Multi-Scale Modification Of Jets In Dense Media, From Single Emission To Full Simulation

The Quark-Gluon Plasma, an extreme state of matter at temperatures above 2 Trillion K, is now regularly produced in relativistic heavy-ion collisions. High transverse momentum ($p_T$) partons (quarks and gluons), produced in the collision, with energies far above the temperatures of the QGP, lose their energy while traversing the QGP via scattering and emissions. Parton energy loss for these hard partons is studied in Deep inelastic scattering (DIS) of an electron on a large nucleus, where the produced quark traverses the cold nuclear medium. We present a complete reanalysis of the next-to-leading twist single gluon emission rate. The contributions from a power expansion of the phase terms, $N_c$ suppressed terms, and finite gluon momentum fraction terms, which were neglected in previous efforts, are now included prior to the collinear expansion. These additional terms introduce a new momentum fraction dependence to the medium-modified kernel (thereby changing the splitting function in a medium). In addition, the medium-modified kernel is discovered to be positive-definite and increasing with path length. The medium modified DGLAP evolution equation was studied in greater detail by using the jet function and a proposed di-prong function. A partially developed framework to study jet modification, with and without grooming, using the factorized jet function and the di-prong function, which can be evolved using the DGLAP evolution equation, is discussed. The importance of the high virtuality stage of the partonic shower was investigated by means of a multistage event generator. The concept of switching virtuality, which is used to switch between the high and low virtuality stages in a multistage event generator, is explored using the vacuum and medium-modified splitting functions. The importance of multistage evolution is demonstrated using the JETSCAPE framework. A variety of observables associated with a range of collision energies and centralities were shown to be explained without having to re-tune any parameter. The multi-stage framework yields a satisfactory and simultaneous description of a wide variety of observables in heavy-ion collisions, especially jet based observables, thereby establishing a baseline from where further theoretical improvements can be systematically studied