CERN Accelerating science

Heavy-ion collisions at ultra-relativistic energies provide sufficient energy densities and/or temperature that allow to transform the confined hadronic matter into a deconfined state of quarks and gluons, commonly known as quark-gluon plasma (QGP). Jet quenching or partonic energy loss in the medium is one of the most important signatures in favour of the existence of QGP in heavy-ion collisions. Jets are cascades of energetic hadrons that result from the fragmentation of hard-scattered quarks and gluons in high-energy collisions. Recent measurements in high-multiplicity proton-proton (pp) collisions show intriguing QGP-like signals which tell that the pp collisions can not be treated as elementary collisions. This brings to the table the possibility of QGP formation in pp collisions as an open question that requires to be addressed and investigated further. In view of this, intra-jet properties such as jet shape and fragmentation functions are promising observables since they are more sensitive to the details of the parton shower and hadronization processes. This thesis presents the first measurement of the multiplicity dependence of intra-jet properties of leading charged-particle jets, the mean charged-particle multiplicity and fragmentation functions in pp collisions at $\sqrt{s}$ = 13 TeV using the ALICE detector. Jets are reconstructed from charged particles produced at midrapidity ($|\eta| < 0.9$) using the sequential recombination anti-$k_{\rm T}$ algorithm with jet resolution parameters $R$ = 0.2, 0.3, and 0.4 for the transverse momentum ($p_{\rm T}$) interval 5-110 GeV/c. A significant modification in jet fragmentation functions in high-multiplicity (HM) events is observed compared to minimum-bias events. The results are compared with predictions from QCD-inspired event generators and PYTHIA 8 qualitatively reproduces the data. A detailed PYTHIA 8 study shows that multiparton interactions and contributions from gluon-initiated jets play significant roles in HM events. This thesis also reports the estimation of jet transport coefficient, $\hat{q}$, for both quark- and gluon-initiated jets in presence and absence of magnetic field using a quasi-particle model. In presence of a finite magnetic field, a significant enhancement in $\hat{q}$ is found for both quark- and gluon-initiated jets at low temperatures, which gradually decreases towards high temperatures.