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 $$\sqrt{s_{\text {NN}}}={5.02}\,\textrm{TeV}$$ s NN = 5.02 TeV and 8.16 TeV, as well as in pp collisions at $$\sqrt{s}={5.02}\,\textrm{TeV}$$ s = 5.02 TeV and 8 TeV. The cross section is obtained at midrapidity $$(|y|<0.7)$$ ( | y | < 0.7 ) using a charged-track based isolation momentum $$p_\textrm{T}^{\text {iso,~ch}}<{1.5}\,\textrm{GeV}/c$$ p T iso, ch < 1.5 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 $$R_{\text {pA}}$$ R pA for both collision energies is consistent with unity for $$p_{\textrm{T}}$$ p T $$>{20}\,\textrm{GeV}/c$$ > 20 GeV / c . However, deviations from unity ( $$R_\textrm{pA}<1$$ R pA < 1 ) of up to 20% are observed for $$p_{\textrm{T}}$$ p T $$<{20}\,\textrm{GeV}/c$$ < 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 $$p_{\textrm{T}}$$ p T with a s