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The hottest stuff on earth! Physics at the Relativistic Heavy Ion Collider

The hottest stuff on earth! Physics at the Relativistic Heavy Ion Collider. Physics Colloquium Princeton University Barbara V. Jacak Stony Brook April 24, 2003. outline. Why collide heavy ions? the QCD phase transition Creating and studying hot, dense matter in the laboratory

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The hottest stuff on earth! Physics at the Relativistic Heavy Ion Collider

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  1. The hottest stuff on earth! Physics at the Relativistic Heavy Ion Collider Physics Colloquium Princeton University Barbara V. Jacak Stony Brook April 24, 2003

  2. outline • Why collide heavy ions? • the QCD phase transition • Creating and studying hot, dense matter in the laboratory • the Relativistic Heavy Ion Collider • experiments and observables • What have we learned so far? • A few interesting mysteries…

  3. The Physics of RHIC • Create very high temperature and density matter • as existed ~1 msec after the Big Bang • inter-hadron distances comparable to that in neutron stars • collide heavy ions to achieve maximum volume • Study the hot, dense medium • is thermal equilibrium reached? • transport properties? equation of state? • do the nuclei dissolve into a quark gluon plasma? • Collide Au + Au ions at high energy • s = 200 GeV/nucleon pair, p+p and d+A to compare • Also polarized p+p collisions to study carriers of p’s spin

  4. + +… Quantum ChromoDynamics • Strong interaction field theory : 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 short distance: force is weak (probe w/ high Q2, Calculate with perturbation theory) large distance: force is strong (probe w/ low Q2, calculations must be non-perturbative) High temperature: force becomes screened by produced color-charges

  5. QCD Phase Transition • Basic (i.e. hard) questions • how does process of quark confinement work? • how nature breaks symmetries  massive particles from ~ massless quarks • transition affects evolution of early universe • latent heat & surface tension  • matter inhomogeneity in evolving universe • equation of state  compression in stellar explosions

  6. Experimental approach pT Central region has max temperature & density Head-on = “central” collisions  max volume Thermalization? particle spectra, yields Pressure developed? particle/energy flows Medium properties? effects upon probe particles Deconfinement? c and anti-c remain bound as J/Y?

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

  9. STAR 4 complementary experiments

  10. The challenge at RHIC • Determine experimentally that conditions are sufficient to search for evidence of new phase of matter • Figure out its properties • Address by correlating different variables • First look at “global” quantities • number, energy flow of produced particles • hadron production and patterns at “end” of collision • Then look at probes of the earliest phase • processes at small distance (high energy) scales • collective behavior • thermal radiation

  11. Energy  to beam direction per unit velocity || to beam pR2 2ct0 Is the energy density high enough? PRL87, 052301 (2001) Colliding system expands: • e 4.6 GeV/fm3 (130 GeV Au+Au) 5.5 GeV/fm3 (200 GeV Au+Au) YES - well above predicted transition!

  12. Central Au+Au collisions (~ longitudinal velocity) Density: a first look sum particles under the curve, find ~ 5000 charged particles in collision final state (6200 in 200 GeV/A central Au+Au) In initial volume ~ Vnucleus

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

  14. Preliminary STAR STAR Preliminary v2 measured by the experiments 200 GeV: 0.2< pt < 2.0 130 GeV: 0.075< pt < 2.0 200 GeV: 0.150< pt < 2.0 4-part cumulants v2=0.05 200 GeV: Preliminary - Consistent results - At 200 GeV better pronounced decrease of v2 for the most peripheral collisions.

  15. Large v2: as predicted by hydrodynamics v2: Much larger than at CERN or AGS! Hydro. Calculations Huovinen, P. Kolb, U. Heinz STAR PRL 86 (2001) 402 pressure buildup  explosion happens fast  early equilibration ! first time hydrodynamic behavior seen…

  16. thermal history of heavy ion collisions v2 builds up g, g* e+e-, m+m- p, K, p, n, f, L, D, X, W, d, Hadrons reflect thermal properties when inelastic collisions stop (chemical freeze-out). Real and virtual photons from quark scattering most sensitive to the early stages.

