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Heavy quarkonia and Quark-Gluon Plasma: a saga with (at least) three-episodes

Heavy quarkonia and Quark-Gluon Plasma: a saga with (at least) three-episodes. E. Scomparin (INFN Torino ). Nikhef , Amsterdam, June 15, 2012. SPS: the discovery of the anomalous suppression. RHIC: puzzling observations f rom America. LHC: towards a new era ?.

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Heavy quarkonia and Quark-Gluon Plasma: a saga with (at least) three-episodes

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  1. Heavy quarkonia and Quark-Gluon Plasma:a saga with (at least) three-episodes E. Scomparin (INFN Torino) Nikhef, Amsterdam, June 15, 2012 SPS: the discovery of the anomalous suppression RHIC: puzzling observations from America LHC: towards a new era ? “The Empire strikes back” “Return of the Jedi” “A new hope”

  2. Heavy quarkonia states Almost 40 years of physics! • Spectroscopy • Decay • Production • In media See 182 pages review On arXiv:1010.5827

  3. Which medium ? • We want to study the phase diagram of strongly interacting matter • Is it possible to deconfine quarks/gluons • and create a Quark-Gluon Plasma (QGP) ? • Only way to do that in the lab  ultrarelativistic HI collisions • Problems ! • Quark-Gluon Plasma is short-lived ! • Only final state hadrons are observed in our detectors • (indirect observation)

  4. Probing the QGP • One of the best way to study QGP is via probes which are • sensitive to the short-lived QGP phase • Ideal properties of a QGP probe • Production in elementary NN collisions • under control • Interaction with cold nuclear matter • under control HADRONIC MATTER VACUUM QGP • Not (or slightly) sensitive to the final-state • hadronic phase • High sensitivity to the properties of the • QGP phase • None of the probes proposed up to now • (including heavy quarkonia!) actually satisfies • all of the aforementioned criteria So what makes heavy quarkonia so attractive ?

  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. How the whole story began… First paper on the topic  1986, Matsui and Satz The most famous paper in our field (1570citations!) Keywords 1)Hot quark-gluon plasma 2)Colour screening 3)Screening radius 4)Dilepton mass spectrum Unambiguous signature of QGP formation

  7. …and how first measurements looked like • NA38: first measurement of J/ suppression at the SPS, • O-U collisions at 200 GeV/nucleon(1986) NJ/ centrali periferiche Peripheral events Central events cont. (2.7-3.5) • J/ is suppressed (factor 2!) moving from peripheral • towards central events Do we see a QGP effect in O-U collisions at SPS? Is this the end of the story ?

  8. ...but the story was not so simple • In particular: • 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 !

  9. Sequential suppression The quarkoniumstates can be characterized by • the binding energy • radius More bound states  smaller size (2S) (2S) (2S) (2S) c c c c J/ J/ J/ J/ Debye screening condition r0 > D will occur at different T T~Tc T~1.1Tc T>>Tc Tc Tc Tc Tc T<Tc thermometer for the temperature reached in the HI collisions Sequential suppression of the resonances

  10. Do we need to measure all the states? Quarkonium production can proceed: • directly in the interaction of the initial partons • via the decay of heavier hadrons (feed-down) Low pT For J/ (at CDF/LHC energies) the contributing mechanisms are: Feed Down 30% Direct 60% Direct production B decay 10% Prompt Feed-down from higher charmonium states: ~ 8% from (2S), ~25% from c B decay contribution is pT dependent ~10% at pT~1.5GeV/c Non-prompt B-decay component “easier” to separate displaced production

  11. (3S) (2S) b(2P) c(1P) (2S) b(1P) J/ (1S) J/  Suppression pattern • Since each resonance should have a typical dissociation • temperature, one should observe «steps» in the suppression • pattern of the measured J/ when increasing T Digal et al., Phys.Rev. D64(2001) 094015 • Ideally, one could vary T • by studying the same system (e.g. Pb-Pb) at various s • by studying the same system for various centrality classes

  12. How can we get Tdiss? • Lattice QCD calculations are our main source of information • on the dissociation temperatures • Early studies showed that the complete disappearance of the J/ • peak occurred at very high temperatures (~2Tc) • However spectral functions expected to change rather smoothly • How to pin down Tdiss?

