Jets in proton-proton, proton-lead and lead-lead collisions at the CERN Large Hadron Collider

This thesis aims to understand the substructure of charged jet in proton-proton and lead-lead collisions. The topics covered in this thesis are as follows, 1. Charged particles of jets are measured in pp collisions at = 5.02 TeV with the ALICE experiment at CERN, Geneva. FastJet package is used to use the final state particle to obtain the inital parton information i.e jet finding. Anti-K T algorithm is used to reconstruct jets with resolution parameter R=0.2, 0.4 and E-scheme. Minimum track p T of 0.15 GeV/c and |η|<0.9 are used as constituents cut and jet acceptance is considered as |η| < 0.7 for R=0.2 and |η| < 0.5 for R=0.4. Three jet shape observables i.e. Angularity (g), Dispersion (p T D), LeSub have been studied in this analysis. The angularity that measures the radial energy profile of the jet, signifies whether jets are collimated or broadened. The dispersion tells how hard or soft the fragmentation is. LeSub describes the hardest splitting, therefore it should not be sensitive to the background. Two dimensional Bayesian unfolding has been used in an iterative way to remove the detector effects on the jet shape variables and to get the corrected observables. To test the stability of the unfolding procedure, a closure test has also been performed where, the unfolding input is filled with Monte Carlo(MC) information. The systematic errors are then estimated for different sources and added in quadrature. Finally, the fully corrected results are shown with model comparison. 2. A phenomenological study of the medium modified jet shape observables in Pb-Pb collisions at = 2.76 TeV using EPOS-3 and JEWEL event generators has been presented. We investigate the in-medium modification to the two jet shape observables i.e., the differential jet shape (ρ(r)) and the angularity (g) in the jet-p T range of 20-40 GeV/c. JEWEL with recoil OFF has been used primarily as a reference system as that has been found to explain the global jet observables satisfactorily but lacks in jet-shape variables at the higher jet-radii. EPOS-3 that explains the bulk properties in such collisions quite well takes into account a hydrodynamically evolving bulk matter, jets and hard-soft interactions. A comparison between the results from these models shows that while JEWEL (recoil OFF) does not explain the distribution of lost energy at higher radii with respect to the jet-axis, EPOS-3 explains the effect quite well. However, in EPOS-3, the implemented partonic energy loss mechanism and secondary hard-soft interactions during hadronization and hadronic cascade phase are different from the conventional jet energy loss models. The current study can, therefore, provide important new insights on mechanisms regarding the modeling of the medium and hard-soft interactions in heavy ion collisions.