Understanding the Jet Quenching and Medium Response
A Survey Study of Di-hadron Correlation From PHENIX
By Jiangyong Jia
A jet is a collimated cluster of particles, which are the fragments of outgoing quarks and gluons produced in hard-scattering processes in high-energy and heavy-ion collisions. Jets are typically produced in pairs and fly back-to-back in the direction transverse to the beam (di-jets). In Au+Au collisions at RHIC, these outgoing quarks and gluons lose energy in the QCD medium, the strongly interacting Quark-Gluon Plasma or sQGP, before fragmenting into jets of hadrons, a phenomena known as jet quenching. The attenuation and broadening of the single jets and di-jets, measured through the leading hadron and di-hadron correlations, can be used to infer the properties of sQGP, such as the opacity and sound speed.
In the last several years, a significant experimental and theoretical effort has been devoted to understand the jet-medium interactions. The observation of large suppression of high pT hadron yield firmly established the jet quenching phenomena. The follow up di-hadron correlation measurements have yielded more information on jet-medium interactions. At high pT region, the pair yield was found to be suppressed but jet-like, confirming that these hadrons mainly come from fragmentation of survived jets. In the low pT region, a rich and non-trivial modification pattern is observed, characterized by a double shoulder structure centered at Δφ = π ± 1.1 (the cone) for the away-side and a flat structure along Δη for the near-side (the ridge), reflecting the response of the medium to the quenched jets.
The PHENIX collaboration has carried out a detailed survey study of di-hadron angular correlations with an eye toward addressing the following questions: Are the correlation patterns consistent with combined contribution from the jet fragmentation and medium response across the whole pT range? and if so, where is the transition region? Are the medium responses of the near-side and away-side related? What can we learn about the jet contribution to intermediate pT where soft physics dominates? The challenge partially lies in the large phase space that need to be covered in the correlation analyses. The di-hadron correlation needed be mapped out as function of trigger pT (pTA), partner pT (pTB), Δφ and Δη, in contrast to the single particle spectra measurements, where the pT is the main dial. The results of this survey study are summarized in an archival Phys. Rev. C paper . This paper is preceded by a short PRC rapid communication published last year .
In this paper, PHENIX collaboration mapped out the detailed evolution of hadron pair distribution in Au+Au collisions in a broad pT (0.4-10 GeV/c) range as shown in Fig. 1. Many distinct features found in previous publications can now be seen in a single plot from the same dataset as indicated by the red circles and lines: the onset of away-side mach-cone [3, 4], the disappearance and reappearance of the away-side jet [5, 6], and the near-side broadening and enhancement [3, 7], etc. A detailed analysis of jet shape and yield, in terms of their pT , centrality and √s dependence, at both the near- and away-side, was carried out in the paper.
Through systematic exploration of several observables, we showed that all the features indicated by Fig.1 can be accounted for by a simple two-component picture, separately for both the near- and away-side: a jet fragmentation component that dominates at high pTA,B > 5 GeV/c, and a medium response component that dominates at pTA , pTB < 4 GeV/c. The rich pT dependent correlation patterns simply reflect the competition between fragmentation of survived jets and medium response to quenched jets on both the near- and away-side. The observed patterns in Fig. 1 are complicated, simply because 1) the medium response and jet fragmentation have different angular shape and different spectral slope, 2) the shapes of the medium response are quite different between the near- and away-side.
We also compared the near- and away-side medium responses (the ridge and the cone), along the line of several studies that found very similar properties between them, i.e. both have similar slope  and bulk like particle compositions [9,10]. Our data further support this similarity: both near- and away-side distributions show enhancement and broadening at pTA,B < 4 GeV/c, above which the jet characteristics qualitatively approach the jet fragmentation. All these observations suggest that the modification mechanisms for the near and away-side may be related.
Previously, the modifications of di-hadron yield were characterized with IAA (ratio of per-trigger yield between Au+Au and p + p). IAA is a good variable at high pT , since most triggers come from jets and most jets fragment into at most one trigger, such that per-trigger yield is a good representation of per-jet yield. However it is not a good variable at low pT when the non-fragmentation triggers from soft production mechanisms or medium response mechanisms become important. We found that these triggers tend to dilute the IAA, by up to factor of 2 in most central Au+Au collisions at intermediate pT.
In this paper, we introduce a new variable, JAA , for characterizing the medium response. JAA quantify the medium modification of hadron pair yield from the expected yield, in a way similar to RAA for describing the modification of single hadron yield. The hadron pair yield is proportional to the initial dijet yield, and in the absence of nuclear effects, it scales with Ncoll , and JAA = 1. Fig. 2 shows the measured JAA as a function of pair proxy energy (pTSum = pTA + pTB ) for the near- (left panel) and away-side (right panel). The pair yields are enhanced at pTSum < 8 GeV/c relative to the constant level from jet fragmentation at larger pTSum .
This enhancement reflects the energy transport that redistributes energy of the quenched jets to low pT hadrons. JAA also allow us to quantify the role of jets at low pT, albeit in pair momentum space. Providing additional constraints, such as the multiplicity for each jet, one may use results in Fig. 2 to estimate the jet contribution (both fragmentation and medium response) in single particle momentum space.
The next step will be to extend this survey study to identified
particle correlation, and their dependence on the angle with
respect to reaction plane. This will combine the resolving power
of PID dependence of the hadron RAA, momentum anisotropy (v2 and
v4 ) and jet-induced correlations, and help to reach an ultimate
understanding of the medium response and its reciprocal relation
with hydrodynamic expansion and recombination.
 A. Adare et al. [PHENIX Collaboration], arXiv: 0801.4545 [nucl-ex], accepTed by Phys. Rev. C
 A. Adare et al. [PHENIX Collaboration], Phys. Rev. C 77, 011901(R) (2008)
 S. S. Adler et al. [PHENIX Collaboration], Phys. Rev. Lett. 97, 052301 (2006)
 A. Adare et al. [PHENIX Collaboration], Phys. Rev. Lett. 98, 232302 (2007)
 C. Adler et al. [STAR Collaboration], Phys. Rev. Lett. 90, 082302 (2003)
 J. Adams et al. [STAR Collaboration], Phys. Rev. Lett. 97, 162301 (2006)
 J. Adams et al. [STAR Collaboration], Phys. Rev. Lett. 95, 152301 (2005)
 A. Frantz, [PHENIX Collaboration] Quark Matter 2008 Proceedings.
 J. Bielcikova, J. Phys. G 34, S929 (2007)
 S. Afanasiev et al. [PHENIX Collaboration], arXiv:0712.3033 [nucl-ex].