Photon pair distributions and event-mixing at low invariant mass

Abstract

Neutral meson cross-section and direct photon HBT measurements in high energy pp, pA, and AA collisions rely on either subtracting or dividing out the background underlying the $M_{\mathrm{inv}}$ or $Q_{\mathrm{inv}}$ distributions. In this paper we investigate the photon pair distributions for simulated proton-proton collisions at $\sqrt{s}=13$TeV, and study why the background in $M_{\mathrm{inv}}$ gets poorly described by the conventional event-mixing method implemented in experimental analyses. We show that it fails to describe the background as it gives a qualitatively different shape especially at invariant masses around the ${\pi }^0$ and $\eta$ meson mass, which is mainly caused by the correlations originating from parton fragmentation.