Pion and photon production in heavy ion collisions

TL;DR

This paper reviews measurements of neutral pions and direct photons in heavy ion collisions, focusing on nuclear modification factors, energy dependence, and implications for parton energy loss models, highlighting the importance of comprehensive experimental comparisons.

Contribution

It provides a detailed analysis of neutral pion and photon production data from RHIC, emphasizing the need for systematic studies across different collision systems and energies.

Findings

Nuclear modification factor varies with energy and system size.

Photon measurements complement pion data in probing collision dynamics.

Results support models of parton energy loss in quark-gluon plasma.

Abstract

Measurement of neutral pions and direct photons are closely connected experimentally, on the other hand they probe quite different aspects of relativistic heavy ion collisions. In this short review of the $\pi^0$ results from the PHENIX experiment at RHIC our focus is on the $\phi$-integrated nuclear modification factor, its energy and system size dependence, and the impact of these results on parton energy loss models. We also discuss the current status of high $p_T$ and thermal direct photon measurements both in $p$ + $p$ and Au+Au collisions. Recognizing the advantages of measuring not only the "signal", but also all the "references" needed for proper interpretation in the same experiments (with same or similar systematics) we argue that RHIC should regularly include $d$ + A and even $d$ + $d$ collisions into its system size and energy scan.