Isolating final state effects in high p_T π⁰ production using direct photons in small system collisions with PHENIX

Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11790, USA

^* e-mail: daniel.firak@stonybrook.edu

Published online: 26 June 2024

Abstract

PHENIX observed a 20% suppression in the production of high p_T neutral pions in the most central (0-5%) d+Au collisions at 200 GeV. Through the simultaneous measurement of high p_T direct photons (γ^dir) and π⁰ production for event samples selected by event activity, the final state effects could be disentangled from cold-nuclear-matter effects and event-selection biases that are inherent in using the standard Glauber model. This isolation of final state effects is achieved by approximating the nuclear modification factor by the double ratio R_xA = (γ^dir/π⁰)_pp/(γ^dir/π⁰)_xA. While the cold-nuclear-matter effects in x+A collisions cancel in the (γ^dir/π⁰) ratio, the effective number of binary collisions is given by N_Coll^EXP = γ_xAu^dir / γ_pp^dir, which eliminates the dependence on the Glauber model. In addition, many systematic uncertainties cancel in the double ratio.