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QGP Formation Signals and Quark Recombination Model

QGP Formation Signals and Quark Recombination Model. Chunbin Yang Central China Normal University Wuhan. Outline. Heavy ion collisions and QGP formation Anomalies at RHIC Physics ideas in the recombination model Fragmentation in the recombination model Applications to Au+Au collisions

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QGP Formation Signals and Quark Recombination Model

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  1. QGP Formation Signals and Quark Recombination Model Chunbin Yang Central China Normal University Wuhan

  2. Outline • Heavy ion collisions and QGP formation • Anomalies at RHIC • Physics ideas in the recombination model • Fragmentation in the recombination model • Applications to Au+Au collisions • NCQ scaling of flow v2 • Violation of the scaling • Particle species dependence of Cronin effect • Discussions

  3. time Hot and Dense Cooling down freezing out Initial conditions and interactions

  4. QGP formation signals • Strangeness enhancement • Suppression of J/Ψ • Dilepton enhancement • Direct photon • … Parton degree of QGP? QGP signal from the bulk? Experimental probes: 1) Penetrating probes:“jets” energy loss 2) Bulk probes :Elliptic flow, radial flow …

  5. Evidence for the formation of QGP Dihadron Single hadron Jet quenching Energy loss ofjets in medium No suppression for p spectrum

  6. HOW? Hadron production mechanisms • Partons are produced in high energy collisions like e++e-, e+p, p+p, p+A,A+A • Partons in the final stage of evolution are converted into hadrons

  7. Traditional models • String formation and break for low p T • Fragmentation for high p T The string model may not be applicable to heavy ion collisions Fragmentation failed for central Au+Au collisions

  8. Anomalies at intermediate pT • B/M • v2(pT) • Jet structure • Cronin effect p/≈1 v2(baryons) > v2(mesons) not the same as in pp • RCPp > RCP Hard to be understood in traditional models

  9. Hadronization by recombination • The colliding system generates quarks and gluons in the phase space • The quarks get dressed • The dressed quarks recombine into hadrons to the detector

  10. Why Recombination? meson momentum p p q p1+p2 Parton distribution (log scale) (recombine) (fragment) higher yield heavy penalty

  11. Features • quark momenta add, higher yield for high produced pT hadrons • soft parton density depends on medium • more quarks for baryons than for mesons • enhanced dependence on centrality for baryons when thermal partons are involved

  12. No anomalies in recombination • At intermediate pT, aplenty soft quarks are more important for proton production than for pionsp/1 • For baryons, three quarks contribute to the flow, while only two quarks for mesons  v2(baryons) > v2(mesons), quark number scaling • Soft and semi-soft recombination  Cronin effect • Process dependence of soft partons different jet structure in dA and AA

  13. Recombination models • Use just the lowest Fock state i.e. valence quarks • qqqB q qbarM • Gluons converted to quarks first • The probability for two (three) quarks to form a meson (baryon) is given by a process independent recombination function R

  14. Different implementations • Duke group etc: • 6-dimensional phase space • using Wigner function from density matrix • Oregon group: • one-dimensional momentum space • using phenomenological recombination function

  15. Duke approach • Low pT recombination • high pT fragmentation

  16. Texas/Ohio approach Texas A&M/Budapest (Ko, Greco, Levai, Chen) • Monte Carlo implementation (with spatial overlap) • Soft and hard partons • Soft-hard coalescence allowed Ohio State (Lin, Molnar) • ReCo as a solution to the opacity puzzle

  17. Basic formulas in Oregon approach

  18. Recombination functions Given by the valon distribution of the hadrons

  19. Determining R • R p was determined from CTEQ • From the parton distributions in proton • a=b=1.755, c=1.05 at Q2=1GeV2 • R  was determined from Drell-Yan processes • a=b=0 • See Phys. Rev. C 66, 025204

  20. Fragmentation? Recombination? Answer:NO FRAGMENTATION only RECOMBINATION • Fragmentation is not a description of the hadronization process. It uses phenomenological functions D(z) that give the probability of momentum fraction z of a hadron in a parton jet

  21. Fragmentation D(z) q A A

  22. fragmentation h q recombination Initiating parton (hard) Parton shower (semi-hard) Parton shower

  23. Recombination for fragmentation Recombination function known in the recombination model Fragmentation function known from fitting e+e- annihilation data S  V  G  S K G K Hwa, Phys. Rev. D (1980). Shower parton distributions K, L, G, Ls, Gs BKK KKP etc

  24. Fitted results

  25. Shower parton distributions

  26. Application to Au+Au collisions • Thermalized low pT (soft) partons • Hard partons (semi-hard) shower partons • Three types of recombination for mesons • thermal parton & thermal parton • thermal parton & shower parton • shower parton & shower parton • Joint parton distribution is not factorizable

  27. Parton sources Thermal parton distribution is assumed Hard parton distributions fi(k) can be calculated from • pQCD • nuclear shadowing • nuclear geometry

  28. Parton sources Single shower parton distribution is Joint two (three) shower parton distribution can also be written down

  29.  Spectrum (0-10%)

  30. Nuclear modification RAA

  31. p spectrum

  32. p/

  33. Centrality dependence

  34. New physics • Thermal-thermal recombination makes p/ increase from very small value to about 1 at pT3GeV/c • Thermal-shower recombination plays an important role • This recombination can be equivalently regarded as modification of the fragmentation functions

  35. NCQ scaling • AMPT model results: • Scaling in v2: partonic dof dominant; • No scaling in v2 : hadronic dof dominant • => • A tool to search for the possible phase boundary! • The beam energy • dependence of the partonic cross sections will not affect the v2 scaling argument. • => • Important for Beam Energy Scan program.

  36. NCQ scaling violation

  37. Why NCQ scaling ? Assumptions: F(p1,p2)=F(p1)F(p2) collinear φdependence joint distribution Validity of the assumptions?

  38. Why NCQ scaling violates? • Because of quark interactions, joint distributions are not products of quark distributions • Recombined quarks not necessarily have the same momentum • Fluctuations: large n=1,3 terms appears in quarks distributions. They contribute to v2 • NCQ at RHIC may be coincident

  39. Application to d+Au collisions • Basic formulas the same as for Au+Au collisions • Soft parton distribution the same form, T not temperature but inverse slope • No jet quenching • Nuclear shadowing a little different from that in Au+Au case

  40. Pion spectrum

  41. Centrality dependence

  42. Cronin effect Enhancement of hadron spectrum in pA collisions at high pT Traditional explanation: initial interactions Many soft collisions before the last hard one, each gives a kT kick

  43. Cronin effect Shadowing effect is cancelled partially

  44. Puzzles • If Cronin effect is really due to initial interactions, dilepton spectrum should show similar effect. • Experimentally, the effect for dilepton is very small, no definite conclusion • Species dependence of the Cronin effect

  45. From recombination Medium density depends on centrality Medium effects are different in mesonand baryon production

  46. Proton spectrum T different for different centralities

  47. RCP for proton

  48. RCP for p & 

  49. Discussions • QGP signal can be found from the bulk • Hadronization of partons can be described by ReCo for d+Au and Au+Au collisions • ReCo naturally explains species dependence, such as baryon enhancement, v2 scaling... • Cronin effect can be interpreted as from final state interactions

  50. Discussions • Combination with other models, such as hydrodynamics etc, is needed and under development • Recombination formulism from pQCD How to calculate the joint distributions?

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