1. Exploring superdense matter at RHIC
Barbara V. Jacak
Stony Brook
June 12, 2002

2. Goals of experiments at RHIC
Collide Au + Au ions at high energy
130 GeV/nucleon c.m. energy in 2000
s = 200 GeV/nucleon in 2001
Achieve highest possible temperature and density
as existed ~1 msec after the Big Bang
inter-hadron distances comparable to that in neutron stars
heavy ions to achieve maximum volume
Study the hot, dense matter
do the nuclei dissolve into a quark gluon plasma?
do partons/hadrons thermalize?
characteristics of the phase transition?
transport properties of the quark gluon plasma? equation of state?

3. Use RHIC to study QCD Hadron properties governed by QCD
force between quarks: exchange of colored gluons
How does confinement work?
What are the properties of deconfined matter?
QCD is non-abelian:
gluons can interact
with gluons
calculations challenging
at short distance:
force is weak
(probe w/ high Q2,
perturbative)
at large distance:
force is strong
(probe w/ low Q2,
non-perturbative)

4. Deconfinement temperature, energy density?
QCD on the lattice predicts:
Karsch, Laermann, Peikert ‘99
e/T4
T/Tc
Tc ~ 170 ± 10 MeV (1012 °K)
e ~ 3 GeV/fm3

5. Evolution of a heavy ion collision
104 gluons, q, q’s
Initial collision probability given by nuclear structure functions
followed by parton cascade

6. Experiments ask: did something new happen?
Collision dynamics (via hadronic final state)
Probe the early (hot) phase
Equilibrium?
hadron spectra, yields
Collective behavior
i.e. pressure and expansion?
elliptic, radial flow
matter box
Particles created early
in predictable quantity
interact differently with
QGP and normal matter
fast quarks, J/Y, strange
quark content, thermal radiation
vacuum
QGP

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

8. 4 complementary experiments
STAR

9. Address via experiment:
Temperature
early in the collision during plasma phase
Density
also 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?

10. Density: a first look Central Au+Au collisions
(~ longitudinal velocity)
summing particles under the curve, find
~ 5000 charged particles in collision final state
initial volume ~ Vnucleus

11. Is energy density high enough?
PRL87, (2001)
Colliding system expands:
Energy  to
beam direction
pR2
2ct0
per unit
velocity || to beam
 e  4.6 GeV/fm3
YES - well above predicted transition!
50% higher than seen before

12. elliptic flow as “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
Almond shape overlap region in coordinate space

13. Large v2: the matter can be modeled by hydrodynamics
v2 = 6%: larger than at CERN or AGS!
Hydro. Calculations
Huovinen,
P. Kolb and
U. Heinz
STAR
PRL 86 (2001) 402
pressure buildup  explosion
pressure generated early!
 early equilibration !
first hydrodynamic behavior seen

14. charged hadron spectra
mT2 = pT2 + m02
mT - m0 = transverse kinetic energy
Protons are flatter  velocity boost

15. Many high pt baryons! As many baryons as pions at pT> 2 GeV/c
nucl-ex/
As many baryons as pions at pT> 2 GeV/c
hydrodynamical
calculation
agrees with data
Teaney, Lauret, Shuryak
nucl-th/

16. Conditions in hadronic phase at RHIC_
 s B
---
Collisions at
RHIC approach
zero net baryon
density
Braun-Munzinger, Magestro, Redlich, Stachel,
hep-ph/
Tch = 175 MeV
mB = 51 MeV
Analyze with Grand Canonical Ensemble:
fit particle ratios for mB, T

17. Locate RHIC on phase diagram
Baryonic Potential B [MeV]
200
250
150
100 50
400
600
800
1000
1200
AGS
SIS
SPS
RHIC
quark-gluon plasma
hadron gas
neutron stars
early universe
thermal freeze-out
deconfinement
chiral restauration
Lattice QCD
atomic nuclei
At the time of chemical equilibrium among
hadrons

18. Mystery #1
 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)
Possible explanations:
non-uniform particle density distribution!
(i.e. Hydrodynamics is not explosive enough
middle not depopulated)
Shape of correlation function different at RHIC

19. Hard scattered partons as probe of early collision stage
hadrons 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”

20. hadron pT spectra Should be dominated by leading hadrons from jets
PHENIX data
STAR data
Should be dominated
by leading hadrons
from jets
Baseline: inclusive pt distribution in p+p collision
Fit power law: pp = d2N/dpt2 = A (p0+pt)-n

21. Both h & p0 below p+p Peripheral (60-80% of sgeom):
PRL 88, (2002)
Peripheral (60-80% of sgeom):
<N binary collisions> = 20  6
central (0-10%): <N bin coll> = 905  96

22. Jet quenching in central Au + Au collisions?
Phys. Rev. Lett. 88, (2002)
charged
p0 lower
as h ½ baryons
transverse momentum (GeV/c)
Charged deficit
seen by both
STAR & PHENIX
STAR
preliminary

23. A closer look at high pT Yield scales with Nbin.coll? NO
PHENIX preliminary
Yield scales with Nbin.coll? NO
Yield scales with Npart?
high pT : should be from hard
processes, but see scaling with # of binary
NN collisions decrease with increasing
collision centrality (quenching effect!?)

