Measurement of isolated prompt photon production in pp and p-Pb collisions at the LHC

Abstract

This paper presents the measurement of the isolated prompt photon inclusive production cross section in pp and p-Pb collisions by the ALICE Collaboration at the LHC. The measurement is performed in p-Pb collisions at centre-of-mass energies per nucleon pair of $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV and $8.16$ TeV, as well as in pp collisions at $\sqrt{s}=5.02$ TeV and $8$ TeV. The cross section is obtained at midrapidity $(|y|<0.7)$ using a charged-track based isolation momentum $p_{\mathrm{T}}^{\mathrm{iso},\mathrm{ch}}<1.5$ GeV/$c$ in a cone with radius $R=0.4$. The data for both collision systems are well reproduced by perturbative QCD (pQCD) calculations at next-to-leading order (NLO) using recent parton distribution functions for free (PDF) and bound (nPDF) nucleons. Furthermore, the nuclear modification factor $R_{\text{pA}}$ for both collision energies is consistent with unity for $p_{\mathrm{T}}>20$ GeV/$c$. However, deviations from unity ($R_{\mathrm{pA}}<1$) of up to 20% are observed for $p_{\mathrm{T}}<20$ GeV/$c$ with limited significance, indicating the possible presence of nuclear effects in the initial state of the collision. The suppression increases with decreasing \pt with a significance of $2.3\sigma$ for a non-zero slope and yields $R_{\mathrm{pA}}<1$ with a significance of $1.8\sigma$ at $\sqrt{s_{\mathrm{NN}}}=8.16$ TeV for $p_{\mathrm{T}}<20$ GeV/$c$. In addition, a significance of $1.1\sigma$ is observed for $R_{\mathrm{pA}}<1$ at the lower collision energy $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV for $p_{\mathrm{T}}<14$ GeV/$c$. The magnitude and shape of the suppression are consistent with pQCD predictions at NLO using nPDFs that incorporate nuclear shadowing effects in the Pb nucleus.

29 pages, 6 captioned figures, 2 tables, authors from page 23, published version, figures at http://alice-publications.web.cern.ch/node/11890