Dijet invariant mass spectrum in pp and p-Pb collisions at √sNN=5.02 TeV

Abstract

In this thesis, I present the dijet invariant mass analysis in proton–proton and proton–lead collisions at the center-of-mass energy per nucleon of 5.02 TeV. The measurement of the data was conducted with the ALICE detector, which is one of the four experiments along the CERN Large Hadron Collider accelerator, which is the largest particle collider in the world. The dijets were reconstructed from charged tracks in the ALICE central detectors in minimum bias proton–proton (pp) and proton–lead (p–Pb) collisions. I reconstructed the jets using anti-<i>k</i>_T algorithm with radius parameter <i>R</i> = 0.4 implemented by the FastJet [1] package and subtracted the underlying event background with the 4-momentum background subtraction method. A notable new analysis technique I developed for this analysis is the correction of the background fluctuations for the dijet invariant mass spectrum. I unfolded the measured distributions for background fluctuations together with the detector response in a similar way as in earlier jet pT cross section measurements. I determined the nuclear modification factor <i>R</i>_pA from the dijet invariant mass cross sections in proton–proton and proton–lead collisions. The final statistical and systematic uncertainties remained at 10–20%. The <i>R</i>_pA is compatible with no modifications from the cold nuclear matter effects within the uncertainties. This is the first measurement of the dijet invariant mass involving a heavy ion environment. I used Pythia and POWHEG+Pythia MC event generators to calculate the dijet invariant mass with nuclear parton distribution functions (nPDF), which show a slight enhancement in <i>R</i>_pA. According to nPDFs sets of EPPS21 [2], the hard parton momentum fraction distributions of the dijet events could be attributed to the anti-shadowing domain, which is x ∼ 0.01–0.1 for gluons. However, the final uncertainties in the measurement are too large to make definite conclusions. Keywords: jet, dijet, invariant mass, heavy ion, p–Pb, ALICE, CERN, LHC