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

Abstract We compute the leading order (LO) q g → q γ and next-to-leading order (NLO) g g → q q ¯ γ 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 γ ⊥ 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 ⊥ -dependent unintegrated gluon distributions in both of the protons. We predict a maximal 10% modification of the cross section at low k γ ⊥ as a direct consequence of the violation of k ⊥ -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 γ ⊥ ≤ 20 GeV in p+p collisions that can be tested in the future at the LHC.