  13. strong binding weak binding Recent results on Tdiss • Binding energies for the • various states can be obtained • from potential models too • Assume a state “melts” when • Ebind< T  Tdiss~1.2 Tc • Latest results from lattice: • No clear sign of bound state • beyond T=1.46 Tc • Region close to Tc now under study New! O.Kaczmarek@HP12

  14. target beam hadron absorber Muon Other Measurement via  decays Approach adopted by NA50,PHENIXand ALICE muon trigger andtracking Iron wall magnetic field • Place a huge hadron absorber to reject hadronic background • Implement a trigger system, based on fast detectors, to select  • Reconstruct tracks in a spectrometer • Correct for multiple scattering and energy loss • Extrapolate muon tracks back to the target •  Vertex reconstruction is usually rather poor(z~10 cm)

  15. muon trigger and tracking Iron wall magnetic field hadron absorber or ! Muon Other Measurement via  decays Approach adopted by NA60, CMS and foreseen in PHENIX and ALICE upgrades Dipole magnet vertex tracker target • Use a silicon tracker in the vertex region to track muons before they suffer multiple scattering and energy loss in the hadron absorber. Improve mass resolution Determine origin of the muons

  16. Experiments From fixed target at the SPS (muons only)… NA60 to RHIC collider (muons+electrons)… PHENIX STAR

  17. Experiments ALICE (low pT) …to the LHC (electrons+muons) ALICE  dedicatedHI experiment CMS+ATLAS  mainly pp, but very good capability for charmonia and bottomonia in HI ATLAS (high pT) CMS (high pT)

  18. Quantifying the suppression (1) • High temperature should indeed induce a suppression of the • charmonia and bottomonia states • How can we quantify the suppression ? • Low energy (SPS) • Normalize the charmonia yield to the Drell-Yan dileptons + • Advantages • Same final state, DY is • insensitive to QGP • Cancellation of syst. • uncertainties g c J/ g - c q + * • Drawbacks • Different initial state • (quark vs gluons) - q

  19. Quantifying the suppression (2) • At RHIC, LHC Drell-Yan is no more “visible” in the dilepton • mass spectrum  overwhelmed by semi-leptonic decays of • charm/beauty pairs • Solution: directly normalize to elementary collisions (pp), via • nuclear modification factor RAA = RAA<1 suppression RAA>1 enhancement • Advantages • same process in nuclear environment and in vacuum • Drawbacks • Systematics more difficult to handle (no cancellations) An ideal normalization would be the open heavy quark yield However, this poses several practical problems (see later)

  20. Mechanisms for suppression • We have seen earlier that a suppression of the J/ was observed • already in low-energy O-U collisions at the SPS • Was this a sign of J/ dissociation by QGP-related effects ? • Not really. Look at what happens in p-A collisions, where no QGP • formation would be expected ! • = 1  no nuclear effects • <1  nuclear effects N.B.: J/pA/(A J/pp) is equivalent to RpA NA50, pA 450 GeV • A significant reduction of the yield per NN collision is observed, usually parametrized by effective quantities (, abs)

  21. L Nuclear absorption The most direct interpretation of the previous observations is nuclear absorption  once the J/ is produced, it must cross a thickness L of nuclear matter, where it may interact and dissociate If the cross section for nuclear absorption is absJ/, one expects … a behaviour indeed seen in the data … which may affect J/ production in AA collisions too, and mask genuine QGP-related effects

  22. J/ c p c g J/ vs (2S) • Less bound quarkonium states • should be easier to break…. • and indeed this is the case At SPS energies • 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

  23. Is nuclear absorption the whole story ? Collection of results from many fixed target pA experiments Nuclear effects show a strong variation vs the kinematic variables J/ Very likely we observe a combination of several nuclear effects higher s J/ vs (2S) lower s Fast (2S) as absorbed as J/!

