Consistent simulation of direct-photon production in hadron collisions including associated two-jet production

Abstract: We have developed an event generator for direct-photon production in hadron collisions, including associated two-jet production in the framework of the GR@PPA event generator. The event generator consistently combines $\gamma$ + 2-jet production processes with the lowest-order $\gamma$ + jet and photon-radiation (fragmentation) processes from QCD 2-jet production using a subtraction method. The generated events can be fed to general-purpose event generators to facilitate the addition of hadronization and decay simulations. Using the obtained event information, we can simulate photon isolation and hadron-jet reconstruction at the particle (hadron) level. The simulation reasonably reproduces measurement data obtained at the LHC concerning not only the inclusive photon spectrum, but also the correlation between the photon and jet. The simulation implies that the contribution of the $\gamma$ + 2-jet is very large, especially in low photon-$p_{T}$ ($\lesssim$ 50 GeV) regions. Discrepancies observed at low $p_{T}$, although marginal, may indicate the necessity for the consideration of further higher-order processes. Unambiguous particle-level definition of the photon-isolation condition for the signal events is desired to be given explicitly in future measurements.