Measurement of prompt photon production in TeV + Pb collisions with ATLAS

The inclusive production rates of isolated, prompt photons in $p+\mathrm{Pb}$ collisions at $\sqrt{{s}_{\mathrm{NN}}}=8.16$ 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 ${R}_{p\mathrm{Pb}}$ 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, $\left(-2.83,-2.02\right)$, $\left(-1.84,0.91\right)$, and $\left(1.09,1.90\right)$. The cross-section and ${R}_{p\mathrm{Pb}}$ 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.