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Making a Little Bang with heavy ion collisions

This talk discusses the goals and structure of nuclear matter, as well as the Relativistic Heavy Ion Collider and its experimental observables. It also explores the possibility of a phase transition and the properties of the quark gluon plasma.

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Making a Little Bang with heavy ion collisions

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  1. Making a Little Bang with heavy ion collisions Barbara V. Jacak Stony Brook May 8, 2002

  2. outline • Science questions which define our goals • Structure of nuclear matter and theoretical tools • Super-dense matter in the laboratory • the Relativistic Heavy Ion Collider • experimental observables & • what have we learned already? • Next steps…

  3. Goal of experiments at RHIC • Collide Au + Au ions at high energy • 130 GeV/nucleon c.m. energy in 2000 • s = 200 GeV/nucleon this year • Achieve highest possible temperature and density • as existed ~1 msec after the big bang • inter-hadron distances comparable to that in neutron stars • Study the hot, dense matter • do the nuclei dissolve into a quark gluon plasma? • what are the transport properties?

  4. Phase Transition • we don’t really understand • how process of quark confinement works • how symmetries are broken by nature  massive particles from ~ massless quarks • transition affects evolution of early universe • latent heat & surface tension  • matter inhomogeneity in evolving universe? • why more matter than antimatter today? • equation of state of nuclear matter compression in stellar explosions

  5. Early Universe plasma of free quarks & gluons T (MeV) RHIC 200 SPS AGS baryons <qqq> mesons <q q> Color superconductor? hadrons m (MeV) Baryon density Phase diagram of hadronic matter

  6. + +… Quantum ChromoDynamics • Field theory • for strong • interactions • among • colored quarks - by exchange of gluons • Parallels Quantum Electrodynamics (QED) • in electromagnetic interactions • the exchanged particles are photons • electrically uncharged • QCD: exchanged gluons have “color charge” •  a curious property: they interact among themselves (i.e. theory is non-abelian) This makes interactions difficult to calculate!

  7. Transition temperature? Lattice QCDpredicts a phase transition: Karsch, Laermann, Peikert ‘99 e/T4 T/Tc Tc ~ 170 ± 10 MeV (1012 °K) e ~ 3 GeV/fm3

  8. Collide two nuclei Look at region between the two nuclei for T/density maximum Sort collisions by impact parameter head-on = “central” collisions RHIC is first dedicated heavy ion collider 10 times the energy previously available!

  9. RHIC at Brookhaven National Laboratory Relativistic Heavy Ion Collider started operations in summer 2000

  10. STAR 4 complementary experiments

  11. In Heavy Ion Collisions When nuclei collide at near the speed of light, have a cascade of quark & gluon scatterings 104 gluons, q, q’s

  12. Questions to address in experiments • 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?

  13. 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

  14. 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

  15. schematic view of jet production hadrons leading particle q q hadrons leading particle Observables IIDensity - use a unique probe 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”

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

  17. look at inclusive pt distribution • p-p data available over wide range of s, but not for 130 GeV power law: pp = d2N/dpt2 = A (p0+pt)-n interpolate A, p0, n to 130 GeV

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

  19. An experimental artifact? • NO! • pt data from PHENIX + STAR agree well:

  20. Deficit is indeed observed in central Au + Au collisions Phys. Rev. Lett. 88, 022301 (2002) charged p0 transverse momentum (GeV/c) Charged deficit seen by both STAR & PHENIX STAR preliminary

  21. u, d, s u, d, s c c Observables IIIConfinement • J/Y (cc bound state) • produced early, traverses the medium • if medium is deconfined (i.e. colored) • Debye screening by colored medium • J/Y screened by quark gluon plasma • binding dissolves  2 D mesons

  22. 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...

  23. How about 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” uses optical “boom” when vpart. > cmedium

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

  25. Observables IV: Propertieselliptic flow “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

  26. 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

  27. charged hadron spectra mT2 = pT2 + m02 Protons are flatter  velocity boost

  28. hydrodynamical calculation agrees with data Teaney, Lauret, Shuryak nucl-th/01100037 Many high pt baryons! nucl-ex/0203015 Explains difference between h++h- and p0 not the expected jet fragmentation function D(z)!

  29. 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 Energy loss at RHIC? scaled pp shadowing + initial mult. scattering energy loss <dE/dx> = 0.25 GeV/fm

  30. q e-, m- g * Thermal dilepton radiation e+, m+ q q Thermal photon radiation g q, g Observables VTemperature Look for “thermal” radiation processes producing thermal radiation: Rate, energy of the radiated particles determined by maximum T (Tinitial) NB: g, e, m interact only electromagnetically  they exit the collision without further interaction

  31. At RHIC we don’t know yet But it should be higher since the energy density is larger At CERN, photon and lepton spectra consistent with T ~ 200 MeV Initial temperature achieved? NA50 WA98 photon pT m+ m- pair mass

  32. _ ¯ early universe 250 RHIC 200  s quark-gluon plasma 150 SPS Lattice QCD AGS deconfinement chiral restauration Conditions when hadrons freeze out - by fitting yields vs. mass (grand canonical ensemble) Tch = 175 MeV mB = 51 MeV 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 Collisions at RHIC approach zero net baryon density

  33. What have we learned so far? • unprecedented energy density at RHIC! • e > ecrit • freeze-out near the phase transition • high density, probably high temperature • very explosive collisions  matter has a stiff equation of state • new features: hints of quark gluon plasma? • elliptic flow  early thermalization, high p • suppression of high pT particles • J/Y suppression at CERN? • Not yet at appropriate standard of proof (but I think we see QGP at RHIC)

  34. 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???

  35. Mysteries... How come hydrodynamics does so well on elliptic flow and momentum spectra of mesons & nucleons emitted D. Teaney & J. Burward-Hoy … but FAILS to explain correlations between meson PAIRS? pT (GeV) Hydrodynamics is not explosive enough: non-uniform particle density distribution!

  36. Mysteries II  If jets from light quarks are quenched, shouldn’t charmed quarks be suppressed too? nucl-ex/0202002

  37. 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

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

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

  40. vacuum matter box QGP Did something new happen? • Study collision dynamics • Probe the early (hot) phase Do the particles equilibrate? Collective behavior i.e. pressure and expansion? Particles created early in predictable quantity interact differently with QGP and normal matter fast quarks, bound cc pairs, s quarks, ... + thermal radiation!

  41. 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.

  42. Quark confinement • Hadron properties governed by QCD • force between quarks: exchange of colored gluons • 2 fundamental puzzles of QCD •  confinement of quarks and gluons •  broken chiral symmetry • (gives hadrons mass) QCD is non-abelian: gluons can interact with gluons at short distance: force is weak (probe w/ high Q2, perturbative) at large distance: force is strong (probe w/ low Q2, non-perturbative)

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