Probing QCD via dijet invariant mass in pp and p–Pb collisions and performance of forward jets with ALICE FoCal

Jets, collimated sprays of particles emerging from hard parton processes, are powerful probes of the strong interaction and of nuclear matter under extreme conditions at the CERN Large Hadron Collider (LHC). In this thesis, I present two studies of jets in the ALICE experiment at the LHC. The first analysis reports an updated measurement of the invariant mass of charged dijets in proton-proton and proton-lead collisions at a nucleon-nucleon center-of-mass energy of √ sNN = 5.02 TeV, using Run 2 data. I determine the dijet mass spectra and the nuclear modification factor RpA in the mass range of 75 − 150 GeV/c2. The results, extending the earlier work presented in Ref. [1], are compared with predictions from Monte Carlo simulations of PYTHIA and POWHEG+PYTHIA. While the measured RpA is found to be consistent with unity within experimental uncertainties, it lies systematically slightly above unity. Simulations suggest that this enhancement may result from dijets probing the anti-shadowing region of the nuclear PDFs of lead. While the current measurement uncertainties are too large for a definite conclusion, future studies with Run 3 data can provide greater sensitivity. This measurement also establishes a baseline for corresponding future analyses in Pb–Pb collisions. The second analysis is the study of jet performance in the upcoming ALICE Forward Calorimeter (FoCal) upgrade. Using data and detector simulations, the achievable transverse momentum range as well as the jet energy scale and resolution are quantified. The feasibility of forward jet, dijet and photon-jet measurements is demonstrated. Such measurements will enable FoCal to probe the gluon saturation regime of QCD matter at low Bjorken-x down to x ∼ 10−6. In addition, results from test beam campaigns for the FoCal prototype are presented, validating the detector concept and performance in preparation for the construction and installation of FoCal for data taking in Run 4.