  24. Nuclear shadowing PDF in nuclei are strongly modified with respect to those in a free nucleon i(x,Q2)=Ri(A,x,Q2) xi(x,Q2) free proton PDF nPDF: PDF of proton in a nucleus valence quarks sea quarks gluons Various parameterizations available RHIC LHC Significant uncertainties for the gluon modifications, the more relevant for quarkonia production From enhancement to suppression, moving towards higher energy SPS

  25. Consequences 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 Shadowing can be factorized: is nuclear absorption what remains ?

  26. Results on d-Au from RHIC PHENIX, J/  , J/  ee Data favourrather small absorption cross sections, ~2-3 mb (depending on pdf parameterization), much lower than at fixed target

  27. s-dependence of nuclear absorption • Tendency towards vanishing J/abs when s increases • Global interpretation of cold nuclear matter effects not easy • (other ingredients such as initial state energy loss can play a role) • Collect pA data in the same kinematic domain of AA data

  28. PbPb results at sNN =17.2 GeV (SPS) • NA50 and the discovery of the anomalous J/ suppression • N.B.: the cold nuclear matter effects were extrapolated from • pA results obtained at higher s (27.4 GeV)

  29. Cold nuclear matter effects at 17.2 vs 27.4 GeV s = 27.4 GeV s = 17.2 GeV • When finally pA collisions were studied at the very same energy • of the nuclear collisions (s=17.2 GeV), it was found thatcold • nuclear matter effects are stronger at that energy •  Need to re-normalize Pb-Pb suppression to the new reference

  30. B. Alessandro et al., EPJC39 (2005) 335 R. Arnaldi et al., Nucl. Phys. A (2009) 345 SPS “summary” plot Let’s compare NA50 (Pb-Pb)and NA60 (In-In) results: Size of anomalous suppression smaller wrt first estimates (due to stronger than expected cold nuclear matter effects) In-In 158 GeV (NA60) Pb-Pb 158 GeV (NA50) Anomalous suppression for central PbPbcollisions (up to ~30%, compatible with (2S) and c melting) Agreement between PbPb and InIn in the common Npart region PbPb data not precise enough to clarify the details of the pattern! After correction for EKS98 shadowing

  31. Is (2S) suppressed too ? • Yes, but already for • light-nuclei projectiles • (S-U collisions) • Makes sense, the less bound • (2S) state may need lower • temperatures to melt • Up to now, the most • accurate set of results • on (2S) production in • nuclear collisions New results from LHC in a few minutes….

  32. Moving to RHIC: expectations • Two main lines of thought We gain one order of magnitude in s. In the “color screening” scenario we have then two possibilities We reach T>TdissJ/ suppression becomes stronger than at SPS We do not reach T>TdissJ/  suppression remains the same 2) Moving to higher energy, the cc pair multiplicity increases A (re)combination of cc pairs to produce quarkonia may take place at the hadronization  J/ enhancement ?!

  33. J/ RAA: SPS vs RHIC • Let’s simply compare RAA • (i.e. no cold nuclear effects taken into account) • Qualitatively, very similar • behaviour at SPS and • RHIC ! • Do we see (as at SPS) • suppression of (2S) and • c ? • Or does (re)generation • counterbalance a larger • suppression at RHIC ? • PHENIX experiment • measured RAA at both • central and forward • rapidity: what can we • learn ?

  34. RHIC: forward vs central y Comparison of results obtained at different rapidities Mid-rapidity Forward-rapidity Stronger suppression at forward rapidities • Not expected if suppression • increases with energy density • (which should be larger at • central rapidity) • Are we seeing a hint of • (re)generation, since there are • more pairs at y=0?

