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STAR focus: Jet-like correlations with direct-photon and neutral-pion triggers at √sNN = 200 GeV

The STAR experiment recently published “Jet-like correlations with direct-photon and neutral-pion triggers at √sNN = 200 GeV” in . Physics Letters B 760 (2016) 689.

Direct photons are produced during the early stage of a heavy-ion collision, through QCD processes such as quark-gluon Compton scattering and quark-antiquark pair annihilation (at leading order). In these processes the transverse momentum of the trigger photon approximates the initial transverse momentum of the recoil parton. The away-side parton (resulting in a spray of collimated hadrons called a “jet”) is expected to lose energy while traversing the medium. Jet-like charged-hadron yields on the recoil side of the trigger photon are calculated from the azimuthal angular correlation functions. The suppression of these jet-like yields in central Au+Au collisions is then quantified by comparing to the per-trigger yields measured in p+p collisions, denoting the ratio of integrated yields IAA

It is also compelling to compare the suppression of jet-like yields on the recoil side of direct-photon triggers with the suppression of jet-like yields on the recoil side of neutral-pion (or hadron) triggers. Differences in the suppression are expected from two effects. 1) There is a trigger bias for neutral pions (since they are themselves subject to medium interaction and energy loss) to be from the surface of the medium, maximizing the path length of the recoil parton through the medium; whereas direct-photon triggers can originate from anywhere within the medium (since the direct-photon mean-free-path is much larger than the size of the medium). 2) The recoil side of direct-photon triggers is dominated by quark jets, while the recoil of neutral-pion triggers can be either quark or gluon jets. Both of these effects naively should result in a larger suppression, on average, for the recoil jet-like yields associated with neutral-pion triggers than those associated with direct-photon triggers. One would expect to get information about both the path-length and the color-factor dependence of parton energy loss through the comparison.

Left: The IAA for direct-photon and neutral-pion triggers are plotted as a function of zT. The points for IAA for direct-photon triggers are shifted by +0.03 in zT for visibility. Right: IAA for direct-photon triggers as a function of transverse momentum of the jet-like associated hadrons. The vertical lines represent statistical errors and the vertical extent of the boxes represents systematic errors. The curves represent different energy-loss models.

The IAA for direct-photon (red) and neutral-pion (blue) triggers are plotted as a function of zT = pTassoc/pTtrig or the ratio of transverse momentum carried by the away-side hadron relative to that of the trigger) in the left panel of the figure. The results suggest that for jet-like associated hadrons, with transverse momentum greater than 1.2 GeV/c, the suppression factor is similar for direct-photon and neutral-pion triggers, within measurement uncertainties. The expected effects due to differences in path length and color-factor are not observed, within uncertainties, within our kinematic range. There is a hint of less suppression (for both types of triggers) at low zT, but this effect is more significant when IAA is plotted as a function of the transverse momentum of the jet-like associated hadrons pTassoc (shown in the right panel of the figure).

In contrast to these results, PHENIX (Phys. Rev. Lett. 111 (2013) 032301) has measured an enhancement in jet-like yields (IAA > 1), at large angles, for zT = 0.1-0.25. In the PHENIX measurement, the trigger photon has transverse momentum 5-9 GeV/c, and the associated hadrons have transverse momentum as low as 0.5 GeV/c (compared to the 1.2 GeV/c lower cut in the STAR measurement). Our measurement, in comparison with the PHENIX result, has led to the important conclusion that the modified fragmentation function is not a universal function of zT.

Posted Nov. 3, 2016

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