Study of isolated prompt photon production in p−Pb collisions for the ALICE kinematics

Abstract

Prompt photon production is known as a powerful tool for testing perturbative QCD predictions and also the validity of parton densities in the nucleon and nuclei especially of the gluon. In this work, we have performed a detailed study on this subject focusing on the isolated prompt photon production in p-Pb collisions at forward rapidity at the LHC. The impact of input nuclear modifications obtained from different global analyses by various groups on several quantities has been investigated to estimate the order of magnitude of the difference between their predictions. We have also studied in detail the theoretical uncertainties in the results due to various sources. We found that there is a remarkable difference between the predictions from the nCTEQ15 and other groups in all ranges of photon transverse momentum p γ T . Their differences become more explicit in the calculation of the nuclear modification ratio and also the yield asymmetry between the forward and backward rapidities rather than single differential cross sections. We emphasize that future measurements with ALICE will be very useful not only for decreasing the uncertainty of the gluon nuclear modification, but also to accurately determine its central values, especially in the shadowing region.

Probing nuclear modifications of parton distribution functions through the isolated prompt photon production at energies available at the CERN Large Hadron Collider

“…Therefore, in inverse, the accurate measurements of some particle production cross sections in nuclear collisions can bring us new useful information to judge about the behavior of nuclear modifications in different kinematic regions and also decrease their uncertainties. The measurements of the prompt photon production is one of the most useful ones to obtain information about the gluon nuclear modifications [34][35][36][37][38][39], since the gluon directly contribute to it through the Compton scattering q(q)g → γq(q) subprocesses at the Born-level.…”

“…From past to present, several QCD global analyses of nPDFs have been done which are different from various aspects such as the amount and kind of the experimental data included, the perturbative order of the analysis, the parametrization of the nuclear modifications, the method for calculating uncertainties and treatment of the heavy quark masses (for review see Refs. [39,70]). In most of the analyses performed so far, like the HKN07 [24], EPS09 [26], DSSZ [27], KA15 [29] and EPPS16 [30] analyses, the bound-proton PDFs have been defined in terms of nuclear modification factors R A i that characterize the relation of the PDFs of a proton inside a nucleus with atomic number A, f p/A i , with respect to free-proton PDFs f p i ,…”

“…Recently, EPPS16 [30] has also included, for the first time, the fixed-target DY data in pion-nucleus collisions and new LHC proton-lead (p-Pb) data on dijet and heavy gauge-boson production. There are also some studies show that important information about nPDFs can be achieved by analyzing the prompt photon production in nuclear collisions [34][35][36][37][38][39], jet and dijet photoproduction measurements at a future electron-ion collider (EIC) [40,41], single inclusive jet production at very forward rapidity [42], and heavy-flavor production in p-Pb collisions [43].The main reason for such efforts to include more accurate experimental data as much as possible in the global analysis of nPDFs is to achieve more valid nuclear modifications of PDFs with less uncertainties. In fact, most of experimental data that are used in the nPDFs analyses can just put well constraints on the quark nuclear modifications at fairly large values of x, while the sea quarks and gluon distributions cannot be controlled as well, especially at smaller values of x.…”

Probing cold nuclear matter effects with the productions of isolated- ${\gamma} $ and ${\gamma} $+jet in p+Pb collisions at ${\sqrt{{s}_{{NN}}}}= $ 8.16 TeV

“…Note the nuclear modification factor is sensitive to the nucleon PDF(nPDF) and p+p baseline [52], we may calculate the ratio of the photon production at forward and backward rapidity, which could get rid of the large uncertainty in free nucleon PDFs, which could be used to probe the CNM effects with less arbitrariness [12][13][14]. We define the forward-backward yield asymmetry as:…”

“…In high energy nuclear physics, productions of prompt photon and photon associated jet with high transverse momentum are valuable observations of the shortdistance dynamics of quarks and gluons [1]. Since prompt photon is precisely calculable by perturbative quantum chromodynamics (pQCD) at higher order corrections and carries no color charge like other gauge bosons, it has been widely regarded as an optimal probe of the initial state of the collisions [2][3][4][5][6], as well as an excellent tag of inversive parton (jet) to quantify the mechanisms of jet quenching in ultra-relativistic heavy ion collisions [7][8][9][10][11][12][13][14]. In the past few years, ATLAS [15][16][17] and CMS [18][19][20] collaborations have made lots of measurements on isolated prompt photon and γ+jet productions in proton-proton, proton-nucleus and nucleus-nucleus collisions.…”