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The densest stuff on earth: what we learn from RHIC

The densest stuff on earth: what we learn from RHIC. Barbara V. Jacak Stony Brook September 4, 2002. outline. Why collide heavy ions? the QCD phase transition Creating and studying super-dense matter in the laboratory the Relativistic Heavy Ion Collider experimental observables &

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The densest stuff on earth: what we learn from RHIC

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  1. The densest stuff on earth:what we learn from RHIC Barbara V. Jacak Stony Brook September 4, 2002

  2. outline • Why collide heavy ions? • the QCD phase transition • Creating and studying super-dense matter in the laboratory • the Relativistic Heavy Ion Collider • experimental observables & • what have we learned so far? • Conclusions • Next steps…

  3. Goals of 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? • thermalization? • characteristics of the phase transition? • transport properties of the quark gluon plasma? equation of state?

  4. QCD 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? • 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 normal nuclei

  6. + +… Quantum ChromoDynamics • Field theory of the strong interaction: • colored quarks exchange gluons • Parallels QED but gluons have color charge • unlike E&M where g are uncharged •  they interact among themselves (i.e. theory is non-abelian): curious properties at short distance: force is weak (probe w/ high Q2, Calculate with perturbation theory) at large distance: force is strong (probe w/ low Q2, calculations must be non-perturbative)

  7. Phase transition temperature From lattice QCD Karsch, Laermann, Peikert ‘99 e/T4 T/Tc Tc ~ 170 ± 10 MeV (1012 °K) e ~ 3 GeV/fm3

  8. Experimental approach Look at region between the two nuclei for T/density maximum Sort collisions by impact parameter head-on = “central” collisions

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

  10. STAR 4 complementary experiments

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

  12. 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?

  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 (130 GeV) 5.5 GeV/fm3 (200 GeV) • YES - well above predicted transition!

  14. Density: a first look Central Au+Au collisions (~ longitudinal velocity) summing particles under the curve, find ~ 5000 charged particles in collision final state (6200 in 200 GeV/A central Au+Au) 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 • 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>

  17. Baseline: p+p collisions Agrees with pQCD predictions (next to leading order)

  18. Is Au+Au different? PHENIX Preliminary Yes!!

  19. Same ratio for charged particles Suppression stronger in central collisions and higher pT

  20. So, is there jet quenching? • Suppression to 9 GeV/c! (in 3 independent measurements) • Difference in charged hadron ratio and neutral pion ratio accounted for by particle composition at high momentum agrees with theory when quark/gluon energy loss is included

  21. Observables III: a barometer called “elliptic flow” 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

  22. 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

  23. charged hadron spectra Look at “transverse mass” mT2 = pT2 + m02 — is distribution e-E/T? i.e. Boltzmann distribution from thermal gas? Protons are flatter  velocity boost

  24. 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)!

  25. _ ¯ 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 Antibaryon/baryon

  26. min bias 200 GeV Au+ Au Elliptic flow at very high pT v2 > 0.15 at high pT interpretation? 15% jets per STAR flow vs. hard processes contribution unclear PHENIX STAR

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

  28. 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...

  29. 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

  30. All tracks Electron enriched sample (using RICH) We do find the electrons Energy/Momentum

  31. What have we learned so far? • unprecedented energy density at RHIC! • e > ecrit • freeze-out near the phase transition T • 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 pressure • suppression of high pT particles • modified composition at high pT • J/Y suppression??? • Not yet at appropriate standard of proof (but I think we see QGP at RHIC)

  32. 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???

  33. 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!

  34. Mysteries II What’s this? protons?? Particle composition at high momentum very different than in p-p or in “typical” jets Must understand if modification is initial or final state effect…

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

  36. 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

  37. At RHIC we don’t know yet But it should be higher since the energy density is larger s=17 GeV/A: photon and lepton spectra consistent with T ~ 200 MeV Initial temperature achieved? NA50 WA98 photon pT m+ m- pair mass

  38. 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

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

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

  41. 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!

  42. 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.

  43. + +… Quantum ChromoDynamics • Field theory • of strong • interaction • colored quarks exchange of gluons • Parallels Quantum Electrodynamics (QED) • but in electromagnetic interactions • the exchanged 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!

  44. v2 Negatives pi-&K-,pbar Positives pi+&K+,p PHENIX Preliminary PHENIX Preliminary pT (GeV/c) pT (GeV/c) v2 of identified hadrons Au+Au at sNN=200GeV r.p. |h|=3~4 (min. bias) p cross p,K not expected from hydro p modified and p not??

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