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Jiangyong Jia is an assistant professor of Chemistry at Stony Brook University and a member of the PHENIX Collaboration.

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 p_T 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 p_T 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 p_T 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 p_T 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 p_T 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 p_T (p_T^A), partner p_T (p_T^B), Δφ and Δη, in contrast to the single particle spectra measurements, where the p_T is the main dial. The results of this survey study are summarized in an archival Phys. Rev. C paper [1]. This paper is preceded by a short PRC rapid communication published last year [2].

In this paper, PHENIX collaboration mapped out the detailed evolution of hadron pair distribution in Au+Au collisions in a broad p_T (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 p_T , centrality and √s dependence, at both the near- and away-side, was carried out in the paper.

FIG. 1: The Δφ distribution in fine bin of trigger and partner p_T in p + p (open symbols) 0-20% central Au+Au (solid symbols) collisions, and thin lines indicates the systematic uncertainties. Several important features are indicated by the thick red lines and red circles.

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 p_T^A,B > 5 GeV/c, and a medium response component that dominates at p_T^A , p_T^B < 4 GeV/c. The rich p_T 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 [8] and bulk like particle compositions [9,10]. Our data further support this similarity: both near- and away-side distributions show enhancement and broadening at p_T^A,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 I_AA (ratio of per-trigger yield between Au+Au and p + p). I_AA is a good variable at high p_T , 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 p_T when the non-fragmentation triggers from soft production mechanisms or medium response mechanisms become important. We found that these triggers tend to dilute the I_AA, by up to factor of 2 in most central Au+Au collisions at intermediate p_T.

In this paper, we introduce a new variable, J_AA , for characterizing the medium response. J_AA quantify the medium modification of hadron pair yield from the expected yield, in a way similar to R_AA 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 N_coll , and J_AA = 1. Fig. 2 shows the measured JAA as a function of pair proxy energy (p_T^Sum = p_T^A + p_T^B ) for the near- (left panel) and away-side (right panel). The pair yields are enhanced at p_T^Sum < 8 GeV/c relative to the constant level from jet fragmentation at larger p_T^Sum .

FIG. 2: The modification factor for hadron pair yield (JAA ) as a function of p_T^Sum = p_T^A + p_T^B for the near-side (left) and away-side (right). p_T^sum condenses the 2-D correlation data in p_T^A and p_T^B space into a one dimensional plot. The right panel is reproduced from the published data table in [1].

This enhancement reflects the energy transport that redistributes energy of the quenched jets to low p_T hadrons. J_AA also allow us to quantify the role of jets at low p_T, 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 R_AA, 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.

[1] A. Adare et al. [PHENIX Collaboration], arXiv: 0801.4545 [nucl-ex], accep_Ted by Phys. Rev. C

[2] A. Adare et al. [PHENIX Collaboration], Phys. Rev. C 77, 011901(R) (2008)

[3] S. S. Adler et al. [PHENIX Collaboration], Phys. Rev. Lett. 97, 052301 (2006)

[4] A. Adare et al. [PHENIX Collaboration], Phys. Rev. Lett. 98, 232302 (2007)

[5] C. Adler et al. [STAR Collaboration], Phys. Rev. Lett. 90, 082302 (2003)

[6] J. Adams et al. [STAR Collaboration], Phys. Rev. Lett. 97, 162301 (2006)

[7] J. Adams et al. [STAR Collaboration], Phys. Rev. Lett. 95, 152301 (2005)

[8] A. Frantz, [PHENIX Collaboration] Quark Matter 2008 Proceedings.

[9] J. Bielcikova, J. Phys. G 34, S929 (2007)

[10] S. Afanasiev et al. [PHENIX Collaboration], arXiv:0712.3033 [nucl-ex].

The Relativistic Heavy Ion Collider at Brookhaven National Laboratory is a world-class scientific research facility primarily funded by the U.S. Department of Energy Office of Science. Hundreds of physicists from around the world use RHIC to study what the universe may have looked like in the first few moments after its creation. What physicists learn from these collisions may help us understand more about why the physical world works the way it does, from the smallest subatomic particles, to the largest stars.