Hard scattering and QCD Fundamentals at RHIC
Abstract
In 1998, at the 4th QCD workshop, Rolf Baier asked me whether jets could be measured in Au+Au collisions because he had a prediction of a QCD mediumeffect (energy loss via soft gluon radiation induced by multiple scattering) on colorcharged partons traversing a hotdensemedium composed of screened colorcharges. I reviewed the possibilities in a talk explaining that there was a general consensus that for Au+Au central collisions at $\sqrt{s_{NN}}=200$ GeV, leading particles are the only way to find jets because of the large particle density. The good news was that hardscattering in pp collisions was originally observed by the method of leading particles and that these techniques could be used to study hardscattering and jets in Au+Au collisions. Notably, I described ``How everything you want to know about jets can be found using 2particle correlations''. In fact, the predicted ``jet quenching'' and other new phenomena were discovered by this method. However, this past year, I had to soften the statement to {\em almost everything} because we found by explicit calculation in PHENIX that the awayside twoparticle correlation $p_{T_a}$ spectrum from a $\pi^0$ trigger with $p_{T_t}$ is not sensitive to the fragmentation functionoverturning a belief dating from the seminal paper of Feynman, Field and Fox in 1977. A new formula was derived which showed that the shape of the $p_{T_a}$ spectrum depends only on the power $n$ of the parton invariant cross section so that NLO pQCD calculations of the two particle awayside correlations are, in fact, less sensitive to the shape of the fragmentation function than calculations of the inclusive $\pi^0$ cross section.
 Publication:

arXiv eprints
 Pub Date:
 June 2008
 DOI:
 10.48550/arXiv.0806.0301
 arXiv:
 arXiv:0806.0301
 Bibcode:
 2008arXiv0806.0301T
 Keywords:

 High Energy Physics  Experiment
 EPrint:
 7 pages, 5 figures, Talk given at 9th Workshop on NonPerturbative Quantum Chromodynamics, Paris, France, 48 Jun 2007