Hard scattering and QCD Fundamentals at RHIC

TL;DR

This paper discusses the measurement of jets in Au+Au collisions at RHIC, highlighting the discovery of jet quenching and the limitations of two-particle correlation methods in probing fragmentation functions.

Contribution

It reveals that two-particle correlations are less sensitive to fragmentation functions than previously believed, providing a new formula relating correlation spectra to the parton cross section power.

Findings

Jet quenching observed in Au+Au collisions.

Two-particle correlations depend mainly on the parton cross section power.

Explicit calculations show limited sensitivity to fragmentation functions.

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 medium-effect (energy loss via soft gluon radiation induced by multiple scattering) on color-charged partons traversing a hot-dense-medium composed of screened color-charges. 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 hard-scattering in p-p collisions was originally observed by the method of leading particles and that these techniques could be used to study hard-scattering and jets in Au+Au collisions. Notably, I described ``How everything you want to know about jets can be found using 2-particle 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 away-side two-particle correlation $p_{T_a}$ spectrum from a $\pi^0$ trigger with $p_{T_t}$ is not sensitive to the fragmentation function--overturning 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 away-side correlations are, in fact, less sensitive to the shape of the fragmentation function than calculations of the inclusive $\pi^0$ cross section.