Successes and failures with hard probes

Abstract

The two major pillars of searches for the Quark Gluon Plasma have been: J/$Ψ$ suppression, proposed in 1986, and observed at both SPS fixed target energies and at RHIC; and, more recently, the suppression of $π^0$ with $p_T\geq 3$ GeV/c by a factor $\sim 5$ in Au+Au central collisions, observed at RHIC in 2001, which had been predicted in advance as a consequence of Landau-Pomeranchuck-Migdal coherent (gluon) bremsstrahlung by the outgoing hard-scattered partons traversing the medium. However, new effects were discovered and the quality of the measurements greatly improved so that the clarity of the original explanations has become obscured. For instance: J/$Ψ$ suppression is the same at SpS and RHIC. Is it the QGP, comovers, something else? QCD provides beautiful explanations of $π^0$ and direct $γ$ measurements in p-p collisions but precision fits of the best theories of $π^0$ suppression barely agree with the Au+Au data. Better data are needed for $10< p_T <20$ GeV/c, systematic errors are needed in theory calculations, the values of parameters of the medium such as $\mean{\hat{q}}$ derived from precision fits are the subject of controversy. ... These and other issues will be discussed with a view to identify which conclusions are firm and where further progress towards real understanding is required.