Azimuthally anisotropic emission of low-momentum direct photons in Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV

TL;DR

This paper reports measurements of azimuthal anisotropy coefficients of low-momentum direct photons in gold-gold collisions at 200 GeV, revealing significant flow patterns similar to hadrons and challenging existing theoretical models.

Contribution

It provides the first detailed measurements of both v2 and v3 coefficients of direct photons at low transverse momentum in heavy-ion collisions, offering new constraints for theoretical models.

Findings

v2 coefficients are similar to those of hadrons at low pT

v3 coefficients are nonzero and nearly independent of centrality

Models struggle to simultaneously explain large yields and anisotropies

Abstract

The PHENIX experiment at the Relativistic Heavy Ion Collider has measured 2nd and 3rd order Fourier coefficients of the azimuthal distributions of direct photons emitted at midrapidity in Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV for various collision centralities. Combining two different analysis techniques, results were obtained in the transverse momentum range of $0.4<p_{T}<4.0$ GeV/$c$. At low $p_T$ the second-order coefficients, $v_2$, are similar to the ones observed in hadrons. Third order coefficients, $v_3$, are nonzero and almost independent of centrality. These new results on $v_2$ and $v_3$, combined with previously published results on yields, are compared to model calculations that provide yields and asymmetries in the same framework. Those models are challenged to explain simultaneously the observed large yield and large azimuthal anisotropies.