# Measurement of prompt photon production in $\sqrt{s_\mathrm{NN}} = 8.16$ TeV $p$+Pb collisions with ATLAS

## TL;DR

This paper measures isolated prompt photon production in proton-lead collisions at 8.16 TeV using ATLAS, comparing results with QCD predictions and models of parton energy loss to improve understanding of nuclear parton distributions.

## Contribution

It provides the first detailed measurement of prompt photon production in p+Pb collisions at this energy, constraining nuclear parton distribution functions and energy loss models.

## Key findings

- Data disfavors large parton energy loss prior to scattering.
- Results constrain nuclear parton distribution functions.
- Photon production cross-sections agree with NLO QCD predictions without large energy loss.

## Abstract

The inclusive production rates of isolated, prompt photons in $p$+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, (-2.83,-2.02), (-1.84,0.91), and (1.09,1.90). 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.