Making a Little Bang with heavy ion collisions

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

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

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? why more matter than antimatter today? l equation of state of nuclear matter  compression in stellar explosions

Phase diagram of hadronic matter baryons mesons

Quantum ChromoDynamics l Field theory for strong interactions among colored quarks - by exchange of gluons l Parallels Quantum Electrodynamics (QED) in electromagnetic interactions the exchanged particles are photons electrically uncharged l 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!

Transition temperature? Lattice QCD 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

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!

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

4 complementary experiments STAR

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

Questions to address in experiments l Temperature early in the collision during plasma phase l Density also early in the collision, at maximum l Are the quarks confined or in a plasma? Use probes of the medium to investigate l Properties of the quark gluon plasma: equation of state (energy vs. pressure) how is energy transported in the plasma?

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 summing particles under the curve, find ~ 5000 charged particles in collision final state initial volume ~ V nucleus (~ longitudinal velocity) Central Au+Au collisions

Observables II Density - use a unique probe hadrons q q leading particle leading particle schematic view of jet production 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”

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

l p-p data available over wide range of  s, but not for 130 GeV power law:  pp = d 2 N/dp t 2 = A (p 0 +p t ) -n interpolate A, p 0, n to 130 GeV look at inclusive p t distribution

Both h  &  0 below p+p Peripheral (60-80% of  geom ): = 20  6 central (0-10%): = 905  96 PRL 88, (2002)

l NO! l p t data from PHENIX + STAR agree well: An experimental artifact?

Deficit is indeed observed in central Au + Au collisions Charged deficit seen by both STAR & PHENIX 00 charged transverse momentum (GeV/c) Phys. Rev. Lett. 88, (2002) STAR preliminary

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

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

How about at RHIC? PHENIX looks for J/  e+e - and  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 10,000 Ring Imaging Cherenkov counter to tag the electrons “RICH” uses optical “boom” when v part. > c medium

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

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 by 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

m T 2 = p T 2 + m 0 2 Protons are flatter  velocity boost charged hadron spectra

Many high p t baryons! Explains difference between h + +h - and  0 not the expected jet fragmentation function D(z)! hydrodynamical calculation agrees with data Teaney, Lauret, Shuryak nucl-th/ nucl-ex/

Energy loss at RHIC? = 0.25 GeV/fm scaled pp shadowing + initial mult. scattering but we know system is not static! With expansion:  7.3 GeV for 10 GeV/c jets X.N. Wang & E. Wang, hep-ph/ energy loss

Observables V Temperature Thermal dilepton radiation q q e-,  - e+,  +  * Thermal photon radiation  g q, q Look for “thermal” radiation processes producing thermal radiation: Rate, energy of the radiated particles determined by maximum T (T initial ) NB: , e,  interact only electromagnetically  they exit the collision without further interaction

Initial 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 photon p T  pair mass

Locate RHIC on phase diagram Baryonic Potential  B [MeV] AGS SIS SPS RHIC quark-gluon plasma hadron gas neutron stars early universe thermal freeze-out deconfinement chiral restauration Lattice QCD atomic nuclei Conditions when hadrons freeze out - by fitting yields vs. mass (grand canonical ensemble) T ch = 175 MeV  B = 51 MeV ¯ _  s Collisions at RHIC approach zero net baryon density

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

What’s next? l To rule out conventional explanations 9 extend reach of Au+Au data 9 measure p+p reference 9 p+Au to check effect of cold nuclei on observables 9 study volume & energy dependence  are jets quenched & J/  suppressed???

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? p T (GeV) Hydrodynamics is not explosive enough: non-uniform particle density distribution! D. Teaney & J. Burward-Hoy

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

Measure momentum & flight time; calculate particle mass  (dE/dx) =.08 dE/dx pions e kaons protons STAR or measure momentum + energy loss in gas detector also Identify hadrons

PHENIX measures  0 in PbSc and PbGl calorimeters  0 ’s p T >2 GeV, asym<0.8 in PbSc excellent agreement! PRL 88, (2002)

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

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.

Quark confinement l Hadron properties governed by QCD force between quarks: exchange of colored gluons l 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 Q 2, perturbative) at large distance: force is strong (probe w/ low Q 2, non-perturbative)