Trends in Yield and Azimuthal Shape Modification in Dihadron Correlations in Relativistic Heavy Ion Collisions

TL;DR

This study investigates how dihadron correlations in heavy ion collisions are modified by the medium, revealing that shape modifications occur only at lower trigger momenta, while high-momentum correlations are largely unaltered, challenging existing medium response models.

Contribution

It provides the first detailed measurement of azimuthal correlations across a wide momentum range in Au+Au collisions, highlighting the momentum-dependent nature of medium modifications.

Findings

Shape modification of low-momentum hadrons is only observed below 7 GeV/c trigger pT.

High-momentum correlations show minimal shape modification, even at low associated pT.

Associated yield suppression is significant but less than single particle suppression.

Abstract

Fast parton probes produced by hard scattering and embedded within collisions of large nuclei have shown that partons suffer large energy loss and that the produced medium may respond collectively to the lost energy. We present measurements of neutral pion trigger particles at transverse momenta p^t_T = 4-12 GeV/c and associated charged hadrons (p^a_T = 0.5-7 GeV/c) as a function of relative azimuthal angle Delta Phi at midrapidity in Au+Au and p+p collisions at sqrt(s_NN) = 200 GeV. These data lead to two major observations. First, the relative angular distribution of low momentum hadrons, whose shape modification has been interpreted as a medium response to parton energy loss, is found to be modified only for p^t_T < 7 GeV/c. At higher p^t_T, the data are consistent with unmodified or very weakly modified shapes, even for the lowest measured p^a_T. This observation presents a quantitative challenge to medium response scenarios. Second, the associated yield of hadrons opposite to the trigger particle in Au+Au relative to that in p+p (I_AA) is found to be suppressed at large momentum (IAA ~ 0.35-0.5), but less than the single particle nuclear modification factor (R_AA ~0.2).