  17. Protons are flatter  velocity boost  to beam Result of pressure built up hadron spectra: p, K, p and antiprotons Look at “transverse mass” mT2 = pT2 + m02 — is distribution e-E/T? i.e. Boltzmann distribution from thermalized gas? yes ! 130GeV/A

  18. 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 fit yields vs. mass (grand canonical ensemble) Tch = 175 MeV mB = 51 MeV These are the conditions when hadrons stop interacting T Observed particles “freeze out” at/near the deconfinement boundary!

  19. What does theory say about p, K, p data? • To get proper particle yields must “fix” models so they no longer agree with pp collisions •  the N-N collisions are NOT independent! • Must add some kind of a partonic phase with large scattering cross sections to reproduce v2 • Need QGP-type equation of state to get the v2 (and also radial expansion) correctly • Superposition of pp collisions gives insufficient initial pressure as the “strings” don’t scatter.

  20. schematic view of jet production hadrons leading particle q q hadrons leading particle Hot medium - 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”

  21. nucleons Is there something new at RHIC? • Compare to baseline: nucleon-nucleon collisions at the • same energy • To 0’th order: Au + Au collisions start with collisions of quarks & gluons in the individual N-N reactions • (+ effects of • nuclear binding and • collective excitations) • Hard scattering (high p transfer) processes scale as the number of N-N binary collisions <Nbinary> • so expect: YieldA-A = YieldN-N. <Nbinary>

  22. Preliminary Agrees with pQCD predictions (next to leading order) Baseline: p+p collisions p0 spectrum

  23. Is Au+Au different? For p0, plot: PHENIX Preliminary Central Peripheral collisions Yes!!

  24. So, is there jet quenching? • Suppression observed to 8 GeV/c! (in 3 independent measurements) Theory agrees with data when quark, gluon energy loss is included NB: 2 examples here, others also must add some kind of medium modification of the fast quarks/gluons In initial or final state?

  25. trigger-jet pT>4 GeV particles are from jets see not much modification Away side: strong jet suppression STAR Can look for the jet on the other side Centrality  • Strong jet suppression  jet going through medium quenched? • “Colored glass” back-to-back jets simply not created…

  26. probe rest frame r/ ggg gluon saturation colored glass condensate Mueller, McLerran, Kharzeev, … Wavefunction of low x (very soft) gluons overlap and the self-coupling gluons fuse, thus saturating the density of gluons in the initial state, coupling gets weak  treat as a 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)) Predict: suppressed jet cross section and no back-back pairs At RHIC should have saturation effects at higher x than at HERA due to nuclear size.

  27. Vitev & Gyulassy nucl-th/0104066 Combination of hydro. expansion at lower pT with jet quenching at high pT? Medium modifies fragmentation? We see excess protons at high pT Higher than in p+p collisions or fragmentation of gluon jets in e+e- collisions

  28. protons p0, h Do the baryons scale with Nbinary? Baryon yields not suppresed  Ncoll at pT = 2 – 3 GeV/c Looks like hard/soft process interplay… Challenge to initial state explanations (like colored glass)

  29. Other penetrating probes • See that early medium is hot, dense, equilibrated and (probably) induces energy radiation in transiting q,g • What else can we say about it? • J/Y • Test confinement: do bound c + c survive? • Open Charm • Extra heavy quarks from dense gluon gas? • Do the c quarks lose energy like the light quarks? • Dileptons • Tmax from thermal radiation spectrum Need (a lot) more statistics in the data But can take a first look…

  30. J/Y suppression was observed at CERN at s=18 GeV/A NA50 collaboration J/Y yield Fewer J/Y in Pb+Pb than expected! Interpret as color screening of c-cbar by the medium Initial state processes affect J/Y too so interpretation is still debated...

  31. J/Y at RHIC (PHENIX) Energy/Momentum Centrality  Data consistent with: Hadronic comover breakup (Ramona Vogt) w/o QGP Limiting suppression via surface emission (C.Y. Wong) Dissociation + thermal regeneration (R. Rapp)

  32. Total charm quark cross section at RHIC Cross section fits into expected energy dependence No evidence for strong energy loss of charmed quarks…

  33. conclusions • Rapid thermalization! Strong pressure gradients develop early • high gluon density, extensive scattering • flows reproduced by hydrodynamics; EOS beyond hadronic • theories require QGP, “string melting”, large s… • The hot matter is “sticky” – it absorbs energy • High pT and high mass data look like pQCD + something • Seeenergy loss, disappearance of back-to-back jets • Excess of protons, antiprotons at high pT • Colored glass condensate? • Too early to tell, but stuff is dense, hot, and ~ equilibrated

  34. So, • are we seeing quark gluon plasma? • If it looks like a duck, walks like a duck…. • BUT • Serious conclusion should await • results from the “control” experiment d+Au • (cold matter to measure initial state effects) • theoretical description(s) which hangs together • WE ALSO NEED TO KNOW • Tinitial from thermal photon spectrum • Is there deconfinement-driven J/Y suppression?

