Impact of residual contamination on inclusive and direct photon flow

Understand

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.