Geometrical Scaling of Direct Photons in Relativistic Heavy Ion and d+Au Collisions

TL;DR

This paper demonstrates that direct photon spectra in heavy ion and d+Au collisions exhibit geometrical scaling, depending on a combined saturation scale rather than individual variables, aligning with Color Glass Condensate theory predictions.

Contribution

It shows for the first time that direct photon multiplicity spectra follow geometrical scaling across different collision systems and energies, supporting the saturation physics framework.

Findings

Photon spectra depend on a saturation scale combining N_part, energy, and p_T.

Scaling behavior is consistent across various collision centralities.

Results support the Color Glass Condensate model predictions.

Abstract

In this paper, we show that multiplicity spectra of direct photons in A+A and d+Au collisions at different centrality classes and different energies exhibit geometrical scaling, {\em i.e.}, they depend on a specific combination of number of participants $N_{\rm part}$, collisions energy $W$, and transverse momentum $p_{T}$ -- called saturation scale -- rather than on all these three variables separately. In particular, the dependence on the geometry of collisions encoded in the dependence on $N_{\rm part}$ is in agreement with the expectations based on the Color Glass Condensate theory.