Photon Hanbury-Brown–Twiss interferometry for noncentral heavy-ion collisions

Currently, the only known way to obtain experimental information about the space-time structure of a heavy-ion collision is through two-particle momentum correlations. Azimuthally sensitive Hanbury-Brown–Twiss (HBT) 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 two-photon intensity interferometry with transverse momenta up to 2 GeV, in particular the azimuthal angle dependence of the HBT radii in noncentral collisions. We highlight the dual nature of thermal photon emission, in both central and noncentral 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 two-photon correlation function.