Eur. Phys. J. C (2009) 61: 747-759
https://doi.org/10.1140/epjc/s10052-009-0877-6

Regular Article - Experimental Physics

Successes and failures with hard probes

Brookhaven National Laboratory, Upton, NY, 11973-5000, USA

^* e-mail: mjt@bnl.gov

Received: 27 August 2008
Revised: 10 December 2008
Published online: 27 January 2009

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 ≥3 GeV/c by a factor ∼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 derived from precision fits are the subject of controversy. Baryons are much less suppressed than mesons, leading to an anomalous ratio for 2≤p _T ≤4.5 GeV/c, but beautiful theoretical explanations of the effect such as recombination do not work in detail. Heavy quarks seem to be suppressed the same as the light quarks, naively arguing against the bremsstrahlung explanation and suggesting exotic, possible transformational explanations. Di-hadron correlations reveal a trigger side ridge, possible Mach cones on the away side, vanishing and reappearance of away jets, both wide and normal jet correlations with and without apparent loss of energy. Can this all be explained consistently? Preliminary results of direct γ production in Au+Au appear to indicate a suppression approaching that of π ⁰ for p _T ≈20 GeV/c and a possibly thermal component for 1≤p _T ≤ 3 GeV/c. What are the implications? Are fragmentation photons a problem? Regeneration of direct γ by outgoing partons is predicted, leading to negative v ₂—is there evidence for or against it? STAR and PHENIX have different observations relevant to the existence of monojets in d+Au collisions. Will new data clarify the situation? When? etc. 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.