Probing gluon and heavy-quark nuclear PDFs with γ + Q production in pA collisions

Abstract

We present a detailed phenomenological study of direct photon production in association with a heavy-quark jet in pA collisions at the Relativistic Heavy Ion Collider (RHIC) and at the Large Hadron Collider (LHC) at next-to-leading order in QCD. The dominant contribution to the cross-section comes from the gluon-heavy-quark (gQ) initiated subprocess, making γ + Q production a process very sensitive to both the gluon and the heavy-quark parton distribution functions (PDFs). Additionally, the RHIC and LHC experiments are probing complementary kinematic regions in the momentum fraction x 2 carried by the target partons. Thus, the nuclear production ratio R γ+Q pA can provide strong constraints, over a broad x-range, on the poorly determined nuclear parton distribution functions which are extremely important for the interpretation of results in heavy-ion collisions.

The impact of the intrinsic charm quark content of a proton on the differential $\gamma +c$ cross section

“…In this way, the photon component includes contributions from gg → γcc, gc → γgc, cq → γqc, cq → γqc, cc → γcc, cc → γcc and the annihilation sub-process qq → γcc [59,60] , which apart from qq → γcc, all sub-processes are g and c PDF initiated. At the LHC, the Compton process dominates for all energies, whereas at the Tevatron the annihilation process qq → γcc dominates for photons with high transverse momentum p γ T [59].…”

Nuclear PDFs in the beginning of the LHC era

“…For instance, different rapidity and transverse momentum distributions of charged leptons from electroweak gauge boson decays have been theoretically explored [45,46,47,48,49] and promise to probe various aspects of nPDFs. Direct photons, inclusive hadrons and jet observables [50,51,52,53,54,55] have been predicted to retain sensitivity to the nuclear modifications in PDFs as well. However, the first hard QCD-process data set from the 2013 proton+lead run that can be directly compared to the nPDF predictions at the NLO level is the shape of the pseudorapidity distribution of dijets measured by the CMS collaboration [56].…”

Prompt photon production and photon-jet correlations at the LHC

“…They allow, e.g., to determine the strong coupling constant (α s ) [1] or the partonic structure of protons, photons [2] and nuclei [3][4][5], and they can serve as a thermometer of the hot Quark-Gluon-Plasma (QGP) created in heavy-ion collisions [6,7]. If produced in association with a hadronic jet, they allow to calibrate the transverse momentum of the latter at its creation and to deduce its subsequent hadronic energy loss or azimuthal decorrelation in the medium [8].…”