Abstract
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.
Highlights
Collisions of heavy nuclei at the Relativistic Heavy Ion Collider (RHIC) produce matter that is sufficiently hot and dense to form a plasma of quarks and gluons [1]
for hadrons associated with lower momentum direct photons
Comparison to theoretical calculations suggests that the hadron excess arises from medium response
Summary
Collisions of heavy nuclei at the Relativistic Heavy Ion Collider (RHIC) produce matter that is sufficiently hot and dense to form a plasma of quarks and gluons [1]. Bound hadronic states cannot exist in a quark gluon plasma, as the temperatures far exceed the transition temperature calculated by lattice quantum chromodynamics (QCD) [2]. Experimental measurements and theoretical analyses have shown that this plasma exhibits remarkable properties, including opacity to traversing quarks and gluons [3,4]. Experimental probes to address these questions include high momentum hadrons, reconstructed jets, and correlations among particles arising from hard partonic scatterings [1] occurring in the initial stages of the collision. Direct photons are produced dominantly via the QCD analog of Compton scattering, q + g → q + γ , at leading order, and do not interact via the strong force as they traverse the plasma. Measuring the correlation of high momentum direct photons with opposing hadrons allows investigation of quark-gluon plasma effects upon transiting quarks and their fragmentation into hadrons
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