Pb! R. Arnaldi et al., PRL99 (2007) 132302

  43. AA results - RHIC • Cold nuclear matter effects poorly known  Results shown as RAA • Systems studied: AuAu, CuCu Strong suppression in Au-Au Forward rapidity J/ are more suppressed Main observations

  44. SPS vs RHIC • Try to plot together SPS and midrapidity RHIC results • (in terms of RAA) The agreement between SPS/NA38+NA50+NA60 and RHIC/PHENIX is more than remarkable....... ...but difficult to understand! • Different s • Different shadowing • Different nuclear absorption

  45. 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) • 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 TdissJ/~ 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

  46. This calc. is for open charm, but J/ similar =0 =2 SPS overall syst (guess) ~17% hep-ph/0402298 0 = 1 fm/c used here PHENIX overall syst~12%& ~7% Sequential suppression • Nuclear absorption taken • (approx) into account • Quantitative comparison • of energy densities not • easy (different formation • times RHIC vs SPS) • 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)

  47. 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

  48. 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!)

  49. rescaled to 158 GeV Charmonium at FAIR The Compressed Baryonic Matter (CBM) experiment will measure charmonia through its decay into de-leptons in the energy regime 10 - 40 AGeV. • 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/y and y'? • (sequential melting /co-mover absorption?) • No regeneration : clear suppression signature • Effect of high baryon density ? measure energy and system-size dependence!

  50. 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 .

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