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Hard Probes

Hard Probes

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Hard Probes

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  1. Hard Probes Roberta Arnaldi INFN Torino 5th International School on QGP and Heavy Ions Collisions: past, present and future Torino, 5-12 March 2011

  2. Outlook: Hard probes: definitions High pT hadrons 3) Heavy Flavours 4) Quarkonia Theoretical expectations SPS results RHIC results 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 SPS RHIC LHC 17 GeV/c 200 GeV/c 2.76 TeV/c √s 1986 years 1990 ~2000 2010 …but, as for the other hard probes, in order to understand quarkoniumbehaviour in the hot matter (AA collisions), its interactions with the cold nuclear matter should be under control (pA/dAu collisions)

  4. What is Quarkonium? Quarkonium is a bound state of and q with q According to the quantum numbers, several quarkonium states exists Bottomonium () family Charmonium () family

  5. Quarkonium At T=0, the binding of the and quarks can be expressed using the Cornell potential: q q Coulombian contribution, induced by a g exchange between and Confinement term What happens to a pair placed in the QGP? The QGP consists of deconfinedcolourcharges  the binding of a pair is subject to the effects of colour screening q q • The “confinement” contribution disappears • The high color density induces a screening of the coulombian term of the potential

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

  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

  8. Sequential screening The quarkoniumstates can be characterized by • the binding energy • radius More bound states have 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>>Tc T~1.1Tc Tc Tc Tc Tc T<Tc thermometer for the temperature reached in the HI collisions Sequential suppression of the resonances

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

  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: 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 Displaced Feed down and J/ from B, if not properly taken into account, may affect physics conclusions

  11. target beam hadron absorber Muon Other Standard way of measuring  pairs Approach adopted by NA50, PHENIX and ALICE (forward region) muon trigger and tracking Iron wall magnetic field 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) Correct for multiple scattering and energy loss • Extrapolate muon tracks back to the target •  Vertex reconstruction is usually rather poor (z~10 cm)

  12. muon trigger and tracking Iron wall magnetic field hadron absorber or ! Muon Other 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 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. Improve mass resolution Determine origin of the muons

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

  14. Models for quarkonium production in pp Color Singlet Model Color Evaporation M. NRQCD Proposed soon after the J/ discovery 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 pair evolves in quarkonium if <mD 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: This approach includes CSM and CEM as special cases Charmonium can be produced also through the creation of a color octet state

  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 factor 50! …but situation still puzzling, because polarization is not described! Recently many step forwards (i.e. NLO and NNLO corrections…) Open questions, to be investigated at LHC!

  16. μ J/ μ p 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: Final state: cc dissociation in the medium, final energy loss Initial state: shadowing, parton energy loss, intrinsic charm provide a reference for the study of charmonia dissociation in a hot medium  approach followed at SPS and similarly at RHIC (with dAu data)

  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: • = 1  no nuclear effects • <1  nuclear effects The larger abs, the more important are the nuclear effects NA50, pA 450 GeV Effective quantities which include al initial and final state effects

  18. 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

  19. Nuclear effects vs. xF Collection of results from many fixed target pA experiments Nuclear effects show a strong variation vs the kinematic variables I. Abt et al., arXiv:0812.0734 higher s lower s Because of the  dependence onxF and energy the reference for the AA suppression must be obtained under the same kinematic/energy domain as the AA data

  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 betaken 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.C55:449-461,2008, arXiv:0907.0043

  21. AA 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 Measured/Expected 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 CNM, evaluated in pA, are extrapolated to AA, in order to build a reference for the J/behaviour in hadronic matter

  22. After correction for EKS98 shadowing B. Alessandro et al., EPJC39 (2005) 335 R. Arnaldi et al., Nucl. Phys. A (2009) 345 J/ in AA collisions @ 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? R.A., P. Cortese, E. ScomparinPhys. Rev. C 81, 014903

  23. J/ @ RHIC PHENIXJ/e+e-|y|<0.35 & J/+- |y| [1.2,2.2] STAR J/e+e-|y|<1 AA collisions Au-Au 200 GeV/nucleon Cu-Cu 200 GeV/nucleon 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

  24. Backward Forward Mid J/ @ RHIC – pp, dAu collisions ppcollisions pp results should help to • understand the J/ production mechanism • provide a reference for AA collisions (RAA) dAucollisions In a similar way as at SPS, CNM effects are obtained from dAu data RHIC data exploit different x2 regions corresponding to  shadowing (forward and midrapidity)  anti-shadowing (backward rapidity)

  25. J/ @ 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?

  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 NPart~200 • For central collisions more important suppression in Au-Au with respect to Pb-Pb

  27. Interpretation of the results Some interpretations Several theoretical models have been proposed in the past, starting from the following observations • RAA 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 Tdiss (J/) ~ 2Tc • 2) Also direct J/ are suppressed at RHIC but cc multiplicity high J/ regeneration ( Ncc2) contributes to the J/ yield The 2 effects may balance: suppression similar to SPS

  28. Recombination Recombination Models including J/ regeneration qualitatively describe the RAA data (X. Zhao, R. Rapp arXiv:0810.4566, Z.Qu et al. Nucl. Phys. A 830 (2009) 335) Direct way for quantitative estimate  accurate measurement of charm  Indirect way some distributions should be affected by regeneration J/ elliptic flow  J/ should inherit the positive heavy quark flow J/ y distribution  should be narrower wrtpp J/ pT distribution  should be softer (<pT2>) wrtpp Results are not precise enough to assess the amount of regeneration

  29. Quarkonium @ 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. ALICE ATLAS LHCb CMS Quarkonium @ 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 (+-)

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

  32.  results @ LHC in pp  hardly seen at RHIC, while now at LHC the  family is fully accessible arXiv:1012.5545 Extremely important measurement:  More robust theory calculation (due to heavy bottom quark and absence of b-hadron feed-down)

  33. First J/ in Pb-Pb collisions! ATLAS: arXiv:10125419 = A centrality dependent suppression is observed We expect few thousands J/ from 2010 statistics J/ with pT>3GeV/c and ||<2.5 no correction for feed-downs, J/ from B

  34. Backup

  35. y 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 A. Andronic et al. arXiv:0805.4781 Good agreement between data and model Recombination should be tested on LHC data!

  36. x2 scaling • Shadowing effects (in the 21 approach) and final state absorption • scale with x2 if parton shadowing and final state absorption were the only relevant mechanisms  should not depend on √s at constant x2

  37. J/ @ 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 Npart range

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