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Searching for the QGP at RHIC

Searching for the QGP at RHIC. Che-Ming Ko Texas A&M University. Signatures of QGP Quark coalescence Baryon/meson ratio Hadron elliptic flows and quark number scaling Effects of resonance decays and hadron wave function Charm flow

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Searching for the QGP at RHIC

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  1. Searching for the QGP at RHIC Che-Ming Ko Texas A&M University • Signatures of QGP • Quark coalescence Baryon/meson ratio Hadron elliptic flows and quark number scaling Effects of resonance decays and hadron wave function Charm flow Higher-order anisotropic flow • Summary

  2. Signatures of quark-gluon plasma • Dilepton enhancement (Shuryak, 1978) • Strangeness enhancement (Meuller & Rafelski, 1982) • J/ψsuppression (Matsui & Satz, 1986) • Pion interferometry (Pratt; Bertsch, 1986) • Elliptic flow (Ollitrault, 1992) • Jet quenching (Gyulassy & Wang, 1992) • Net baryon and charge fluctuations (Jeon & Koch; Asakawa, Heinz & Muller, 2000) • Quark number scaling of hadron elliptic flows (Voloshin 2002) • ……………

  3. MinBias Au-Au thermal - Dilepton spectrum at RHIC • Low mass: thermal dominant • (calculated by Rapp in kinetic model) • Inter. mass: charm decay No signals for thermal dileptons yet

  4. Enhancement of multistrange baryons But enhancement is even larger in HI collisions at lower energies

  5. J/Ψ production at RHIC Au+Au @ 200 GeV Grandchamp, Rapp, Brown, PRL 92, 212301 (04)

  6. Pion interferometry open: without Coulomb solid: with Coulomb STAR Au+Au @ 130 GeV STAR Au+Au @ 130 AGeV Ro/Rs~1 smaller than expected ~1.5

  7. A multiphase transport (AMPT) model Default: Lin, Pal, Zhang, Li & Ko, PRC 61, 067901 (00); 64, 041901 (01); 72, 064901 (05); http://www-cunuke.phys.columbia.edu/OSCAR • Initial conditions: HIJING (soft strings and hard minijets) • Parton evolution: ZPC • Hadronization: Lund string model for default AMPT • Hadronic scattering: ART String melting: PRC 65, 034904 (02); PRL 89, 152301 (02) • Convert hadrons from string fragmentation into quarks and antiquarks • Evolve quarks and antiquarks in ZPC • When partons stop interacting, combine nearest quark and antiquark to meson, and nearest three quarks to baryon (coordinate-space coalescence) • Hadron flavors are determined by quarks’ invariant mass

  8. Zhang’s parton cascade (ZPC) Bin Zhang, Comp. Phys. Comm. 109, 193 (1998) • Using αs=0.5 and screening mass μ=gT≈0.6 GeV at T≈0.25 GeV, • then <s>1/2≈4.2T≈1 GeV, and pQCD gives σ≈2.5 mb and a • transport cross section • σ=6 mb → μ≈0.44 GeV, σt≈2.7 mb • σ=10 mb → μ≈0.35 GeV, σt≈3.6 mb

  9. Two-Pion Correlation Functions and source radii from AMPT Lin, Ko & Pal, PRL 89, 152301 (2002) Au+Au @ 130 AGeV Need string melting and large parton scattering cross section which may be due to quasi bound states in QGP and/or multiparton dynamics (gg↔ggg)

  10. Emission Function from AMPT • Shift in out direction (<xout> > 0) • Strong positive correlation between out position and emission time • Large halo due to resonance (ω) decay and explosion • → non-Gaussian source

  11. ( ) b Á ¢ E r ; ; 1 2 ( = ) = ( = ) E E 1 6 7 5 ¡ + : ¼ ² ¹ ¹ 0 : : ¢ L Z ¡ ¿ ¿ 0 ( ) ~ ~ d b £ + ¿ ½ ¿ r n ¿ g ; ; ¿ ½ 0 0 ¿ 0 High PT hadron suppression Wang & Wang, PRL 87, 142301 (01) Parton energy loss due to radiation and reabsorption ε0=1.07 GeV/fm, μ=1.5 GeV Also Gyulassy, Levai & Titev, PRL 85, 5535 (00) Jet quenching → initial energy density → 5-10 GeV/fm3

  12. Puzzle: Large proton/meson ratio PHENIX, nucl-ex/0304022 PHENIX, nucl-ex/0212014 • Fragmentation leads to p/π ~ 0.2 • Jet quenching affects both • Fragmentation is not the dominant • mechanism of hadronization at • pT < 4-6 GeV π0 suppression: evidence of jet quenching before fragmentation

  13. Surprise: quark number scaling of hadron elliptic flow Except pions, v2,M(pT) ~ 2 v2,q(pT/2) and v2,B(pT) ~ 3 v2,q(pT/3) consistent with hadronization via quark recombination

  14. Unexpected: Appreciable charm flow

  15. Coalescence model in heavy ion collisions • Extensively used for light clusters production. • First used for describing hadronization of QGP by Budapest group • Revived by Oregon: Hwa, Yang (PRC 66 (02) 025205), ……… Duke-Minnesota: Bass, Nonaka, Meuller, Fries (PRL 90 (03) 202303; PRC 68 (03) 044902 ) Ohio and Wayne States: Molnar, Voloshin (PRL 91 (03) 092301; PRC 68 (03) 044901) Texas A&M: Greco, Levai, Rapp, Chen, Ko (PRL (03) 202302; PRC 68 (03) 034904) • Most studies are schematic, based on parameterized QGP parton distributions. • Study based on parton distributions from transport models has been developed by TAMU group (PRL 89 (2002) 152301; PRC 65 (2002) 034904 ) and also pursued by D. Molnar (nucl-th/0406066).

