Relativistic Heavy Ion Physics: the State of the Art

outline l Science goals of the field l Structure of nuclear matter and theoretical tools we use l Making super-dense matter in the laboratory the Relativistic Heavy Ion Collider l experimental observables & what have we learned already? l Next steps...

Studying super-dense matter by creating a little bang! Structure of atoms, nuclei, and nucleons At very high energy shatter nucleons into a cloud of quarks and gluons Expect a phase transition to a quark gluon plasma Such matter existed just after the Big Bang

At high temperature/density l Quarks no longer bound into nucleons ( qqq ) and mesons (  qq ) Phase transition  quarks move freely within the volume they become a plasma * Such matter existed in the early universe for a few microseconds after Big Bang * Probably also in the core of neutron stars 

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

Quantum ChromoDynamics l Field theory for strong interaction among colored quarks by exchange of gluons l Works pretty well... l Quantum Electrodynamics (QED) for electromagnetic interactions exchanged particles are photons electrically uncharged l QCD: exchanged gluons have “color”charge  a curious property: they interact among themselves + +… This makes interactions difficult to calculate!

Transition temperature? QCD “simplified”: a 3d grid of quark positions & summing the interactions predicts a phase transition: Karsch, Laermann, Peikert ‘99  /T 4 T/T c T c ~ 170 ± 10 MeV (10 12 °K)  ~ 3 GeV/fm 3

So, we need to create a little bang in the lab! Use accelerators to reach highest energy v BEAM = x speed of light at RHIC center of mass energy  s = 200 GeV/nucleon SPS (at CERN) has  s  18 GeV/nucleon AGS (at BNL)  s  5 GeV/nucleon Use heaviest beams possible maximum volume of plasma ~ 10,000 quarks & gluon in fireball

Experimental method Look at region between the two nuclei for T/density maximum RHIC is first dedicated heavy ion collider 10 times the energy previously available! Collide two nuclei

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

4 complementary experiments STAR

What do we need to know about the plasma? l Temperature early in the collision, just after nuclei collide l Density also early in the collision, when it is at its maximum l Are the quarks really free or still confined? l Properties of the quark gluon plasma equation of state (energy vs. pressure) how is energy transported in the plasma?

When nuclei collide at near the speed of light, a cascade of quark & gluon scattering results…. In Heavy Ion Collisions 10 4 gluons, q, q’s

Is energy density high enough?    4.6 GeV/fm 3 YES - well above predicted transition! 50% higher than seen before PRL87, (2001) R2R2 2c   Colliding system expands: Energy  to beam direction per unit velocity || to beam

Density: a first look Central Au+Au collisions Adding all particles under the curve, find ~ 5000 charged particles These all started in a volume ~ that of a nucleus! (~ longitudinal velocity)

Observables II Density - use a unique probe hadrons q q leading particle leading particle schematic view of jet production Probe: Jets from 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” See talk by X.N. Wang

Deficit observed in central collisions Charged deficit seen by both STAR & PHENIX 00 charged See talk by F. Messer transverse momentum (GeV/c)

Observables III Confinement J/  (cc bound state) l produced early, traverses the medium l if medium is deconfined (i.e. colored) other quarks “get in the way” J/  screened by QGP binding dissolves  2 D mesons u, d, s c c See talks of D. Kharzeev & J. Nagle

J/  suppression observed at CERN Fewer J/  in Pb+Pb than expected! But other processes affect J/  too so interpretation is still debated...  RHIC data being analyzed now ! J/  yield

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

Large v 2 : the matter can be modeled hydrodynamics STAR PRL 86 (2001) 402 Hydro. Calculations Huovinen, P. Kolb and U. Heinz v 2 = 6%: larger than at CERN or AGS! pressure buildup  explosion pressure generated early!  early equilibration !? first hydrodynamic behavior seen

Observables V Temperature Thermal dilepton radiation: q q e-,  - e+,  +  * Thermal photon radiation:  g q, q Look for “thermal” radiation processes producing it: Rate, energy of the radiated particles determined by temperature NB: , e,  interact electromagnetically only  they exit the collision without further interaction See talk of D. Kharzeev

Temperature achieved? l At RHIC we don’t know yet l But it should be higher since the energy density is larger l At CERN, photon and lepton spectra consistent with T ~ 200 MeV WA98 NA50 photons  pairs

The state of the art (and the outlook…) l unprecedented energy density at RHIC! high density, probably high temperature very explosive collisions  matter has a stiff equation of state l new features: hints of quark gluon plasma? large elliptic flow, suppression of high p T, J/  suppression at CERN? but we aren’t sure yet… l To rule out conventional explanations 9 extend reach of Au+Au data 9 compare p+p, p+Au to check effect of cold nuclei on observables 9 study volume & energy dependence

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? not explosive enough! See talk of J. Nagle If jets from light quarks are quenched, shouldn’t charmed quarks be suppressed too? p T (GeV)

Compare spectra to p+p collisions Peripheral collisions (60-80% of  geom ): ~ p-p scaled by = 20  6 central (0-10%): shape different (more exponential) below scaled p-p! ( = 905  96)

Did something new happen? l Study collision dynamics l 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! matter box vacuum QGP

Thermal Properties measuring the thermal history ,  e + e -,  +    K  p  n  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.