24. Can we confirm jets? Correlation of 4 GeV/c trigger hadron
STAR preliminary
Correlation of 4 GeV/c trigger hadron
With particle of pT > 2 GeV/c
(v2 effect removed)
s = 0.27  0.9 rad (as for jets in pp)

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

26. EM probes at RHIC PHENIX looks for J/Y  e+e- and m+m-
A needle in a haystack
must find electron without mistaking a pion for an electron at the level of one in 10,000
There is the electron.
Ring Imaging
Cherenkov
counter to tag
the electrons
“RICH”
See cherenkov
light in CO2
vpart. > cmedium

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

28. Mystery #2
 If jets from light quarks are quenched, shouldn’t charmed quarks be suppressed too?
nucl-ex/
Theorists: yes (some), no (others)
Enhancement balanced by e loss?

29. Conclusions Unprecedented energy density! e > ecrit
Early thermalization
very explosive collisions  matter at early time has a stiff equation of state
hydrodynamics works (mostly)
Chemical equilibration with Tch ~ Tc
Probe early phase with hard partons
see a deficit  energy loss!
Some mysteries
Hydro misses 2 particle correlations
No energy loss by c,cbar quarks
J/Y to come (from higher L data)
QGP? Most likely… pA reference needed

30. Gluon saturation at RHIC?
Venugopalan, McLerran, Kharzeev, etc.
In nucleus rest frame
r/
ggg
Wavefunction of low x partons overlap and the self-coupling gluons fuse,
thus saturating the density of gluons in the initial state
 treat as 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))
 (A, b dependent) mT scaling of hadrons & suppressed gluon jet production
expect saturation effects at higher x than at HERA effect present in initial state at RHIC?

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

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

33. PHENIX measures p0 in PbSc and PbGl calorimeters
pT >2 GeV, asym<0.8
in PbSc
PRL 88, (2002)
excellent
agreement!

34. J/Y suppression observed at CERN
NA50
J/Y
yield
Fewer J/Y in Pb+Pb than expected!
But other processes affect J/Y too
so interpretation is still debated...

35. Something new at RHIC? Compare to a baseline, or control
use nucleon-nucleon collisions at the
same energy
To zero’th order Au + Au collisions
a superposition
of N-N reactions
(modulo effect of
nuclear binding and
collective excitations)
Hard scattering processes scale as
number of N-N binary collisions <Nbinary>
so expect: YieldA-A = YieldN-N . <Nbinary>
nucleons

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

37. measuring the thermal history
Thermal Properties
measuring the thermal history
g, 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.

38. Known effects pA and AA data at lower energy show excess above unity:
X.N.Wang, nucl-th/
pA and AA data at
lower energy show
excess above unity:
“Cronin effect”
(multiple scattering)

39. In Pb + Pb at CERN
From compilation of X.N. Wang
RAA(pT)
Crossing at
~ 1.5 GeV/c
parton energy loss, if any, is overwhelmed by initial state soft
multiple scattering!

40. Is SPS-RHIC comparison fair?
Same pt implies different x!
RHIC
xT =
if pT(had) / pT(jet) ~ 1
then xT ~ x(parton) at y=0

41. Nuclear shadowing at RHIC?
Zheng Huang, Hung Jung Lu, Ina Sarcevic:
Nucl.Phys.A637:79-106,1998 (hep-ph/ )
quark structure function
Shadowing
of structure
functions small in
RHIC x range!!
Gluon shadowing
should be even less
pt comparison OK
deficit  shadowing!

42. Effect of flow + quenching?
hydro boosts baryons to higher pT
Jet quenching should reduce p yield (by ~3-5)
baryons less depleted as less likely to be leading particles in fragmenting jet
Vitev & Gyulassy Phys. Rev. C65 (2002)
pbar/
pi-

43. Correlations at high pT
Hydrodynamics
no longer dominates
Correlation method on HIJING picks
out back-to-back particles from jets
For data
correlation &
reaction plane
methods agree
J. Rak
jet correlations weak or missing!
Reaction plane results a mystery...