The densest stuff on earth:what we learn from RHIC

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outlineWhy 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?

ConclusionsNext steps

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Goals of RHICCollide Au + Au ions at high energy130 GeV/nucleon c.m. energy in 2000s = 200 GeV/nucleon in 2001

Achieve highest possible temperature and densityas existed ~1 msec after the Big Banginter-hadron distances comparable to that in neutron starsheavy 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?

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QCD Phase Transitionwe dont really understandhow process of quark confinement workshow symmetries are broken by nature massive particles from ~ massless quarkstransition affects evolution of early universelatent heat & surface tension matter inhomogeneity in evolving universe? equation of state of nuclear matter compression in stellar explosions

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Phase diagram of hadronic matternormal nuclei

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Quantum ChromoDynamicsField theory of the strong interaction: colored quarks exchange gluonsParallels QED but gluons have color chargeunlike 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)

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Phase transition temperatureKarsch, Laermann, Peikert 99e/T4T/TcTc ~ 170 10 MeV (1012 K)e ~ 3 GeV/fm3From lattice QCD

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Experimental approachLook at region between the two nuclei for T/density maximumSort collisions by impact parameter head-on = central collisions

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RHIC at Brookhaven National LaboratoryRHIC is first dedicated heavy ion collider10 times the energy previously available!

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4 complementary experiments

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Evolution of a heavy ion collision104 gluons, q, qsInitial collision probability given by nuclear structure functions, followed by parton cascade

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Address via experiment:Temperatureearly in the collision during plasma phase

Densityalso 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?

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Is energy density high enough?e 4.6 GeV/fm3 (130 GeV) 5.5 GeV/fm3 (200 GeV) YES - well above predicted transition!PRL87, 052301 (2001) Colliding system expands:Energy tobeam directionper unitvelocity || to beam

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

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Observables IIDensity - use a unique probeProbe: Jets from hard scattered quarks

Observed via fast leading particles orazimuthal correlations between the leadingparticlesBut, before they create jets, the scatteredquarks radiate energy (~ GeV/fm) in thecolored medium

decreases their momentum fewer high momentum particles beam jet quenching

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Something new at RHIC?Compare to a baseline, or controluse nucleon-nucleon collisions at the same energy

To zeroth order Au + Au collisionsa superpositionof N-N reactions(modulo effect ofnuclear binding andcollective excitations)

Hard scattering processes scale asnumber of N-N binary collisions

so expect: YieldA-A = YieldN-N .

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Baseline: p+p collisionsAgrees with pQCD predictions(next to leading order)

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Is Au+Au different?PHENIX PreliminaryYes!!

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Same ratio for charged particlesSuppression stronger in central collisions and higher pT

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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

agreeswith theorywhenquark/gluonenergy loss isincluded

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Observables III: a barometer called elliptic flowOrigin: spatial anisotropy of the system when created followed by multiple scattering of particles in evolving system spatial anisotropy momentum anisotropyv2: 2nd harmonic Fourier coefficient in azimuthal distribution of particles with respect to the reaction planeAlmond shape overlap region in coordinate space

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Large v2: the matter can be modeled by hydrodynamicsSTARPRL 86 (2001) 402Hydro. CalculationsHuovinen, P. Kolb and U. Heinz

v2 = 6%: larger than at CERN or AGS! pressure buildup explosionpressure generated early! early equilibration !first hydrodynamic behavior seen

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charged hadron spectraLook at transverse mass mT2 = pT2 + m02 is distribution e-E/T?i.e. Boltzmann distribution from thermal gas?Protons are flatter velocity boost

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Many high pt baryons!Explains difference between h++h- and p0not the expected jet fragmentation function D(z)!nucl-ex/0203015

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Locate RHIC on phase diagramCollisions atRHIC approachzero net baryondensityAntibaryon/baryon

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Elliptic flow at very high pTv2 > 0.15 at high pT interpretation? 15% jets per STARflow vs. hard processes contribution unclear

PHENIXSTAR

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Observables IV:ConfinementJ/Y (cc bound state)

produced early, traverses the medium

if medium is deconfined (i.e. colored)expect Debye screening by the colored mediumJ/Y screened by quark gluon plasma binding dissolves 2 D mesons

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J/Y suppression observed at CERNFewer J/Y in Pb+Pb than expected!But other processes affect J/Y tooso interpretation is still debated...NA50J/Yyield

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How about at RHIC?PHENIX looks for J/Y e+e- and m+m-There is the electron.A needle in a haystack

must find electron without mistaking a pion for an electron at the level of one in 10,000Ring ImagingCherenkovcounter to tagthe electronsRICH

uses optical boom whenvpart. > cmedium

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We do find the electronsEnergy/Momentum

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What have we learned so far?unprecedented energy density at RHIC!e > ecritfreeze-out near the phase transition T

high density, probably high temperaturevery explosive collisions matter has a stiff equation of state

new features: hints of quark gluon plasma?elliptic flow early thermalization, high pressuresuppression of high pT particlesmodified composition at high pT J/Y suppression???

Not yet at appropriate standard of proof (but I think we see QGP at RHIC)

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Whats next?To rule out conventional explanations extend reach of Au+Au datameasure p+p reference p+Au to check effect of cold nuclei on observablesstudy volume & energy dependence

are jets quenched & J/Y suppressed???

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Mysteries... 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)Hydrodynamics is not explosive enough:non-uniform particle density distribution!D. Teaney & J. Burward-Hoy

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Mysteries IIWhats this? protons??Particle composition at high momentumvery different than in p-p or in typical jets

Must understand if modification is initialor final state effect

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Mysteries III If jets from light quarks are quenched, shouldnt charmed quarks be suppressed too?nucl-ex/0202002

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Observables VTemperatureLook for thermal radiationprocesses 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

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Initial temperature achieved?

At RHIC we dont know yetBut it should be higher since the energy density is larger s=17 GeV/A: photon and lepton spectra consistent with T ~ 200 MeVWA98NA50photon pTm+ m- pair mass

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Identify hadronsMeasure momentum & flight time;calculate particle massor measuremomentum+ energy loss in gas detectoralso

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PHENIX measures p0 in PbSc and PbGl calorimeters0spT >2 GeV, asym

PHENIX at RHIC2 Central spectrometers2 Forward spectrometers3 Global detectorsPhilosophy: optimize for signals / sample soft physics

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Did something new happen?Study collision dynamics

Probe the early (hot) phaseDo the particles equilibrate?

Collective behaviori.e. pressure and expansion?Particles created earlyin predictable quantityinteract differently withQGP and normal matterfast quarks, bound cc pairs, s quarks, ...+ thermal radiation!

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Thermal Propertiesmeasuring the thermal historyg, 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.

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Quantum ChromoDynamicsField theory of strong interaction colored quarks exchange of gluonsParallels Quantum Electrodynamics (QED)but in electromagnetic interactionsthe 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!

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Au+Au at sNN=200GeV r.p. |h|=3~4(min. bias)v2 of identified hadronsp cross p,K

not expected from hydro

p modifiedand p not??

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