Heavy quarkonium in nuclear collisions from SPS to LHC

1. Heavy quarkonium in nuclear collisionsfrom SPS to LHC E. Scomparin –INFN Torino (Italy) Roma – April 21-23, 2009 • Introduction • why 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

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

3. .. then moving to the New World Gatlinburg, Tennessee 1891

4. The (real) beginning of the story 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. Everything in one slide..... Screening 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

6. ...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 !

7. 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

8. 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

9. target beam hadron absorber Muon Other Standard way of measuring muon pairs(NA50 at SPS, PHENIX at RHIC, ALICE at LHC) muon trigger and tracking Iron wall magnetic field • Place ahuge hadron absorberto reject hadronic background • Implement atrigger 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 aspectrometer(B + tracking detectors) • Correct formultiple scatteringandenergy loss • Extrapolate muon tracks back to the target •  Vertex reconstructionis usually ratherpoor(z~10 cm)

10. muon trigger and tracking Iron wall magnetic field hadron absorber or ! Muon Other Second generation experiment(s):NA60 at SPS, future upgrades at RHIC,LHC 2.5 T dipole magnet beam tracker vertex tracker targets 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

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 Simpler approach: 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

13. Color singlet model (LO) • First “microscopic” model • for quarkonium production g c J/ factor 50! g c • Gluon fusion dominant • Requires the cc pair to be produced • in a color singlet state, with the • same quantum nubers of the • charmonium state under study Ruled out by results from the Tevatron collider (circa 1995)

14. 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) 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)) (singlet) For J/ (octet)

15. Success and open problems Cross section values successfully reproduced! Polarization values completely missed!

16. y x decay plane m+ ϕ  2 z axis  = 1 pprojectile ptarget Viewed from dimuonrest frae  = 0 reaction plane  = -1 Polarization measurements • >0 transverse polarization • <0 longitudinal polarization 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

17. Recent news • Color singlet model revisited • 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 Both cross section AND polarization reasonably reproduced

18. 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

19. Quarkonium production in pA collisions Cold nuclear matter effects

20. J/ / DY: same behaviour 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 NA50, pA 450 GeV

21. 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/)

22. 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)

23. 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

24. I. Abt et al., arXiv:0812.0734 Nuclear effects vs xF 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

25. 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

26. Influence of shadowing SPS Tevatron (FT) RHIC At SPS, the “true” nuclear absorption cross section is larger than the “effective” one • Increasing √s • From anti-shadowing to shadowing

27. 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

28. 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

29. 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

30. pA collisions at fixed target – after QM09 Two effects may help to explain 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)

31. 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

32. Forward Mid Backward d Au Influence of shadowing at RHIC • RHIC data sit in the • Shadowing region • (forward and midrapidity) • Anti-shadowing region • (backward rapidity) d Au Shape of RdAu vs rapidity largely determined by shadowing

33. d-Au collisions – news from QM09 • Considering various production processes, one gets different • results for cold nuclear matter effects Different x2 range “Intrinsic” production “Extrinsic” production gg  J/ + g gg  J/ (following emission of soft gluon(s) does not modify kinematics) (emission of a hard gluon)

34. EPS08 σ = 0 mb σ = 4 mb EKS σ = 0 mb σ = 4 mb d-Au collisions – news from QM09 • Large statistics sample (run-8) • First preliminary results (RCP) EKS shadowing

35. 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

36. Quarkonium production in AA collisions Looking for the QGP

37. cc pair in a deconfined medium 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 ?

38. 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

39. 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

40. (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/() 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)

41. 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 • Calculate spectral functions for the various states • Lattice spectral functions seemed to indicate high dissociation • temperatures These conclusions are now regarded as premature

42. strong binding weak binding Recent results on Tdiss • 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 mass Stronger binding

43. AA results – SPS energy - QM09 • Recent results on pA at 158 GeV (see previous slides) 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

44. Reference curves for InIn and PbPb,including shadowing Role of shadowing In 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 ofanomalous suppression, the use of the standard reference (no shadowing) induces a 5-10% suppression signal  sizeable effect Using the new reference (shadowing in the projectile and target) • Central Pb-Pb: still anomalously suppressed • In-In: almost no anomalous suppression?

45. 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

46. AA results – RHICAnomalous suppression Compare CuCu and AuAu with expected nuclear absorption 1) CuCu compatible with nuclear absorption AuAu 2) Midrapidity: compatible with nuclear absorption 3) Forward rapidity Anomalous suppression at Npart > 100200 Cold matter effects still based on low-statistics d-Au data

47. 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

48. RHIC AA results – news from QM09 Pb-Pb NA50 • 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 pTalready seen at SPS

49. 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

50. 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)