CERN Accelerating science

Heavy-ion collisions at ultra-relativistic energies produce a deconfined state of partonic matter, known as quark-gluon plasma (QGP). Small collision systems, previously thought not to form QGP, show collectivity at the LHC energies, challenging the conventional notion, albeit no observation of jet production suppression, a prominent signature of QGP formation. Study of internal jet properties and their multiplicity dependence in small collision systems, i.e., proton-proton (pp) and proton-lead (p-Pb) collisions, presents a promising approach, given their sensitivity to both the perturbative and non-perturbative aspects of quantum chromodynamics. This thesis reports the measurements of jet properties, including mean number of charged-particle multiplicity within jets and jet fragmentation function in minimum bias pp and minimum bias and central p-Pb collisions at the nucleon-nucleon center-of-mass energy of 5.02 TeV using the ALICE experiment at CERN, for charged-particle leading jets in the transverse momentum ($p_{\rm T}$) interval of 20-100 GeV/c. Jets are reconstructed from charged particles at mid-rapidity using the sequential recombination anti-$k_{\rm T}$ algorithm with jet resolution parameter $R$ = 0.4. The corrected distributions (corrected for instrumental effects and underlying events) are presented in comparison to various theoretical MC model predictions. Jet $p_{\rm T}$-dependent modifications of jet fragmentation are observed between minimum bias p-Pb and pp collisions and between central and minimum bias p-Pb collisions, marking the first experimental observation of jet modification in p-Pb collisions at LHC energies. Surprisingly, MC models without implementation of QGP-medium effects, are also found to qualitatively reproduce the features of jet modification observed in data. This thesis also presents the study of internal jet properties, including jet shape and fragmentation function in minimum bias and high-multiplicity pp collisions at the center-of-mass energy of 13 TeV using PYTHIA 8 Monte Carlo event generator. The study is performed in presence and absence of multiparton interactions and color reconnection mechanisms of PYTHIA. Significant jet modification is observed in high-multiplicity events compared to minimum bias ones for low-$p_{\rm T}$ jets. The main sources contributing to the observed jet modification are found to be multiparton interactions, color reconnection and increase in the fraction of gluon-initiated jets in high-multiplicity events.