Relativistic Heavy Ion Physics: the State of the Art

outline

Science goals of the field

Structure of nuclear matter and theoretical tools we use

Making super-dense matter in the laboratory the Relativistic Heavy Ion Collider

experimental observables &what have we learned already?

Next steps...

Studying super-dense matter by creating a little bang!

Structure ofatoms, nuclei,and nucleons

At very high energyshatter 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

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 universefor a few microseconds after Big Bang

* Probably also in the core of neutron stars

Phase Transition

we don’t really understandhow process of quark confinement workshow 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? more matter than antimatter today?

equation of state compression in stellar explosions

Quantum ChromoDynamics

Field theory for strong interaction among colored quarksby exchange of gluons

Works pretty well...

Quantum Electrodynamics (QED)for electromagnetic interactionsexchanged particles are photons

electrically uncharged 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/T4

T/Tc

Tc ~ 170 ± 10 MeV (1012 °K)

~ 3 GeV/fm3

So, we need to create a little bang in the lab!

Use accelerators to reach highest energy vBEAM = 0.99995 x speed of light at RHICcenter of mass energy s = 200 GeV/nucleonSPS (at CERN) has s 18 GeV/nucleonAGS (at BNL) s 5 GeV/nucleon

Use heaviest beams possiblemaximum 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 knowabout the plasma?

Temperatureearly in the collision, just after nuclei

collide

Densityalso early in the collision, when it is at its

maximum

Are the quarks really free or still confined?

Properties of the quark gluon plasmaequation 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

101044 gluons, q, q’s gluons, q, q’s

Is energy density high enough?

4.6 GeV/fm3

YES - well above predicted transition!50% higher than seen before

PRL87, 052301 (2001)

dy

dE

cRT

Bj 22

11

02

R2

2c

Colliding system expands: Energy tobeam direction

per unitvelocity || to beam

Density: a first look

Central Au+Aucollisions

Adding all particles under the curve, find ~ 5000 charged particles

These all started in a volume ~ that of a nucleus!

(~ longitudinal velocity)

Observables IIDensity - use a unique probe

hadrons

q

q

hadronsleadingparticle

leading particle

schematic view of jet production

Probe: Jets from scattered quarks

Observed via fast leading particles orazimuthal correlations between the leadingparticles

But, before they create jets, the scatteredquarks radiate energy (~ GeV/fm) in thecolored 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

charged

central coll central

pp

/Yield N

Yield

See talk by F. Messer

transverse momentum (GeV/c)

Observables IIIConfinement

J/ (cc bound state)

produced early, traverses the medium

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

cu, d, s

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/ tooso interpretation is still debated...

RHIC data being analyzed now !

J/yield

Observables IV: Propertieselliptic flow “barometer”

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

y2 x2 y2 x2

2cos2 v

x

y

p

patan

Almond shape overlap region in coordinate space

Large v2: the matter can be modeled hydrodynamics

STARPRL 86 (2001) 402

Hydro. 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

Observables VTemperature

Thermal dileptonradiation:

q

q

e-, -

e+, +

*

Thermal photonradiation:

g

q, q

Look for “thermal” radiationprocesses 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?

At RHIC we don’t know yet But it should be higher since the energy

density is larger

At CERN, photon and lepton spectra consistent with T ~ 200 MeV

WA98

NA50

photonspairs

The state of the art (and the outlook…)

unprecedented energy density at RHIC!high density, probably high temperaturevery explosive collisions matter has a stiff

equation of state

new features: hints of quark gluon plasma?large elliptic flow, suppression of high pT,J/ suppression at CERN?but we aren’t sure yet…

To rule out conventional explanations extend reach of Au+Au data compare p+p, p+Au to check effect of

cold nuclei on observables 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?

pT (GeV)

Compare spectra to p+p collisions

Peripheral collisions (60-80% of geom):

~ p-p scaled by <N bin coll> = 20 6

central (0-10%):shape different (more exponential)below scaled p-p!(<N bin coll> = 905 96)

Did something new happen?

Study collision dynamics

Probe the early (hot) phase

Do the particles equilibrate?

Collective behaviori.e. pressure and expansion?

Particles created earlyin predictable quantityinteract differently withQGP and normal matterfast quarks, bound fast quarks, bound ccc pairs, s quarks, ...c pairs, s quarks, ...

+ thermal radiation!

matter box

vacuum

QGP

Thermal Properties

PCM & clust. hadronization

NFD

NFD & hadronic TM

PCM & hadronic TM

CYM & LGT

string & hadronic TM

measuring the thermal history

, e+e-,

+Kpn

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.