CERN Accelerating science

Prompt photons are an exciting and robust probe for the initial state of nuclear collisions, in particular the momentum distribution of gluons bound inside the colliding proton. Measurements of prompt photon production in p-Pb collisions can offer insights into cold nuclear matter effects such as gluon shadowing, as well as non-linear QCD effects such as gluon saturation when measured at forward rapidities. They offer these insights in a robust way, mostly unaffected by final state effects such as fragmentation or interactions with a hot or cold medium. This thesis presents the measurement of the isolated prompt photon production cross section in pp and p-Pb collisions at $\sqrt{s}=8$ TeV and 8.16 TeV, respectively. The measurement is performed using data recorded by the ALICE experiment, where in particular the EMCal is used to measure photons with pseudorapidities of $|\eta|<0.7$. The production cross section is measured in both collision systems for $12$ GeV$/c\leq p_{T}\leq$80 GeV$/c$ and confronted with pQCD calculations at NLO using the JetPhox program. Good agreement between the theoretical calculations and experimental data is observed in both collision systems within the theoretical and experimental uncertainties, showcasing the ability of NLO calculations to adequately describe the experimental data in the inspected phasespace. In order to quantify possible modifications of the partonic substructure in nuclear environments, e.g. due to the presence of the gluon shadowing, the so-called nuclear modification factor $R_{\text{pA}}$ is constructed. While the data is found to be in agreement with unity for the whole covered transverse momentum within the experimental uncertainties, a hint of a suppression is observed for photons with $p_{T}\lesssim$20 GeV/c. This is the first time the isolated prompt photon $R_{\text{pA}}$ is presented for transverse momenta below 25 GeV/c -- extending the low momentum reach of a previous measurement by the ATLAS collaboration in the same collision system by a factor of two. The measurement is in good agreement with the theoretical predictions, which also indicate an increasing suppression of the prompt photon production for $p_{T}\lesssim$20 GeV/c that is commonly attributed to gluon shadowing. Finally, detailed simulation studies for a future Forward Calorimeter (FoCal) are presented, which showcase the capabilities of the FoCal detector to perform isolated prompt photon measurements down to unprecedented low Bjorken-$x$ of $x\sim10^{-6}$.