Photon HBT interferometry for non-central heavy-ion collisions

TL;DR

This paper explores how photon HBT interferometry can provide early-time, detailed spatial and temporal information about non-central heavy-ion collisions, complementing existing hadron-based measurements.

Contribution

It introduces a theoretical analysis of 2-photon intensity interferometry, highlighting the azimuthal dependence of HBT radii and the dual emission sources from QGP and hadron gas.

Findings

Photon HBT radii show azimuthal angle dependence in non-central collisions.

Thermal photon emission exhibits a dual nature from QGP and hadron gas.

The dual emission signature is observable in the source function and correlation functions.

Abstract

Currently, the only known way to obtain experimental information about the space-time structure of a heavy-ion collision is through 2-particle momentum correlations. Azimuthally sensitive HBT interferometry (Hanbury Brown-Twiss intensity interferometry) can complement elliptic flow measurements by constraining the spatial deformation of the source and its time evolution. Performing these measurements on photons allows us to access the fireball evolution at earlier times than with hadrons. Using ideal hydrodynamics to model the space-time evolution of the collision fireball, we explore theoretically various aspects of 2-photon intensity interferometry with transverse momenta up to 2 GeV, in particular the azimuthal angle dependence of the HBT radii in non-central collisions. We highlight the dual nature of thermal photon emission, in both central and non-central collisions, resulting from the superposition of QGP and hadron resonance gas photon production. This signature is present in both the thermal photon source function and the HBT radii extracted from Gaussian fits of the 2-photon correlation function.