Measurement of Jet Modification at RHIC

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Measurement of Jet Modification at RHIC
Fuqiang Wang Purdue University
THE PLAN Physics motivation Results 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
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Physics motivation
Quantum Chromodynamics predicts phase transition
between hadrons and Quark-Gluon Plasma (a state
of thermalized partons) at high energy density.
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Physics motivation contd
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.

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Inclusive high pT yield suppressed
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Jet-like angular correlation disappeared
Peripheral Au Au
near-side well-described
peripheral collision similar to pp
away side hadrons are suppressed!
Central Au Au
central collision strong absorption in medium
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dAu control experiment
Yes, there is nuclear effect. Energy loss ?
suppression of high pT hadron yield.
But, were jets initially there? Maybe the
initial hard scatterings in AA were different
from pp e.g. modified PDFs.
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Reconstructing those jets
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Jet charge multiplicity and energy
STAR Preliminary QM 2004
With the same final leading particle, we are
selecting a larger energy jet in central AA than
in pp.
Estimate of energy loss scale.
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Associated particles pT distributions
STAR Preliminary Quark Matter 2004
Near
softening in spectra equilibration
with medium
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Summary
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.
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Conclusion / Open question

• Have we created a new form of matter?

Definitely! Its dense, strongly interacting,
dissipative.

• Is it a QGP (i.e. a state of deconfined,
  thermalized quarks and gluons)?