Direct Photon-hadron Correlations Research Articles

Measurement of jet-medium interactions via direct photon-hadron correlations in Au+Au and d+Au collisions at sNN=200 GeV

We present direct photon-hadron correlations in 200 GeV/A Au$+$Au, $d$$+$Au and $p$$+$$p$ collisions, for direct photon $p_T$ from 5--12 GeV/$c$, collected by the PHENIX Collaboration in the years from 2006 to 2011. We observe no significant modification of jet fragmentation in $d$$+$Au collisions, indicating that cold nuclear matter effects are small or absent. Hadrons carrying a large fraction of the quark's momentum are suppressed in Au$+$Au compared to $p$$+$$p$ and $d$$+$Au. As the momentum fraction decreases, the yield of hadrons in Au$+$Au increases to an excess over the yield in $p$$+$$p$ collisions. The excess is at large angles and at low hadron $p_T$ and is most pronounced for hadrons associated with lower momentum direct photons. Comparison to theoretical calculations suggests that the hadron excess arises from medium response to energy deposited by jets.

PHENIX results on jet modification with π0- and photon-triggered two particle correlations in p+p, p(d)+Au, and Au+Au collisions

As a colorless probe, direct photons balance the pT of the away-side jet at leading order. Direct photon-hadron correlations are thus an excellent probe for nuclear structure and QCD effects, including parton energy loss in the Quark-Gluon Plasma. PHENIX has measured π0 and direct photon-triggered two-particle azimuthal correlations in a variety of collision systems ranging from p+p to Au+Au at 200 GeV. In p+Au and d+Au collisions, no modification of the per-trigger jet yield or away-side correlation width compared to p+p collisions is observed for direct photon triggered correlations while an increase in the away-side width for π0 triggered correlations in p+Au has been measured. In Au+Au collisions, direct photons have been identified statistically as well as using an isolation cut. Combining data sets from different collision systems allows us to quantify the transition from suppression at high zT(pT,h/pT,γ) to the enhancement of low zT particles relative to p+p, and to study this transition as a function of trigger pT.

PHENIX results on direct photon–hadron correlations

Direct photon-hadron correlations are a golden channel to study parton in-medium energy loss in QGP. The modification of the effective fragmentation function for the away-side jet can be measured by comparing integrated away-side yields of direct photon-hadron pairs in heavy ion collisions to those in p+p. We measured per-trigger-yield of associated hadrons in Au+Au collisions and observed that there is a suppression compared to p+p for higher momentum fraction (zT) hadrons. This can be explained by the opacity of the hot medium: energetic partons from the initial hard scattering lose energy while traversing it. A yield enhancement on the other hand has been found at low zT (high ξ). Medium response is likely to be responsible for the enhanced production of these lower momentum particles.The same measurement is done in d+Au collisions and the result suggests that no modification to the fragmentation function is observed, given the current uncertainties.

Direct photon-hadron correlations with the PHENIX detector

Direct photon tagged jets, in the form of photon-hadron correlations, are well suited to provide unique insight into how jets interact with the quark gluon plasma. Since photons do not interact strongly with medium produced at RHIC, at leading order the measured photon momentum approximately balances that of the away-side parton. The modification to the effective fragmentation function can be measured by comparing integrated away-side yields in direct photon-hadron correlations in Au+Au collisions to those in p+p. By varying the away-side integration range, the angular dependence of the observed modification can also be studied. Direct photon-hadron correlations have been measured with PHENIX in p+p and Au+Au using a statistical subtraction technique to remove the decay photon contribution from the inclusive photon-hadron correlations, with an additional isolation cut applied in p+p to reduce uncertainties. Recently published results showing a significant modification to the fragmentation function with an azimuthal angle dependence indicative of broadening are discussed in the context of related theoretical models and complementary results from other experiments.

