Jets correlated with isolated photons are a promising channel to study jet quenching in heavy-ion collisions, as photons do not participate in the strong interaction and therefore constrain the Q^2 of the initial hard scattering. We present isolated photon-jet correlations measured in Pb–Pb collisions at sqrt(s_NN) = 5.02 TeV by the ALICE collaboration. We study correlations of isolated photons with 28 < pT < 40 GeV/c with charged-particle jets with pT > 10 GeV/c reconstructed with the anti-kT algorithm and report the azimuthal correlation and pT asymmetry. The correlations probe the lowest jet pT range ever measured at LHC energies, and larger modifications due to the QGP are expected in the lower pT regime. We see no trend in the away-side yield with centrality, but do see a change in the distribution of the pT asymmetry consistent with increasing jet energy loss in more central collisions.

Prompt photons, produced via QCD Compton scattering or quark and anti-quark annihilation at leading order, are unique probes to study QCD processes since they do not interact strongly and therefore allow to control the dynamics of the initial hard parton scattering. Isolated photon production in pp collisions is one of the most clear tests of perturbative QCD processes and parton distribution functions. Photon measurement in p–A collisions provides the opportunity to measure possible modifications of the nucleon structure function in nuclei. ALICE has measured isolated photons at low pT, thus extending previous measurements down to small x. In this thesis, the cross section of isolated photons for the range 12 GeV/c < pT < 60 GeV/c is presented in pp and p–Pb collisions at √sNN = 5.02 TeV along with photon nuclear modification factor (RpPb).

This work presents the measurement of isolated photon-hadron correlations and the first study of photon-tagged fragmentation in p–Pb at the Large Hadron Collider using pp and p–Pb data collected by the ALICE detector. Prompt photons produced at leading order in hard scatterings constrain the kinematics of the recoiling parton, enabling the study of parton energy loss and modification to the parton fragmentation function. For photons with |η| < 0.67 and 12 < pT < 40 GeV/c, the associated yield of charged particles in the previously unexplored kinematic range of 0.5 < pT < 8 GeV/c is measured. No significant difference between pp and p–Pb is observed. Pythia 8.2 and cold nuclear matter theoretical models can describe both data sets within uncertainties, setting constraints on cold nuclear matter effects on the parton fragmentation in p–Pb collisions.

Jet substructure observables are powerful tools to search for new physics and test theoretical descriptions of perturbative and non-perturbative processes in QCD. In heavy-ion collisions, jet substructure observables are used to elucidate the structure and dynamics of the quark-gluon plasma. One substructure observable is jet mass, which probes the virtuality of hard-scattered partons and their modified fragmentation. Additionally, generalized jet angularities allow differential measurements of the jet shower and its modification, as two parameters independently vary the weight of the jet constituents’ relative angle and transverse momentum. Previous measurements of the jet mass and jet angularities show conflicting deviations when compared with models. This thesis presents new measurements of the jet mass and jet angularities to resolve this conflict, using charged-particle tracks in pp and Pb-Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV and jet resolution parameters $R = 0.2$ and $0.4$. The results from this work are compared to ALICE measurements of heavy-flavor jets, which provide a high-powered probe of perturbative QCD at low transverse momentum. Jet angularities in jets containing a charm meson are compared to the inclusive measurements of this thesis, revealing a significant narrowing due to the QCD dead cone modifying jet fragmentation. Jet angularity results are also compared to QCD predictions using both folding and shape function corrections for nonperturbative effects. Jet grooming can be used to isolate specific splittings inside the jet fragmentation history, and ALICE measurements of the groomed-jet splitting angle and momentum fraction with soft drop and dynamical grooming are also compared to QCD predictions to test parton branching. The high-precision tracking system of ALICE enables these measurements over a broad range in transverse momentum, with a low-transverse momentum reach that is unique at the LHC.