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

Abstract

We compute the leading order (LO) $qg\to q\gamma$ and next-to-leading order (NLO) $gg\to q\bar{q}\gamma$ 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 $k_{\gamma \perp }$ corresponding to momentum fractions of $x\le 0.01$ 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 $k_{\perp }$-dependent unintegrated gluon distributions in both of the protons. We predict a maximal $10\%$ modification of the cross section at low $k_{\gamma \perp }$ as a direct consequence of the violation of $k_{\perp }$-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 $k_{\gamma \perp }\le 20$ GeV in p+p collisions that can be tested in the future at the LHC.

9 pages, 5 figures

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