Jet Fragmentation via Recombination of Parton Showers and Its Medium Modification in Heavy Ion Collisions

For jets produced in relativistic heavy ion collisions, the presence of the quark-gluon plasma (QGP) formed in the collisions not only affects their energies but is also expected to affect their hadronization. In this dissertation, we have studied the influence of the QGP on jets by treating the jet as shower partons, which are then converted to hadrons via their recombination among themselves as well as with the thermal partons in the quark-gluon plasma. To verify the feasibility of treating the hadronization of energetic jets by recombination of shower partons, we have computed hadron spectra in e^+ + e^− collisions. Including contributions from resonance decays and from the string fragmentation of the remnant partons left after the recombination by using a subroutine of PYTHIA, we have found that the resulting spectra of longitudinal momentum fractions and transverse momenta for pion, kaon, nucleon, and Λ reproduce reasonably those from the string fragmentation obtained in PYTHIA. The medium effect on the conversion of jets to hadrons is then studied for heavy ion collisions at both RHIC and LHC by using the blast-wave model for generating the thermal partons in the QGP. In obtaining the shower partons from jets, we have included their energy loss due to interactions with thermal partons in QGP. We have found that including hadron production from the recombination of shower partons with thermal partons leads to a significant enhancement in the production of hadrons with intermediate transverse momenta at both RHIC and LHC. Our results show that the recombination model can reproduce well the experimental measurements conducted at both RHIC and LHC, thus suggesting the importance of medium modifications in the conversion of jets to hadrons in the presence of a QGP