Impact of residual contamination on inclusive and direct photon flow

Abstract: Direct photon flow is measured by subtracting the contribution of decay photon flow from the measured inclusive photon flow via the double ratio $R_{\rm \gamma}$, which defines the excess of direct over decay photons. The inclusive photon sample is affected by a modest contamination from different background sources, which is often ignored in measurements. However, due to the sensitivity of the direct photon measurement even a residual contamination may significantly bias the extracted direct photon flow. In particular, for measurements using photon conversions, which are very powerful at low transverse momentum, these effects can be substantial. Assuming three different types of correlated background contributions we demonstrate using the Therminator2 event generator that the impact of the contamination on the magnitude of direct photon flow can be on the level of $50\%$, even if the purity of the inclusive photon sample is about $97\%$. Future measurements should attempt to account for the contamination by measuring the background contributions and subtracting them from the inclusive photon flow.