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Exploring superdense matter at RHIC. Barbara V. Jacak Stony Brook June 12, 2002. Goals of experiments at RHIC. Collide Au + Au ions at high energy 130 GeV/nucleon c.m. energy in 2000  s = 200 GeV/nucleon in 2001 Achieve highest possible temperature and density

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Exploring superdense matter at RHIC


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    1. Exploring superdense matter at RHIC Barbara V. Jacak Stony Brook June 12, 2002

    2. Goals of experiments at RHIC • Collide Au + Au ions at high energy • 130 GeV/nucleon c.m. energy in 2000 • s = 200 GeV/nucleon in 2001 • Achieve highest possible temperature and density • as existed ~1 msec after the Big Bang • inter-hadron distances comparable to that in neutron stars • heavy ions to achieve maximum volume • Study the hot, dense matter • do the nuclei dissolve into a quark gluon plasma? • do partons/hadrons thermalize? • characteristics of the phase transition? • transport properties of the quark gluon plasma? equation of state?

    3. Use RHIC to study QCD • Hadron properties governed by QCD • force between quarks: exchange of colored gluons • How does confinement work? • What are the properties of deconfined matter? QCD is non-abelian: gluons can interact with gluons calculations challenging at short distance: force is weak (probe w/ high Q2, perturbative) at large distance: force is strong (probe w/ low Q2, non-perturbative)

    4. Deconfinement temperature, energy density? QCD on the lattice predicts: Karsch, Laermann, Peikert ‘99 e/T4 T/Tc Tc ~ 170 ± 10 MeV (1012 °K) e ~ 3 GeV/fm3

    5. Evolution of a heavy ion collision 104 gluons, q, q’s Initial collision probability given by nuclear structure functions followed by parton cascade

    6. vacuum QGP Experiments ask:did something new happen? • Collision dynamics (via hadronic final state) • Probe the early (hot) phase Equilibrium? hadron spectra, yields Collective behavior i.e. pressure and expansion? elliptic, radial flow matter box Particles created early in predictable quantity interact differently with QGP and normal matter fast quarks, J/Y, strange quark content, thermal radiation

    7. RHIC at Brookhaven National Laboratory RHIC is first dedicated heavy ion collider 10 times the energy previously available!

    8. STAR 4 complementary experiments

    9. Address via experiment: • Temperature • early in the collision during plasma phase • Density • also early in the collision, at maximum • Are the quarks confined or in a plasma? • Use probes of the medium to investigate • Properties of the quark gluon plasma: • equation of state (energy vs. pressure) • how is energy transported in the plasma?

    10. Density: a first look Central Au+Au collisions (~ longitudinal velocity) summing particles under the curve, find ~ 5000 charged particles in collision final state initial volume ~ Vnucleus

    11. pR2 2ct0 Is energy density high enough? PRL87, 052301 (2001) Colliding system expands: Energy  to beam direction per unit velocity || to beam e 4.6 GeV/fm3 YES - well above predicted transition! 50% higher than seen before

    12. elliptic flow as “barometer” Origin: spatial anisotropy of the system when created followed by multiple scattering of particles in evolving system spatial anisotropy  momentum anisotropy v2: 2nd harmonic Fourier coefficient in azimuthal distribution of particles with respect to the reaction plane Almond shape overlap region in coordinate space

    13. Large v2: the matter can be modeled by hydrodynamics v2= 6%: larger than at CERN or AGS! Hydro. Calculations Huovinen, P. Kolb and U. Heinz STAR PRL 86 (2001) 402 pressure buildup  explosion pressure generated early!  early equilibration ! first hydrodynamic behavior seen

    14. charged hadron spectra mT2 = pT2 + m02 mT - m0 = transverse kinetic energy Protons are flatter  velocity boost

    15. hydrodynamical calculation agrees with data Teaney, Lauret, Shuryak nucl-th/0110037 Many high pt baryons! nucl-ex/0203015 As many baryons as pions at pT> 2 GeV/c

