UC Berkeley

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 sNN=5.02TeV$$\sqrt{s_{\text {NN}}}={5.02}\,\textrm{TeV}$$ and 8.16 TeV, as well as in pp collisions at s=5.02TeV$$\sqrt{s}={5.02}\,\textrm{TeV}$$ and 8 TeV. The cross section is obtained at midrapidity (|y|<0.7)$$(|y|<0.7)$$ using a charged-track based isolation momentum pTiso, ch<1.5GeV/c$$p_\textrm{T}^{\text {iso,~ch}}<{1.5}\,\textrm{GeV}/c$$ in a cone with radius R=0.4$$R=0.4$$. 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 RpA$$R_{\text {pA}}$$ for both collision energies is consistent with unity for pT$$p_{\textrm{T}}$$>20GeV/c$$>{20}\,\textrm{GeV}/c$$. However, deviations from unity (RpA<1$$R_\textrm{pA}<1$$) of up to 20% are observed for pT$$p_{\textrm{T}}$$<20GeV/c$$<{20}\,\textrm{GeV}/c$$ with limited significance, indicating the possible presence of nuclear effects in the initial state of the collision. The suppression increases with decreasing pT$$p_{\textrm{T}}$$ with a significance of 2.3σ$$2.3\upsigma $$ for a non-zero slope and yields RpA<1$$R_{\textrm{pA}}<1$$ with a significance of 1.8σ$$1.8\upsigma $$ at sNN=8.16TeV$$\sqrt{s_{\textrm{NN}}}={8.16}\,\textrm{TeV}$$ for pT$$p_{\textrm{T}}$$<20GeV/c$$<{20}\,\textrm{GeV}/c$$. In addition, a significance of 1.1σ$$1.1\upsigma $$ is observed for RpA<1$$R_{\textrm{pA}}<1$$ at the lower collision energy sNN=5.02TeV$$\sqrt{s_{\textrm{NN}}}={5.02}\,\textrm{TeV}$$ for pT<14GeV/c$$p_{\textrm{T}} < {14}\,\textrm{GeV}/c$$. 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.