Parton energy loss in heavy-ion collisions via direct-photon and charged-particle azimuthal correlations

Abstract

Charged-particle spectra associated with direct photon (γ dir ) and π 0 are measured in p + p and Au + Au collisions at center-of-mass energy √ s NN = 200 GeV with the STAR detector at the Relativistic Heavy Ion Collider. A shower-shape analysis is used to partially discriminate between γ dir and π 0 . Assuming no associated charged particles in the γ dir direction (near side) and small contribution from fragmentation photons (γ frag ), the associated charged-particle yields opposite to γ dir (away side) are extracted. In central Au + Au collisions, the charged-particle yields at midrapidity (|η| < 1) and high transverse momentum (3 < p assoc T < 16 GeV/c) associated with γ dir and π 0 (|η| < 0.9, 8 < p trig T < 16 GeV/c) are suppressed by a factor of 3-5 compared with p + p collisions. The observed suppression of the associated charged particles is similar for γ dir and π 0 and independent of the γ dir energy within uncertainties. These measurements indicate that, in the kinematic range covered and within our current experimental uncertainties, the parton energy loss shows no sensitivity to the parton initial energy, path length, or color charge.

Responses of the chiral-magnetic-effect–sensitive sine observable to resonance backgrounds in heavy-ion collisions

“…It generates events to be composed of primordial pions and ρ-decay pions. Their input p T distributions and v 2 (p T ) are obtained from data measurements [36][37][38][39][40][41][42][43][44][45][46]. For simplicity, we use the input harmonic plane Ψ 2 (as well as Ψ 3 discussed in Sec.…”

Constraining the physics of jet quenching

“…From these considerations, it can be deduced that any parton-medium interaction model which is tuned to describe single hadron suppression for a given choice of a medium model should also describe the high-z T region of the γ -h data [107,108] in the same medium model because the integrals over geometry and energy-loss probability involved are just the same. This is indeed found for the Zhang-Owens-Wang-Wang (ZOWW) model [109], for the AMY model [110], for the ASW model, and for the shower code YaJEM [111].…”

Overall momentum balance and redistribution of the lost energy in asymmetric dijet events in 2.76A TeV Pb-Pb collisions with a multiphase transport model

“…It originates from the energy loss experienced by the hard partonic jets initially produced from early scatterings as they traverse and interact with the highly excited nuclear matter created in these energetic collisions. The picture of parton energy loss and jet quenching has been confirmed by a wealth of experimental results observed at the Relativistic Heavy-Ion Collider (RHIC) and the Large Hadron Collider (LHC), such as the suppression of large transverse momentum hadron production [3][4][5][6][7], and the strong modification of dihadron and photon-hadron transverse momenta and azimuthal angle correlations [8][9][10][11], in central nucleus-nucleus collisions as compared to elementary nucleon-nucleon collisions. Various theoretical and phenomenological models have been developed to explain these jet modification phenomena [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26], and the comparisons of theories to experimental data have shown that jet energy loss is due to the combined effects of elastic and inelastic interactions between the propagating hard partons and the constituents of the hot and dense QGP matter.…”