1 5 th International School on QGP and Heavy Ions Collisions: past, present and future Torino, 5-12 March 2011

2 Outlook: 1)Hard probes: definitions 2)High p T hadrons 3) Heavy Flavours 4) Quarkonia 1)Theoretical expectations 2)SPS results 3)RHIC results 4)LHC perspectives and first results

3 Quarkonium: introduction Quarkonium is considered since a long time as one of the most striking signatures for the QGP formation and its study in AA collisions is already a 25 years long story SPSRHIC LHC 17 GeV/c200 GeV/c2.76 TeV/c √s years 1990~ …but, as for the other hard probes, in order to understand quarkonium behaviour in the hot matter (AA collisions), its interactions with the cold nuclear matter should be under control (pA/dAu collisions)

4 What is Quarkonium? q q According to the quantum numbers, several quarkonium states exists

5 Confinement term Quarkonium The “confinement” contribution disappears The high color density induces a screening of the coulombian term of the potential q q q q

6 c c vacuum r J/ r c c Temperature T<Td r c c Temperature T>Td J/ D D The screening radius D (T) (i.e. the maximum distance which allows the formation of a bound qq pair) decreases with the temperature T if resonance radius > D (T)  no resonance can be formed At a given T: if resonance radius < D (T)  resonance can be formed Debye screening

7 Phys.Lett. B178 (1986) 416 This is the idea behind the suggestion (by Matsui and Satz) of the J/ as a signature of QGP formation (25 years ago!) Charmonium suppression Very famous paper, cited ~ 1400 times!

8 Sequential suppression of the resonances The quarkonium states can be characterized by the binding energy radius More bound states have smaller size Debye screening condition r 0 > D will occur at different T Sequential screening (2S) J/cc T<T c TcTc thermometer for the temperature reached in the HI collisions (2S) J/cc T~T c TcTc (2S) J/cc T~1.1T c TcTc (2S) J/cc T>>T c TcTc

J/ (quarkonium) can be studied through its decays: J/   +  - J/  e + e - Quarkonium decay

10 Quarkonium production Quarkonium production can proceed: directly in the interaction of the initial partons via the decay of heavier hadrons (feed-down) For J/ (at CDF/LHC energies) the contributing mechanisms are: Direct production Feed-down from higher charmonium states: ~ 8% from (2S), ~25% from  c B decay contribution is p T dependent ~10% at p T ~1.5GeV/c Prompt Displaced Feed down and J/ from B, if not properly taken into account, may affect physics conclusions Direct 60% B decay 10% Feed Down 30%

11 beam Muon Other hadron absorber and tracking target muon trigger magnetic field Iron wall Place a huge hadron absorber to reject hadronic background Implement a trigger system, based on fast detectors, to select muons Reconstruct muon tracks in a spectrometer (magnetic field + tracking detectors) Extrapolate muon tracks back to the target  Vertex reconstruction is usually rather poor ( z ~10 cm) Correct for multiple scattering and energy loss Standard way of measuring  pairs Approach adopted by NA50, PHENIX and ALICE (forward region)

12 Upgraded way of measuring  pairs Approach adopted by NA60, LHC exp. and foreseen in future PHENIX and ALICE upgrades (in the forward muon) 2.5 T dipole magnet target s vertex tracker or ! hadron absorber Muon Other and trackingmuon trigger magnetic field Iron wall 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

13 J/ is produced in two steps that can be factorized: Quarkonium production in pp Different descriptions of this evolution are at the basis of the various theoretical models 1)Color singlet model 2)Color evaporation model 3)NRQCD

14 Models for quarkonium production in pp Color Singlet ModelColor Evaporation M.NRQCD

15 Production models and CDF results The first CDF results on J/ direct production revealed a striking discrepancy wrt LO CSM factor 50! The agreement improves in NRQCD approach …but situation still puzzling, because polarization is not described! Open questions, to be investigated at LHC! Recently many step forwards (i.e. NLO and NNLO corrections…)

Quarkonium production in pA As the other hard probes, quarkonium may be affected by initial and final state effects 16 allow the understanding the J/ behaviour in the cold nuclear medium  complicate issue, because of many competing mechanisms: provide a reference for the study of charmonia dissociation in a hot medium  approach followed at SPS and similarly at RHIC (with dAu data) pA collisions Final state: cc dissociation in the medium, final energy loss p μ μ J/ Initial state: shadowing, parton energy loss, intrinsic charm  Useful to investigate initial state effects

17 These effects can be quantified, in pA collisions, in two ways: Cold Nuclear Matter effects Effective quantities which include al initial and final state effects In pA collisions, no QGP formation is expected NA50, pA 450 GeV = 1  no nuclear effects <1  nuclear effects The larger  abs, the more important are the nuclear effects  in principle, no J/ suppression.  however a reduction of the yield per nucleon-nucleon collisions is observed

18 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  abs J/, one expects L

19 Nuclear effects vs. x F Collection of results from many fixed target pA experiments Nuclear effects show a strong variation vs the kinematic variables Because of the  dependence on x F and energy  the reference for the AA suppression must be obtained under the same kinematic/energy domain as the AA data I. Abt et al., arXiv: lower s higher s

20 Nuclear effects anti-shadowing (with large uncertainties on gluon densities!) final state absorption… Interpretation of results not easy  many competing effects affect J/ production/propagation in nuclei  need to disentangle the different contributions Size of shadowing effects may be large and has to be taken into account when comparing results at different energies C. Lourenco, R. Vogt and H.Woehri, JHEP 0902:014,2009 F. Arleo and Vi-Nham Tram Eur.Phys.J.C55: ,2008, arXiv: Clear tendency towards stronger absorption at low √s

