Abstract
The inclusive production rates of isolated, prompt photons in p+Pb collisions at sNN=8.16 TeV are studied with the ATLAS detector at the Large Hadron Collider using a dataset with an integrated luminosity of 165 nb−1 recorded in 2016. The cross-section and nuclear modification factor RpPb are measured as a function of photon transverse energy from 20 GeV to 550 GeV and in three nucleon–nucleon centre-of-mass pseudorapidity regions, (−2.83,−2.02), (−1.84,0.91), and (1.09,1.90). The cross-section and RpPb values are compared with the results of a next-to-leading-order perturbative QCD calculation, with and without nuclear parton distribution function modifications, and with expectations based on a model of the energy loss of partons prior to the hard scattering. The data disfavour a large amount of energy loss and provide new constraints on the parton densities in nuclei.
Highlights
Measurements of particle and jet production rates at large transverse energy are a fundamental method of characterising hard-scattering processes in all collision systems
The data are compared with an NLO perturbative QCD (pQCD) calculation similar to that used in Ref. [3], where the data is underestimated
√prsoNmN p=t, 8.16 TeV, using a dataset corresponding to an integrated luminosity of 165 nb−1 recorded by the ATLAS experiment at the LHC
Summary
Measurements of particle and jet production rates at large transverse energy are a fundamental method of characterising hard-scattering processes in all collision systems. In collisions involving large nuclei, production rates are modified from those measured in proton + proton (pp) collisions due to a combination of initial- and final-state effects The former arise from the dynamics of partons in the nuclei prior to the hard-scattering process, while the latter are attributed to the strong interaction of the emerging partons with the hot nuclear medium formed in nucleus–nucleus collisions. Measurements of prompt photon production rates offer a way to isolate the initial-state effects because the final-state photons do not interact strongly These initial-state effects include the degree to which parton densities are modified in a nuclear environment [1,2,3], as well as potential modification due to an energy loss arising through interactions of the partons traversing the nucleus prior to the hard scattering [4,5]. The data are compared with predictions from a model including initial-state energy loss [4,5,26]
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