PHENIX Hard Scattering PWG
Hard scattering in p-p collisions was discovered at
the CERN ISR by the observation of a very large flux of high transverse
momentum pions with a power-law tail which varied systematically with
the c.m. energy of the collision[1]. This
observation in 1972 proved that the partons of deeply inelastic
scattering were strongly interacting. Further ISR measurements
utilizing inclusive single or pairs of hadrons established that high
transverse momentum particles are produced from states with two roughly
back-to-back jets which are the result of scattering of constituents of
the nucleons as described by Quantum Chromodynamics. Later experiments
at the CERN and FNAL $\bar{p}=p$ colliders employed hadron calorimeters
to directly measure the jets of hard-scattering. However, because the
huge multiplicity of the underlying event in central Au+Au collisions
deposits an energy comparable to the kinematic limit in a typical
calorimetric jet finding cone, there is general agreement[2] that inclusive single and di-hadron production
offer the best---if not the only---method of detecting hard-scattering
effects in Relativistic Heavy Ion collisions. According to leading
theorists[3] , ``Hard processes are considered good
tools to study ultra relativistic heavy-ion collisions because they
happen early in the reaction processes and thus can probe the early
stage of the evolution of a dense system, during which a quark-gluon
plasma (QGP) could exist for a short period of time.''
Of particular interest at present is the recent considerable
theoretical activity on how the medium at RHIC should affect partons.
Substantial energy loss and re-scattering are predicted in a hot dense
deconfined QGP
[4] leading to enhanced acoplanarity and energy
imbalance of two back-to-back jets. This would have dramatic effects on
both single and di-hadron spectra. As it has been demonstrated that
high p_T single and di-hadrons can accurately measure the properties of
jets and hard-scattering in p-p collisions[5,6], the task of the hard-scattering working group
will be to build on this experience and establish the methods for using
this technique in relativistic heavy ion collisions. Year one running
with a full EM calorimeter in a single arm should allow measurements of
pizeroes to beyond 10 GeV/c in transverse momentum
(Fig 1). Di-hadron measurements require full tracking and/or EMcal
in both central arms. Systematic measurements of Drell-Yan production
of dileptons will also be made.
References:
- e.g. see P. Darriulat, Ann. Rev.
Nucl. Part. Sci. 30, 159 (1980)
- e.g. see Quark Gluon Plasma
Signatures, Eds. V. Bernard, et al. (1990)
- X-N. Wang and Z. Huang, Phys. Rev.
C55, 3047 (1997)
- Baier, Dokshitzer, Mueller, Peigne
and Schiff, Nucl. Phys. B483, 291 (1997)
- A. L. S. Angelis, et al., Phys.
Lett. 97B, 163 (1980)
- C. N. Brown, et al., Phys. Rev. C54,
3195 (1996), and references therein.
Michael J. Tannenbaum 7/28/99 - contact: mjt@bnl.gov