    16. B --- B _ ¯  s Conditions in hadronic phase at RHIC Collisions at RHIC approach zero net baryon density Braun-Munzinger, Magestro, Redlich, Stachel, hep-ph/0105229 Tch = 175 MeV mB = 51 MeV Analyze with Grand Canonical Ensemble: fit particle ratios for mB, T

    17. early universe 250 RHIC 200 quark-gluon plasma 150 SPS Lattice QCD AGS deconfinement chiral restauration thermal freeze-out 100 SIS hadron gas 50 neutron stars atomic nuclei 0 0 200 400 600 800 1000 1200 Baryonic Potential B [MeV] Locate RHIC on phase diagram At the time of chemical equilibrium among hadrons

    18. Mystery #1 How come hydrodynamics does so well on elliptic flow and momentum spectra of mesons & nucleons emitted … but FAILS to explain correlations between meson PAIRS? pT (GeV) Possible explanations: non-uniform particle density distribution! (i.e. Hydrodynamics is not explosive enough middle not depopulated) Shape of correlation function different at RHIC

    19. schematic view of jet production hadrons leading particle q q hadrons leading particle Hard scattered partons as probeof early collision stage Probe: Jets from hard scattered quarks Observed via fast leading particles or azimuthal correlations between the leading particles But, before they create jets, the scattered quarks radiate energy (~ GeV/fm) in the colored medium  decreases their momentum  fewer high momentum particles  beam  “jet quenching”

    20. hadron pT spectra PHENIX data STAR data Should be dominated by leading hadrons from jets Baseline: inclusive pt distribution in p+p collision Fit power law: pp = d2N/dpt2 = A (p0+pt)-n

    21. Both h & p0 below p+p PRL 88, 022301 (2002) Peripheral (60-80% of sgeom): <N binary collisions> = 20  6 central (0-10%): <N bin coll> = 905  96

    22. Jet quenching in central Au + Au collisions? Phys. Rev. Lett. 88, 022301 (2002) charged p0 lower as h ½ baryons transverse momentum (GeV/c) Charged deficit seen by both STAR & PHENIX STAR preliminary

    23. A closer look at high pT PHENIX preliminary Yield scales with Nbin.coll? NO Yield scales with Npart? high pT : should be from hard processes, but see scaling with # of binary NN collisions decrease with increasing collision centrality (quenching effect!?)

    24. Can we confirm jets? STAR preliminary Correlation of 4 GeV/c trigger hadron With particle of pT > 2 GeV/c (v2 effect removed) s = 0.27  0.9 rad (as for jets in pp)

    25. but we know system is not static! With expansion: <dE/dx> 7.3 GeV for 10 GeV/c jets X.N. Wang & E. Wang, hep-ph/0202105 How much energy loss at RHIC? scaled pp shadowing + initial mult. scattering energy loss <dE/dx> = 0.25 GeV/fm

    26. EM probes at RHIC • PHENIX looks for J/Y  e+e- and m+m- A needle in a haystack must find electronwithout mistaking a pion for an electron at the level of one in 10,000 There is the electron. Ring Imaging Cherenkov counter to tag the electrons “RICH” See cherenkov light in CO2 vpart. > cmedium

    27. g conversion Electron enriched sample (using RICH) We do find the electrons All tracks p=0.8-0.9 GeV Energy/Momentum PHENIX sees some “extra” electrons they come from charm quarks c  D meson  e + K + n J/Y analysis is underway now

    28. Mystery #2  If jets from light quarks are quenched, shouldn’t charmed quarks be suppressed too? nucl-ex/0202002 Theorists: yes (some), no (others) Enhancement balanced by e loss?

