1. Comments on RHIC Results
Huan Zhong Huang
Department of Physics and Astronomy
University of California Los Angeles
@ShanDong University
Oct , 2003

2. Outline
High pT Physics – disappearance of back-to-back angular correlation and suppression of high pT yield
Multi-parton Dynamics

3. Nucleus-Nucleus Collisions and Volcanic Eruption
Volcanic high pT -- Strombolian eruption
Volcanic mediate pT – Spatter (clumps)
Volcanic low pT – Bulk matter flows

4. Disappearance of back-to-back angular correlationsy
ptrig
pss
Ptrig – pss same side f correlation
ptrig> 4 GeV/c, pss pos 2<pT<ptrig x
Ptrig – pos opposite side f corr.
pos

5. Suppression of high pT particles
pT Spectra Au+Au and p+p
RAA=(Au+Au)/[Nbinaryx(p+p)]
Au+Au 0-5%
p+p
Strong high pT suppression by a factor of 4-5 in central Au+Au collisions !
The suppression sets in gradually from peripheral to central Au+Au collisions !

6. Two Explanations for High pT Observations
Energy Loss:
Particles lose energy while traversing high density medium after the hard scattering.
Energy loss quenches back-to-back angular correlations.
J. Bjorken, M. Gyulassy, X-N Wang et al….
Parton Saturation:
The parton (gluon) structure function in the relevant region (saturation scale) is modified. Not enough partons available to produce high pT particles.
Parton fusion produces mono-jet with no back-to-back angular correlations.
D. Kharzeev, L. McLerran, R. Venugopalan et al…..

7. d+Au Collisions Au+Au Geometry d+Au Geometry d+Au collisions:q q q
Au+Au Geometry
d+Au Geometry
d+Au collisions:
Little energy loss from the dense medium created, But
Parton saturation from Au nuclei persists!

8. Data from d+Au collisions
No disappearance of
back-to-back correlations!
No high pT suppression !

9. High pT Phenomena at RHIC
Very dense matter has been created
in central Au+Au collisions!
This dense matter is responsible for the
disappearance of back-to-back correlation
and the suppression of high pT particles !

10. What More Measurements?
1) Precise measurements of charged hadrons, identified particles above 6 GeV/c
2) Heavy quark measurements at high pT
3) Elliptic flow of identified particles above 6 GeV/c
4) Elliptic flow and high pT suppression for different colliding systems
5) High pT suppression at lower beam energy
6) The connection between the dE/dx magnitude and the nature of high dense matter

11. End of Talk