Constraining unintegrated gluon distributions from inclusive photon production in proton-proton collisions at the LHC

We compute the leading order (LO) qg -> q gamma and next-to-leading order (NLO) gg -> q (q) over bar gamma contributions to inclusive photon production in proton-proton (p+p) collisions at the LHC. These channels provide the dominant contribution at LO and NLO for photon transverse momenta k(gamma perpendicular to) corresponding to momentum fractions of x <= 0.01 in the colliding protons. Our computations, performed in the dilute-dense framework of the Color Glass Condensate effective field theory (CGC EFT), show that the NLO contribution dominates at small-x because it is sensitive to k(perpendicular to)-dependent unintegrated gluon distributions in both of the protons. We predict a maximal 10% modification of the cross section at low k(gamma perpendicular to) as a direct consequence of the violation of k(perpendicular to)-factorization. The coherence effects responsible for this modification are enhanced in nuclei and can be identified from inclusive photon measurements in proton-nucleus collisions. We provide numerical results for the isolated inclusive photon cross section for k(gamma perpendicular to) <= 20GeV in p+p collisions that can be tested in the future at the LHC. (c) 2019 The Authors. Published by Elsevier B.V.