One-, two-, and three-dimensional photon femtoscopy

Abstract

Femtoscopy with photon pairs is a particularly attractive tool for studying high-energy nuclear collisions. Proposed and extensively discussed in several influential theory articles, it has seen only few applications in experiment because of statistics limitations. With the progress of detectors and electronics, only now it is coming within reach. In this paper we discuss the choice of kinematic variables for two-photon correlation functions. In particular, we argue against $C(Q_{\rm inv})$ and in favor of $C(ΔE,Q_{\rm inv})$.

Figures (2)

• Figure 1: Mapping of $(q_{\rm out},\sqrt{q_{\rm side}^2+q_{\rm long}^2})$ on the $(\Delta E,Q_{\rm inv})$ plane in LCMS. Lines of constant $q_{\rm out}$ and those of constant $\sqrt{q_{\rm side}^2+q_{\rm long}^2}$ are shown in green, red, and black for the pair momentum $K$ of 0.3, 0.6, and 1.2 GeV/$c$, respectively. In the region below 0.1 GeV/$c$, where the Bose-Einstein peak is expected to be located for direct photons from heavy-ion collisions, the two representations are very close to each other.
• Figure 2: The correlation strength $\lambda$ obtained by fitting a Gaussian to $C(Q_{\rm inv})$ (Monte Carlo simulation). The reduction of $\lambda$ is particularly strong at high collision energies and high pair momenta.