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

Abstract

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 root s(NN) = 5.02 TeV and 8.16 TeV, as well as in pp collisions at root s = 5.02 TeV and 8 TeV. The cross section is obtained at midrapidity (|y| 20 GeV/c. However, deviations from unity (R-pA < 1) of up to 20% are observed for p(T) < 20 GeV/c with limited significance, indicating the possible presence of nuclear effects in the initial state of the collision. The suppression increases with decreasing p(T) with a significance of 2.3 sigma for a non-zero slope and yields R-pA < 1 with a significance of 1.8s at root s(NN) = 8.16 TeV for pT < 20 GeV/c. In addition, a significance of 1.1 sigma is observed for R-pA < 1 at the lower collision energy root s(N)N = 5.02 TeV for p(T) < 14 GeV/c. 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.