Prompt photon production and photon-jet correlations at the LHC

Abstract

Next-to-leading order predictions matched to parton showers are compared with recent ATLAS data on isolated photon production and CMS data on associated photon and jet production in pp and pPb collisions at different centre-of-mass energies of the LHC. We find good agreement and, as expected, considerably reduced scale uncertainties compared to previous theoretical calculations. Predictions are made for the ratio of inclusive photons over decay photons R γ , an important quantity to evaluate the significance of additional photon sources, e.g. thermal radiation from a Quark-Gluon-Plasma, and for distributions in the parton momentum fraction in lead ions x obs Pb , that could be determined by ALICE, ATLAS, CMS and LHCb in ongoing analyses of photon+jet production in pPb collisions at √ s N N = 5.02 TeV. These data should have an important impact on the determination of nuclear effects such as shadowing at low x.

Prompt photon-jet angular correlations at central rapidities in p+A collisions

“…6 in Refs. [14]. This may potentially be related to high gluon density effects of the kind considered here, especially if confirmed by more symmetric configurations with smaller Q ⊥ (say, by going to lower k γ ⊥ > 30 GeV).…”

“…[12]. References [13,14] considered photon-jet production in collinear factorization at NLO supplemented by a parton shower generator. Lastly, such angular correlations from qq → γg annihilation in pp collisions have been discussed in Ref.…”

Direct photon production in pp, p-Pb and Pb-Pb collisions: results from ALICE

“…1. The direct photon excess at high p T (> 3 − 4 GeV) is described by NLO pQCD calculations [5]. At both collision energies no thermal photon excess at low-p T region is visible within uncertainties.…”

Photon-jet correlations at RHIC and the LHC

“…While the former suffer from large theoretical scale uncertainties, the latter have only up to three partons in the final state and thus insufficient detail to describe all of the experimental observables. We have recently combined NLO calculations with parton showers (PS) for photon production using the POWHEG method [4] and applied them to data from RHIC [5] and the LHC [6].…”

“…Differential distribution in the observed parton momentum fraction in lead ions in pPb collisions at the LHC[6].…”