  35. Suppression vs recombination Do we have other hints telling us that recombination can play a role at RHIC ? J/ elliptic flow  J/ should inherit the heavy quark flow Recombination could bemeasured in an indirect way J/ y distribution  should be narrower wrtpp J/ pT distribution  should be softer (<pT2>) wrtpp Open charm Closed Difficult to conclude

  36. <pT2> vs system size • No clear decrease of • <pT2>wrtpp at RHIC, • as expected in case of • recombination …still, at the SPS, there was a very clear increase from elementary to nucleus-nucleus collisions  Difficult to conclude

  37. Comparisons with models • In the end, models can • catch the main features • of J/ suppression at • RHIC, but no quantitative • understanding • In particular, no clear • conclusion on (2S) and c only suppression vs All charmonia suppressed + (re)generation

  38. An interesting comparison • Up to now we concentrated on RAA at RHIC • What happens if we try taking into account cold nuclear matter • effects and compare with the same quantity at the SPS Nice “universal” behavior Note that charged multiplicity is proportional to the energy density in the collision Maximum suppression ~40-50% (still compatible with only (2S) and c melting) Go to the LHC and get more data!

  39. First results at the LHC • Great expectation (as for many other observables) from the • first LHC heavy-ion runs (Pb-Pb @ 2.76 TeV) • Complementary acceptance • ALICE  low pTJ/ • CMS/ATLAS  high pTJ/, excellent resolution • Solve/clarify issue with J/ suppression/(re)generation • Advent of upsilon family (seen also at RHIC with small statistics) • (Re)generation negligible • Observe suppression of less bound (2S), (3S) wrt(1S) Mass r0

  40. Very first result • A couple of weeks after the end of the 2010 data taking, • ATLAS published the first LHC result on J/ suppression! • Interesting, but a bit deceiving! ~ Same suppression • as at RHIC,SPS • However this comparison is not sound. Different pT explored!

  41. ALICE results • Results from both 2010 and 2011 (high luminosity ~ 100 b-1) • now available. 2011 results public since just a couple of weeks • (Hard Probes conference, Cagliari) New! e+e- +-

  42. RAA vspT(0<pT<8 GeV/c) New! • RAAat forward rapidity flattens for Npart >100 • Similar behaviour at central rapidity (increse for central ?)

  43. ALICE vs CMS, low vs high pT • Less suppression at ALICE than at PHENIX (low pT dominated) • Larger suppression at high pT • Are we observing an effect of (re)generation ? • Indeed (re)generated J/ should sit predominantly at low pT • (where the bulk of the charm yield is)  Let’s then look at the suppression vspT

  44. J/ suppression vspT New! • Less suppression at lowpT (where regeneration is expected) • Effect not present at RHIC energy • Models reproduce this low-pT enhancement • Do J/ observed at LHC inherit the charm quark flow?

  45. Open charm and J/ flow New! • 3 indication for D-meson flow • fromALICE • Hint of non-zero flow for J/ • at LHC energy (2.2 significance)

  46. A word of caution • At both SPS and RHIC energies studies of cold nuclear matter • effects were very important to establish a “full picture” New! • A priori, nuclear absorption should be negligible (crossing time • of the two nuclei extremely fast at LHC energies) • Shadowing plays a role • Quantitative estimate from 2012 pPb LHC run

  47. From charmonia to bottomonia: CMS New! • Evident suppression of the less bound (2S), (3S) states !

  48. Relative suppression and RAA New! • (2S) almost disappears for central Pb-Pb events • Also (1S) is suppressed, compatible with feed-down from 2S+3S

  49. Comparison with RHIC • STAR measures an inclusive • RAA(all the states together) and • sees a suppression, compatible • with the one observed by CMS • ALICE can complement CMS • measurement by studying • forward rapidity s • (results expected soon)  STAR CMS

  50. Not everything is clear (1) • Thanks to the very good resolution CMS can accurately • measure (2S) in Pb-Pb New! • Double ratio(2S)/J/ in Pb-Pbvspp • Striking difference between “low pT, large y” and “large pT, low y” • To be understood

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