  35. A few mysteries…

  36. Hydro describes single + multi-particles But FAILS to reproduce two-particle correlations! • How to increase R without increasing Rout/Rside??? • EOS? • initial T & rprofiles? • emissivity?

  37. Elliptic flow of high momentum particles min. bias v2 p cross p,K (not expected from hydro) v2 Negatives pi-&K-,pbar Positives pi+&K+,p pT (GeV/c) Still flowing at pT = 8 GeV/c? Unlikely! Geometry effect? Hard to reproduce quantitatively!

  38. Why no big energy loss for heavy quarks? no x4 suppression from peripheral to central, as predicted for dE/dx=-0.5GeV/fm! But (we squirm) - Is 40-70% peripheral enough? error bars still big!

  39. Centrality dependence of charm quarks Compare the measurement to (PYTHIA) an event generator tuned for pp collisions… no large suppression as for light quarks! Spectra of electrons from c e + anything

  40. Still flowing at pT = 8 GeV/c? Unlikely!! A puzzle at high pT Nu Xu Adler et al., nucl-ex/0206006

  41. Vitev: they can get v2 right • There is a quantitative difference • Calculations/fits with flat • or continuously growing Check against high-pT data (200 AGeV) b=7 fm b~7 fm C. Adler et al. [STAR Collab.], arXiv: nucl-ex/0206006 Same for 0-50% • The decrease with pT is now • supported by data • For minimum bias this rate is • slightly slower K. Filimonov [STAR Collab.], arXiv: nucl-ex/0210027 See: N.Borghini, P.Dinh, J-Y.Ollitrault, Phys.Rev. C 64 (2001)

  42. v2 of mesons & baryons Au+Au at sNN=200GeV 1) High quality M.B. data!!! 2) Consistent between PHENIX and STAR pT < 2 GeV/c v2(light) > v2(heavy) pT > 2.5 GeV/c v2(light) < v2(heavy) Model: P.Huovinen, et al., Phys. Lett. B503, 58 (2001) v2

  43. Nbinary ? 2003 ? PHENIX 130 BRAHMS PRL88(02) STAR 130 Npart/2 hch 15% too many particles, baryons over-quenched, but predicted the suppression BUT: dE/dx =2 GeV/fm or 0.5 GeV/fm or not linear with x?

  44. kT dependence of R Centrality is in top 30% • Broad <kT> range : 0.2 - 1.2 GeV/c • All R parameters decrease as a function of kT •  consistent with collective expansion picture. • Stronger kT dependent in Rlong have been observed. kT : average momentum of pair

  45. Comparison of kaon to pion In the most 30% central

  46. Comparison with hydrodynamic model Centrality is in top 30% Recent hydrodynamic calculation by U.Heinz and P. F. Kolb (hep-ph/0204061) Hydro w/o FS • Standard initialization and freeze out which reproduce single particle spectra. Hydro at ecrit • Assuming freeze out directly at the hadronization point. (edec = ecrit) kT dependence of Rlong indicates the early freeze-out?

  47. kT dependence of Rout/Rside A. Enikizono QM2002 C.M. Kuo, QM2002 poster (PHOBOS) 200 GeV: @0.25 GeV/c

  48. HBT PUZZLE Small Rout implies small Dt P.Kolb Small Rbeam implies small breakup t, ~10 fm/c Large Rside implies large R

  49. near-side correlation of charged tracks (STAR) trigger particle pT = 4-6 GeV/c Df distribution for pT > 2 GeV/c signature of jets also seen in g (p0) triggered events (PHENIX) trigger particle pT > 2.5 GeV/c Df distribution for pT = 2-4 GeV/c Jet Evidence in Azimuthal Correlations at RHIC QM2002 summary slide (Peitzmann) M. Chiu, PHENIX Parallel Saturday

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