  16. PRL 90, 202102 (2003); PRC 68, 034904 (2003) Coalescence model Number of hadrons with n quarks and/or antiquarks Spin-color statistical factor e.g. Quark distribution function Coalescence probability function For baryons, Jacobi coordinates for three-body system are used.

  17. Thermal QGP Power-law minijets Choose T=170 MeV quenched soft hard L/l=3.5 Parton transverse momentum distributions Consistent with data (PHENIX) P. Levai et al., NPA 698 (02) 631

  18. Other inputs and assumptions • Minijet fragmentation via KKP fragmentation functions (Kniehl, Krammer, Potter, NPB 582, 514 (2000)) • Gluons are converted to quark-antiquark pairs with equal probabilities in all flavors. • Quark-gluon plasma is given a transverse collective flow velocity of β=0.5 c, so partons have an additional velocity v(r)=β(r/R). • Minijet partons have current quark masses mu,d=10 MeV and • ms=175 MeV, while QGP partons have constituent quark masses • mu,d=300 MeV, ms=475 MeV (Non-perturbative effects, Levai & • Heinz, PRC 57, 1879 (1998)) • Use coalescence radii Δp=0.24 GeV for mesons and 0.36 GeV for baryons

  19. Pion and proton spectra Au+Au @ 200AGeV (central) • Similar results from other groups • Oregon: parton distributions extracted from pion spectrum • Duke group: no resonances and s+h but uses harder parton spectrum

  20. Baryon/Meson ratio TAMU r-> pp OREGON DUKE

  21. Elliptic flow Quark v2 extracted from pion and kaon v2 using coalescence model

  22. Momentum-space quark coalescence model Only quarks of same momentum can coalescence, i.e., Δp=0 Quark transverse momentum distribution Meson elliptic flow Quark number scaling of hadron v2 (except pions): Baryon elliptic flow same for mesons and baryons

  23. Effects of hadron wave function and resonance decays Effect of resonance decays Wave function effect Higher fock states → similar effect (Duke) Wave fun.+ res. decays

  24. Charm spectra D meson J/ψ Charm quark D e- T=0.72 GeV T=0.35-0.50GeV Au+Au @ 200 A GeV Bands correspond to flow velocities between 0.5 and 0.65 NJ/ψ =2.7 . 10-3 NJ/ψ =0.9 . 10-3

  25. S. Kelly,QM04 Charmed meson elliptic flow Smaller charm v2 than light quark v2 at low pT due to mass effect v2 of electrons Data consistent with thermalized charm quark with same v2 as light quarks Greco, Rapp, Ko, PLB595 (04) 202

  26. Charm quark elliptic flow from AMPT • PT dependence of charm quark v2 is different from that of light quarks. • At high pT, charm quark has similar v2 as light quarks. • Charm elliptic flow is also sensitive to parton cross sections

  27. Charm elliptic flow from AMPT Zhang, Chen & Ko, PRC 72, 024906 (05) 15 10 5 0 -5 Current light quark masses are used in AMPT. Charmed meson elliptic flow will be larger if constituent quark masses are used.

  28. Higher-order parton anisotropic flows Kolb, Chen, Greco, Ko, PRC 69 (2004) 051901 Including 4th order quark flow Meson elliptic flow Baryon elliptic flow

  29. Higher-order anisotropic flows Data can be described by a multiphase transport (AMPT) model with large parton cross sections. Parton cascade gives v4,q~v2,q2 Data

  30. Summary on quark coalescence • Quark coalescence can explain observed Large baryon/meson ratio at pT~ 3GeV Quark number scaling of hadron v2 → signature of deconfinement? • Coalescence of minijet partons with thermal partons is significant → medium modification of minijet fragmentation. • Scaling violation of pion v2 can be explained by resonance decays. • Coalescence of thermalized charm quarks can explain preliminary charmed meson spectrum and v2 as well as J/ψ yield. • Required quark v2 is consistent with that from parton cascade. • Appreciable parton v4 is seen in parton cascade. • Entropy violation (~16%) is not as large as one naively thinks and is related to corresponding energy violation.

  31. Summary on QGP search • Most proposed QGP signatures are observed at RHIC. • Strangeness production is enhanced and is consistent with formation of hadronic matter at Tc. • Large elliptic flow requires large parton cross sections in transport model or earlier equilibration in hydrodynamic model. • HBT correlation is consistent with formation of strongly interacting partonic matter. • Jet quenching due to radiation requires initial matter with energy density order of magnitude higher than that of QCD at Tc. • Quark number scaling of elliptic flow of identified hadrons is consistent with hadronization via quark coalescence or recombination. • Studies are needed for electromagnetic probes and heavy flavor hadrons. • Theoretical models have played and will continue to play essential roles in understanding RHIC physics.

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