    29. Conclusions • Unprecedented energy density! e > ecrit • Early thermalization • very explosive collisions  matter at early time has a stiff equation of state • hydrodynamics works (mostly) • Chemical equilibration with Tch ~ Tc • Probe early phase with hard partons • see a deficit  energy loss! • Some mysteries • Hydro misses 2 particle correlations • No energy loss by c,cbar quarks • J/Y to come (from higher L data) • QGP? Most likely… pA reference needed

    30. In nucleus rest frame r/ ggg Gluon saturation at RHIC? Venugopalan, McLerran, Kharzeev, etc. Wavefunction of low x partons overlap and the self-coupling gluons fuse, thus saturating the density of gluons in the initial state  treat as classical field! 1 J.P Blaizot, A.H. Mueller, Nucl. Phys. B289, 847 (1987). The saturation scale: pT2 ~ sNc 1/p A2/3 dNg/dy (a G(x,pT2))  (A, b dependent) mT scaling of hadrons & suppressed gluon jet production expect saturation effects at higher x than at HERA effect present in initial state at RHIC?

    31. What’s next? • To rule out conventional explanations • extend reach of Au+Au data • measure p+p reference • p+Au to check effect of cold nuclei on observables • study volume & energy dependence • are jets quenched & J/Y suppressed???

    32. dE/dx s (dE/dx) = .08 protons kaons pions e STAR Identify hadrons Measure momentum & flight time; calculate particle mass also or measure momentum + energy loss in gas detector

    33. PHENIX measures p0 in PbSc and PbGl calorimeters 0’s pT >2 GeV, asym<0.8 in PbSc PRL 88, 022301 (2002) excellent agreement!

    34. J/Y suppression observed at CERN NA50 J/Y yield Fewer J/Y in Pb+Pb than expected! But other processes affect J/Y too so interpretation is still debated...

    35. nucleons Something new at RHIC? • Compare to a baseline, or control • use nucleon-nucleon collisions at the • same energy • To zero’th order Au + Au collisions • a superposition • of N-N reactions • (modulo effect of • nuclear binding and • collective excitations) • Hard scattering processes scale as • number of N-N binary collisions <Nbinary> • so expect: YieldA-A = YieldN-N. <Nbinary>

    36. PHENIX at RHIC 2 Central spectrometers 2 Forward spectrometers 3 Global detectors Philosophy: optimize for signals / sample soft physics

    37. Thermal Properties measuring the thermal history g, g* e+e-, m+m- p, K, p, n, f, L, D, X, W, d, Real and virtual photons from quark scattering is most sensitive to the early stages. (Run II measurement) Hadrons reflect thermal properties when inelastic collisions stop (chemical freeze-out). Hydrodynamic flow is sensitive to the entire thermal history, in particular the early high pressure stages.

    38. Known effects X.N.Wang, nucl-th/0104031 pA and AA data at lower energy show excess above unity: “Cronin effect” (multiple scattering)

    39. In Pb + Pb at CERN From compilation of X.N. Wang RAA(pT) Crossing at ~ 1.5 GeV/c parton energy loss, if any, is overwhelmed by initial state soft multiple scattering!

    40. Is SPS-RHIC comparison fair? • Same pt implies different x! RHIC xT = if pT(had) / pT(jet) ~ 1 then xT ~ x(parton) at y=0

    41. Nuclear shadowing at RHIC? Zheng Huang, Hung Jung Lu, Ina Sarcevic: Nucl.Phys.A637:79-106,1998 (hep-ph/9705250 ) quark structure function Shadowing of structure functions small in RHIC x range!! Gluon shadowing should be even less pt comparison OK deficit  shadowing!

    42. Effect of flow + quenching? • hydro boosts baryons to higher pT • Jet quenching should reduce p yield (by ~3-5) • baryons less depleted as less likely to be leading particles in fragmenting jet Vitev & Gyulassy Phys. Rev. C65 (2002) 041902 pbar/ pi-

    43. Correlation method on HIJING picks out back-to-back particles from jets J. Rak For data correlation & reaction plane methods agree Hydrodynamics no longer dominates Correlations at high pT jet correlations weak or missing! Reaction plane results a mystery...