Low p_T direct photon results with PHENIX and their bearing on the direct photon puzzle

Stony Brook University

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Published online: 17 April 2026

Abstract

Despite decades of theoretical and experimental achievements, a coherent picture of the evolution in colliding heavy ion systems is still missing, particularly in the non-perturbative regime. Since photons are penetrating probes, the combined information of direct photon yields at low transverse momenta (p_T) and their azimuthally asymmetric emission (flow) provides important constraints on theories. Simultaneous observation of high yields and significant (almost hadron-like) flow of low p_T direct photons in Au+Au collisions by PHENIX, dubbed the "direct photon puzzle" still eludes a coherent explanation. A closely related issue is the apparent universality of photon production, that depends only on charged particle multiplicity, but not on the size of the colliding ions. We will present new results on direct photon yields and flow in Au+Au and discuss their impact on the "direct photon puzzle".