Measurement of isolated prompt photon production in pp and p–Pb collisions at the LHC

This paper presents the measurement of the isolated prompt photon inclusive production cross section in pp and p–Pb collisions by the ALICE Collaboration at the LHC. The measurement is performed in p–Pb collisions at centre-of-mass energies per nucleon pair of and 8.16 TeV, as well as in pp collisions at and 8 TeV. The cross section is obtained at midrapidity using a charged-track based isolation momentum in a cone with radius . The data for both collision systems are well reproduced by perturbative QCD (pQCD) calculations at next-to-leading order (NLO) using recent parton distribution functions for free (PDF) and bound (nPDF) nucleons. Furthermore, the nuclear modification factor for both collision energies is consistent with unity for . However, deviations from unity ( ) of up to 20% are observed for with limited significance, indicating the possible presence of nuclear effects in the initial state of the collision. The suppression increases with decreasing with a significance of for a non-zero slope and yields with a significance of at for . In addition, a significance of is observed for at the lower collision energy for . The magnitude and shape of the suppression are consistent with pQCD predictions at NLO using nPDFs that incorporate nuclear shadowing effects in the Pb nucleus.