Medium Modification of Jet Fragmentation inAu+AuCollisions atsNN=200 GeVMeasured in Direct Photon-Hadron Correlations

The jet fragmentation function is measured with direct photon-hadron correlations in p+p and Au+Au collisions at √[s(NN)]=200 GeV. The p(T) of the photon is an excellent approximation to the initial p(T) of the jet and the ratio z(T)=p(T)(h)/p(T)(γ) is used as a proxy for the jet fragmentation function. A statistical subtraction is used to extract the direct photon-hadron yields in Au+Au collisions while a photon isolation cut is applied in p+p. I(AA), the ratio of hadron yield opposite the photon in Au+Au to that in p+p, indicates modification of the jet fragmentation function. Suppression, most likely due to energy loss in the medium, is seen at high z(T). The associated hadron yield at low z(T) is enhanced at large angles. Such a trend is expected from redistribution of the lost energy into increased production of low-momentum particles.

Direct photon-hadron correlations measured with the PHENIX detector

Photon tagged jets are an excellent probe of the hot dense matter created in heavy ion collisions at RHIC. Since photons interact weakly and are not modified by the strongly coupled medium, the measured photon momentum approximately balances that of the away-side parton. Therefore, the modification of the fragmentation function can be obtained by comparing the spectra of the away-side yields in direct photon-hadron correlations in Au+Au collisions to those in p+p. Direct photon-hadron correlations are measured with the PHENIX detector in p+p and Au+Au collisions at s N N = 200 GeV at RHIC. We employ a statistical subtraction method to remove the contribution of decay photons from the inclusive photon-hadron correlations. For the p+p analysis, we developed event by event techniques to improve the systematic uncertainty in the baseline measurement. Compared to previous measurements, this Au+Au result extends to higher ξ (lower z T ) where a medium-induced modification of jet fragmentation is observed.

Direct Photon-Hadron Correlations Measured with PHENIX

Direct photon-hadron correlations greatly improve our ability to perform jet tomography in heavy-ion collisions because the momentum of the direct photon can be used to constrain the initial momentum of the opposing jet. By comparing the spectrum of away-side hadrons observed in heavy ion collisions to the spectrum seen in nucleon collisions we can quantify the medium modification to the fragmentation function due to energy loss of the away-side parton. High p T direct photon-hadron correlations have been measured with the PHENIX detector using a statistical subtraction method to remove the photon contribution from meson decays. The increased integrated luminosity in the most recent Au+Au RHIC run at s N N = 200 GeV provides substantially improved statistical precision and enhances the kinematic reach. These measurements are compared to PHENIX p+p results and several theoretical models of energy loss. In addition, we compare direct photon-hadron and π 0 -hadron correlations.

High direct photon–hadron correlations using the PHENIX detector

Jet tomography, the study of differential energy loss of hard scattered partons to infer the density profile of the medium, is greatly improved by precise knowledge of the initial energy of the hard probe. As photons are not strongly interacting, the momentum of the recoil jet from a direct photon trigger is balanced, to a good approximation, by the momentum of the photon. The energy loss of the away-side jet may be viewed as an effective modification of the fragmentation function. Direct photon–hadron correlations in A + A collisions should be sensitive to modified jet fragmentation as well as to medium response effects. Complementary measurements from collisions are necessary to benchmark jet fragmentation expectations at GeV as well as to constrain perturbative calculations in the channel. Here we present new results from and collisions which use a statistical method for subtracting the background from decay photons.

PHENIX measurement of high-pT hadron–hadron and photon–hadron azimuthal correlations

High-pT hadron–hadron correlations have been measured with the PHENIX experiment in and collisions at GeV. A comparison of the jet widths and yields between the two colliding systems allows us to study the medium effect on jets. We also present a first measurement of direct photon–hadron correlations in and collisions. We find that the near-side yields are consistent with zero in both systems. By comparing the jet yields on the away side, we observe a suggestion of the expected suppression of hadrons associated with photons in collisions.