Pion and photon production in heavy ion collisions

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.