Outlook: 1) Hard probes: definitions. 2) High p T hadrons. 3) Heavy Flavours

Transcription

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

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 2

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 SPS RHIC LHC s 17 GeV/c 200 GeV/c 2.76 TeV/c years ~ 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) 3

4 What is Quarkonium? Quarkonium is a bound state of q and q with m qq < 2m D (m B ) q According to the quantum numbers, several quarkonium states exists q Charmonium (cc ) family Bottomonium (bb ) family 4

5 Quarkonium At T=0, the binding of the q and q quarks can be expressed using the Cornell potential: V ( r) kr r q q Coulombian contribution, induced by a g exchange between q and q Confinement term What happens to a qq pair placed in the QGP? The QGP consists of deconfined colour charges the binding of a qq pair is subject to the effects of colour screening The confinement contribution disappears The high color density induces a screening of the coulombian term of the potential V ( r) kr r / D V ( r) e r r q q 5

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

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

8 Sequential screening The quarkonium states can be characterized by the binding energy radius state J/ c (2S) Mass(GeV) E (GeV) r o (fm) More bound states have smaller size Debye screening condition r 0 > D will occur at different T state (1S) (2S) (3S) Mass(GeV) E (GeV) r o (fm) (2S) J/ c T c Sequential suppression of the resonances thermometer for the temperature reached in the HI collisions T<T T~T T~1.1T T>>T c c c 8

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

10 Displaced Prompt 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 cs Feed Down 30% B decay 10% Direct 60% B decay contribution is p T dependent ~10% at p T ~1.5GeV/c Feed down and J/ from B, if not properly taken into account, may affect physics conclusions 10

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

12 Upgraded way of measuring pairs Approach adopted by NA60, LHC exp. and foreseen in future PHENIX and ALICE upgrades (in the forward muon) target s 2.5 T dipole magnet vertex tracker hadron absorber muon trigger and tracking Use a silicon tracker in the vertex region to track muons before they suffer multiple scattering and energy loss in the hadron absorber. magnetic field Muon Other Iron wall Improve mass resolution Determine origin of the muons or! 12

13 Quarkonium production in pp J/ is produced in two steps that can be factorized: Production of the QQ pair perturbative Evolution of QQ pair into a bound quarkonium state non perturbative Different descriptions of this evolution are at the basis of the various theoretical models 1) Color singlet model 2) Color evaporation model 3) NRQCD 13

14 Models for quarkonium production in pp Color Singlet Model Color Evaporation M. NRQCD Proposed soon after the J/ discovery QQ pair is produced in a color singlet state, with the same quantum numbers of the final quarkonium Unable to describe Tevatron data. However, recently NLO and NNLO corrections have been included to improve the agreement QQ pair evolves in quarkonium if m QQ <m D independently of its color and spin Probability to evolve into a certain quarkonium state depends by a constant F which is energy and process independent Works rather well, but no detail on the hadronization of the qq pair towards the bound state Inclusive quarkonium production cross section is a sum of short distance coeff. and long distance matrix elements: ˆ ( ij J / ) n J / n This approach includes CSM and CEM as special cases Charmonium can be produced also through the creation of a cc color octet state n C Q O ij Q 14

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

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

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

18 Nuclear absorption Once the J/ has been produced, it must cross a thickness L of nuclear matter, where it may interact and disappear L If the cross section for nuclear absorption is abs J/, one expects pa ~ pp Ae L abs 18

19 Nuclear effects vs. x F Collection of results from many fixed target experiments Nuclear effects show a strong variation vs the kinematic variables I. Abt et al., arxiv: higher s lower s 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 19

20 Nuclear effects Interpretation of results not easy many competing effects affect J/ production/propagation in nuclei anti-shadowing (with large uncertainties on gluon densities!) final state absorption 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 Clear tendency towards stronger absorption at low s C. Lourenco, R. Vogt and H.Woehri, JHEP 0902:014,2009 F. Arleo and Vi-Nham Tram Eur.Phys.J.C55: ,2008, arxiv:

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 J/ /N coll /nucl. Abs. Measured/Expected J/ /N coll pa 1 Anomalous suppression! AA L It is very important to measure cold nuclear matter effects before any claim of an anomalous suppression in AA collisions L CNM, evaluated in pa, are extrapolated to AA, in order to build a reference for the J/ behaviour in hadronic matter 21

22 J/ in AA SPS CNM, evaluated in pa, are extrapolated to AA, in order to build a reference for the J/ behaviour in hadronic matter 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? 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,

23 PHENIX J/ e + e - y <0.35 & J/ + - y [1.2,2.2] STAR J/ e + e - y <1 RHIC AA collisions Au-Au 200 GeV/nucleon PHENIX, PRL (2007) Nucl.Phys.A 830 (2009) 331 Cu-Cu 200 GeV/nucleon PHENIX, PRL (2008) STAR, Phys. Rev. C (2009) pp, da collisions pp 200 GeV/nucleon PHENIX, PRL 98, (2007) STAR, Phys. Rev. C (2009) dau 200 GeV/nucleon PHENIX, Phys.Rev.C (2008) Nucl.Phys.A 830 (2009) 227 All data have been collected with the same collision energy ( s = 200 GeV) and kinematics 23

24 RHIC pp, dau collisions pp collisions pp results should help to understand the J/ production mechanism provide a reference for AA collisions (R AA ) 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) Forward Mid Backward 24

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

26 Comparison with SPS results Comparison with SPS results Results are shown as a function of the multiplicity of charged particles (~energy density, assuming SPS ~ RHIC ) 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 26

27 Interpretation of the results Some interpretations 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: 1) Only J/ from and c decays are suppressed at SPS and RHIC same suppression is expected at SPS and RHIC reasonable if T diss (J/) ~ 2T c 2) Also direct J/ are suppressed at RHIC but cc multiplicity high SPS RHIC LHC s (GeV) N cc J/ regeneration ( N cc2 ) contributes to the J/ yield The 2 effects may balance: suppression similar to SPS 27

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) Indirect way some distributions should be affected by regeneration Recombination Direct way for quantitative estimate accurate measurement of charm 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 (<p T2 > ) wrt pp Results are not precise enough to assess the amount of regeneration 28

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? 29

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 ( + - ) ALICE ATLAS CMS LHCb 30

31 Quarkonium LHC results in pp Differential distributions New results presented by the 4 experiments Fraction of J/ from B Preliminary theory comparison 31

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

33 First J/ in Pb-Pb collisions! ATLAS: arxiv: R CP = dn central N dn peripheral central coll peripheral N coll We expect few thousands J/ from 2010 statistics A centrality dependent suppression is observed J/ with p T >3GeV/c and <2.5 no correction for feed-downs, J/ from B 33

34 Backup 34

35 Statistical hadronization 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! 35

36 x 2 scaling x 2 y m s e / T Shadowing effects (in the 2 1 approach) and final state absorption 1 x 2 s ~ m scale with x J N J 2 x 2 if parton shadowing and final state absorption were the only relevant mechanisms should not depend on s at constant x 2 36

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

38 High p T J/ in Cu-Cu p T dependence PHENIX (minimum bias) STAR (centrality 0-20% & 0-60%) R CuCu up to p T = 9 GeV/c suppression looks roughly constant up to high p T R CuCu =1.4±0.4±0.2 (p T >5GeV/c) R AA increases from low to high p T NA50: Pb-Pb 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 38