Jet Quenching's Effect on the Medium Produced at RHIC
By Saskia Mioduszewski
Since the operation of RHIC began in 2000, we have been pursuing the use of "hard" processes as a means to probe the matter created in heavy-ion collisions.
A "hard scattering" is a collision between the elementary constituents of the nucleon ("partons", i.e. quarks or gluons) with a large transfer of momentum, resulting in fragmentation of the parton into a spray of hadrons called a "jet". Such collisions occur early in the evolution of the heavy-ion collision and the medium formed, so that the scattered parton carries information of the properties of the medium that it traversed and thus acts as a probe of the medium.
The goal of colliding heavy ions at RHIC is to study the medium produced and ultimately quantify its properties. The observation of a suppression of high-momentum hadrons ("jet quenching")  revealed a large effect of the medium on the hard-scattered partons and has led us to conclude that the density of the medium is very high. Energy loss calculations have estimated densities as large as 100 times the density of normal nuclear matter .
Transcending beyond the conclusion of having formed a very dense medium, to be able to make more quantitative conclusions about the properties of the medium, has been the focus of more recent studies of hard scattering in heavy-ion collisions.
Although the single-particle measurements at high transverse momentum first revealed the sizable effect of the medium on hard-scattered partons, as shown via the nuclear modification factor in Fig. 1, it is difficult to quantify the details of the energy loss mechanism on any given parton. This is in part because the measured particles are those that actually emerge from the dense medium with significant energy and thus have a strong bias for having been produced close to the surface of the medium. If one wants to go beyond the measure of an overall suppression factor, then both sides of the hard scattering must be measured simultaneously (i.e. "di-jets", which are back-to-back jets, or a direct photon on the opposite side of a jet). The "near-side'' of the di-jet is the side that is triggered on, and is thus again somewhat surface-biased (not having been absorbed in the medium), and the ``away-side'' jet particles can then reveal the effect of the medium relative to the expectation in vacuum (p+p collisions). With the interaction rates provided by RHIC, PHENIX and STAR are recording high-statistics data sets of events containing products of these rare hard-scattering processes. This allows us to make increasingly discriminating measurements.
The ultimate goal for hard scattering probes is to obtain a direct measure of the effect of the medium on the parton. To achieve this goal, knowledge of the parton's original energy (before losing energy in the medium) is required. This is possible via a photon-hadron correlation measurement, where the hard scattering resulted in a direct photon on one side and a jet on the opposite side (γ-jet) . In this process, the trigger particle is a high-energy photon, which is essentially not affected by the medium, and the hadrons on the away-side measure the medium-modified jet. Figure 2 shows the first result of the jet yields associated with a direct-photon trigger compared to those associated with a π0 trigger .
The yields for direct photon triggers are smaller even in peripheral collisions, which approximate the vacuum (similar to p+p collisions). The π0 triggers come from a fragmentation process where the parton possesses a larger energy, thus resulting in more particle production, while the direct photon does not come from a fragmentation. In central (head-on) collisions, there can be additional differences due to a combination of possible effects: energy loss of the π0 trigger, quark vs. gluon fragmentation, and the amount of the medium that is probed when triggering on a direct photon which is not surface biased. In order to disentangle these effects, the systematic uncertainties on the measurement need to be improved, in addition to including theoretical input for interpretation of the data. It is clear that more work is required before reaching the goal of quantifying the properties of the medium, but the first such measurement is testament to the transition to more discriminating methods of using hard processes as probes of the matter we create at RHIC. Another such measurement is full jet reconstruction, which has long been considered a difficult problem in heavy-ion collisions where the multiplicities are large. However, preliminary results from full jet reconstruction in heavy-ion collisions were presented for the first time at the Hard Probes 2008 conference (see RHIC news article on full jet reconstruction for more details).
Measurements of jet quenching at RHIC have reached a new level. The direct measurement of the effect of the medium on a hard-scattered parton, through γ-jet and/or full jet reconstruction, is the future of hard scattering studies in heavy-ion collisions at RHIC.
 K. Adcox et al., Phys. Rev. Lett. 88, (2002) 022301.
 I. Vitev, J. Phys. G30, S791-S800 (2004).
 S.S. Adler et al., Phys. Rev. C 75, 024909 (2007).
 I. Vitev and M. Gyulassy, Phys. Rev. Lett. 89, 252301 (2002).
 X.-N. Wang et al., Phys. Rev. Lett. 77, (1996) 231-234.
 A.M. Hamed for the STAR Collaboration, Hard Probes 2008 proceedings, arXiv:0809.1462.