Fuqiang WangPurdue University
– THE PLAN –
Physics motivationResults on high pT inclusive yield
Results on high pT angular correlation
Results on high pT + low pT angular correlation
Summary and open question
thanks to the Collaboration
Measurement of Jet Modification at RHIC
Fuqiang Wang – Moriond QCD 2004 2
Physics motivationQuantum Chromodynamics predicts phase transition between
hadrons and Quark-Gluon Plasma (a state of thermalized partons) at high energy density.
Lattice QCD prediction at B = 0:
F. Karsch, Nucl. Phys. A698, 199c (2002).
TC ~ 170 15 MeVC ~ 0.5 GeV/fm3
BRecent progress is made for 0.
T
5-10
~200MeV
Quark-GluonPlasma
Fuqiang Wang – Moriond QCD 2004 3
Physics motivation – cont’d
What medium are we creating in heavy ion collisions: QGP or hadron gas?Want to probe the density of the medium.
self-generated, penetrating probe: large pT hadrons, jets
they are generated early (by initial hard-scatterings); they need some time to escape from the collision zone – the size of
nuclei; during this time frame, a QGP (or whatever medium) is expected to
form; they interact with the medium, giving us information about the
medium.
Fuqiang Wang – Moriond QCD 2004 4
Inclusive high pT yield – suppressed
Large pT hadrons are mainly from jets produced by initial hard-scattering.Small cross-sections, scale with number of binary collisions.
If no nuclear effect, AA = binary-scaled pp; AA binary-scaled pp nuclear effect.
Fuqiang Wang – Moriond QCD 2004 5
Jet-like angular correlation – disappeared
Peripheral Au + Au
near-side well-described
away side hadrons are suppressed!
22 2 2(Au+Au) (p+p) [1 2 cos(2 )]C C A v
Central Au + Au
central collision:strong absorption in medium
peripheral collision:similar to p+p
4 6 GeV/c, 2trig assoc trigT T Tp p p
Fuqiang Wang – Moriond QCD 2004 6
d+Au control experiment
Phys. Rev. Lett. 91, No. 7, August 15, 2003
Yes, there is nuclear effect. Energy loss suppression of high pT hadron yield.
4 6 GeV/c, 2trig assoc trigT T Tp p p
But, were jets initially there? Maybe the initial hard scatterings in AA were different from pp: e.g. modified
PDF’s.
Fuqiang Wang – Moriond QCD 2004 7
Reconstructing those jets…
High pT particle
Au+Au
High pT particle
p+p
High pT:
1/N
trig
ger d
N/d
()
(radians)
Low pT:
(1/N
trig)
dN
/d(
)
STAR Preliminary: Quark M 2004
background
Signal
p+pJet-like structures
Au+Au top 5%
4 6 GeV/c, 0.15 4 GeV/ctrig assocT Tp p
Fuqiang Wang – Moriond QCD 2004 8
assoc
away
1.5 Tp
trig assoc
near
1.5T Tp p
Jet charge multiplicity and “energy”
With the same final leading particle,we are selecting a larger energy jet
in central AA than in pp.
AA pp
For the same final leading particle
(4 < < 6 GeV/c):
near side "jet" energy difference:
1.4 0.2 0.2 GeV
away diff. in TPC 2.2 0.2 0.3 GeV
trigTp
E E
STAR Preliminary: QM 2004
p+p
Leading particle: 4 6 GeV/c
4.5 GeV/c
Associated particle: 0.15 4 GeV/c
trigT
trigT
T
p
p
p
Estimate of energy loss scale.
Fuqiang Wang – Moriond QCD 2004 9
Away
syst. error
Associated particles pT distributionsSTAR Preliminary: Quark Matter
2004
Near side:
Overallenhancementfrom pp to AA
larger initialparton energy(and modestenergy loss)
4 6 GeV/ctrigTp
Away side:
energy from theinitial parton has
been converted tolower pT particles
energy loss inmedium!
Near
softening in spectra:
equilibration with medium
Fuqiang Wang – Moriond QCD 2004 10
Summary
pT (GeV/c)
syst. error
away side
1/N
trig
ger d
N/d
()
(radians)
Large pT hadrons are suppressed in AuAu relative to pp.
Large pT angular correlation disappeared at away side.
Low pT angular correlation is there and stronger than pp.
energy from theinitial parton has
been converted tolower pT particles
by final stateinteractions
dense mediumin AuAu collisions
approachingthermalization
Fuqiang Wang – Moriond QCD 2004 11
Conclusion / Open question
Have we created a new form of matter?
Definitely! It’s dense, strongly interacting, dissipative.
Is it a QGP (i.e. a state of deconfined, thermalized quarks and gluons)?
