Probing the evolution of heavy-ion collisions using direct photon interferometry

TL;DR

This paper explores how direct photon interferometry can differentiate various photon production sources in heavy-ion collisions, revealing the sensitivity of longitudinal correlations and non-Gaussian features to different emission scenarios.

Contribution

It demonstrates the potential of photon HBT correlations to distinguish photon sources and assesses the feasibility of measuring these correlations at the LHC.

Findings

Longitudinal correlations are most sensitive to photon sources.

Including anisotropic pre-equilibrium rates increases non-Gaussianities.

Measurement of HBT correlations is feasible with upcoming high-luminosity LHC data.

Abstract

We investigate the measurement of Hanbury Brown-Twiss (HBT) photon correlations as an experimental tool to discriminate different sources of photon enhancement, which are proposed to simultaneously reproduce the direct photon yield and the azimuthal anisotropy measured in nuclear collisions at RHIC and the LHC. To showcase this, we consider two different scenarios in which we enhance the yields from standard hydrodynamical simulations. In the first, additional photons are produced from the early pre-equilibrium stage computed from the \textit{bottom-up} thermalization scenario. In the second, the thermal rates are enhanced close to the pseudo-critical temperature $T_c\approx 155\,\text{MeV}$ using a phenomenological ansatz. We compute the correlators for relative momenta $q_o, \,q_s$ and $q_l$ for different transverse pair momenta, $K_\perp$, and find that the longitudinal correlation is the most sensitive to different photon sources. Our results also demonstrate that including anisotropic pre-equilibrium rates enhances non-Gaussianities in the correlators, which can be quantified using the kurtosis of the correlators. Finally, we study the feasibility of measuring a direct photon HBT signal in the upcoming high-luminosity LHC runs. Considering only statistical uncertainties, we find that with the projected $\sim 10^{10}$ heavy ion events a measurement of the HBT correlations for $K_\perp<1\, \text{GeV}$ is statistically significant.