21 Why CNM are important? The cold nuclear matter effects present in pA collisions are of course present also in AA and can mask genuine QGP effects It is very important to measure cold nuclear matter effects before any claim of an “anomalous” suppression in AA collisions L  J/ /N coll L  J/ /N coll /nucl. Abs. 1 Anomalous suppression! pA AA CNM, evaluated in pA, are extrapolated to AA, in order to build a reference for the J/ behaviour in hadronic matter Measured/Expected

22 CNM, evaluated in pA, are extrapolated to AA, in order to build a reference for the J/ behaviour in hadronic matter J/ in AA SPS After correction for EKS98 shadowing B. Alessandro et al., EPJC39 (2005) 335 R. Arnaldi et al., Nucl. Phys. A (2009) 345 R.A., P. Cortese, E. Scomparin Phys. Rev. C 81, In-In 158 GeV (NA60) Pb-Pb 158 GeV (NA50) Using the previously defined reference: Central Pb-Pb:  Anomalous suppression ~ 30% effect In-In:  almost no anomalous suppression?

23 RHIC PHENIX J/e + e - |y|<0.35 & J/ +  - |y| [1.2,2.2] STAR J/e + e - |y|<1 pp, dA collisions pp 200 GeV/nucleon dAu 200 GeV/nucleon All data have been collected with the same collision energy (√s = 200 GeV) and kinematics AA collisions Au-Au 200 GeV/nucleon Cu-Cu 200 GeV/nucleon

24 pp results should help to understand the J/ production mechanism provide a reference for AA collisions (R AA ) RHIC – pp, dAu collisions In a similar way as at SPS, CNM effects are obtained from dAu data RHIC data exploit different x 2 regions corresponding to  shadowing (forward and midrapidity)  anti-shadowing (backward rapidity) Backward MidForward dAu collisions pp collisions

25 RHIC – AuAu collisions Comparison at different rapidities Stronger (unexpected) suppression at forward rapidities Coalescence of charm pairs in the medium? Different CNM effects? Mid-rapidity Forward-rapidity

26 Comparison with SPS results Results are shown as a function of the multiplicity of charged particles (~energy density, assuming  SPS ~ RHIC ) Comparison with SPS results  Both Pb-Pb and Au-Au seem to depart from the reference curve at N Part ~200  For central collisions more important suppression in Au-Au with respect to Pb-Pb

27 Interpretation of the results Several theoretical models have been proposed in the past, starting from the following observations R AA at forward y is smaller than at midrapidity similar suppression at SPS and RHIC Different approaches proposed: SPSRHICLHC s (GeV) N cc ≈ 0.2≈10≈ ) Only J/ from ’ and  c decays are suppressed at SPS and RHIC  The 2 effects may balance: suppression similar to SPS 2) Also direct J/ are suppressed at RHIC but cc multiplicity high  J/ regeneration ( N cc 2 ) contributes to the J/ yield  same suppression is expected at SPS and RHIC  reasonable if T diss (J/) ~ 2T c Some interpretations

28 Recombination Models including J/ regeneration qualitatively describe the R AA data (X. Zhao, R. Rapp arXiv: , Z.Qu et al. Nucl. Phys. A 830 (2009) 335) J/ elliptic flow  J/ should inherit the positive heavy quark flow J/ y distribution  should be narrower wrt pp J/ p T distribution  should be softer ( ) wrt pp Results are not precise enough to assess the amount of regeneration Indirect way  some distributions should be affected by regeneration Direct way for quantitative estimate  accurate measurement of charm  Recombination

29 LHC 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 (almost) unexplored in HI collisions Regeneration? Further suppression?

30 LHC Investigated by the 4 LHC experiments: ATLAS  mid-rapidity ||<2.5 ( +  - ) CMS  mid-rapidity ||<2.5 ( +  - ) ALICE  mid rapidity ||<0.9 (e + e - channel) forward rapidity 2.5<  <4 ( +  - ) LHCb  forward rapidity 2.5<  <4 ( +  - ) LHCb CMS ATLAS ALICE

31 Quarkonium LHC results in pp New results presented by the 4 experiments Differential distributions (y, p T ) Fraction of J/ from B Preliminary theory comparison

32  LHC in pp  hardly seen at RHIC, while now at LHC the  family is fully accessible Extremely important measurement:  More robust theory calculation (due to heavy bottom quark and absence of b-hadron feed-down) arXiv:

33 First J/ in Pb-Pb collisions! We expect few thousands J/ from 2010 statistics no correction for feed-downs, J/ from B ATLAS: arXiv: J/ with p T >3GeV/c and ||<2.5 A centrality dependent suppression is observed

34 Backup

35 Statistical hadronization J/ production by statistical hadronization of charm quarks (Andronic, BraunMunzinger, Redlich and Stachel, PLB 659 (2008) 149) 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 y A. Andronic et al. arXiv: Good agreement between data and model Recombination should be tested on LHC data! Statistical hadronization

36 x 2 scaling Shadowing effects (in the 2  1 approach) and final state absorption scale with x 2 if parton shadowing and final state absorption were the only relevant mechanisms   should not depend on √s at constant x 2

37 RHIC – AuAu collisions Comparison at different rapidities Comparison between different systems CuCu explores a smaller N part range Stronger (unexpected) suppression at forward rapidities Coalescence of charm pairs in the medium? Different CNM effects?

38 High p T J/  in Cu-Cu R CuCu up to p T = 9 GeV/c  suppression looks roughly constant up to high p T PHENIX (minimum bias) STAR (centrality 0-20% & 0-60%) R CuCu =1.4±0.4±0.2 (p T >5GeV/c)  R AA increases from low to high p T Difference between high p T results, but strong conclusions limited by poor statistics Both results in contradiction with AdS/CFT+Hydro Increase at high p T already seen at SPS NA50: Pb-Pb p T dependence