Jet Evolution In Medium

Quantum chromodynamics (QCD) is the field theory based on a non-abelian gauge group SU(3). At present QCD is well-established as the theory of strong interaction. QCD has two elementary and important properties: asymptotical freedom and confinement. The first property allows us using perturbative expansion to deal with the high energy transfer processes. The latter property is the origin why we cannot observe free quarks or gluons.However, theoretical calculation of QCD predict there exist a new state of matter in high temperature or high baryon density which can be reached in high energy heavy-ion collisions. In this new matter phase, quarks and gluons can be de-confinement in a space much larger than hadrons. We call it as Quark-Gluon-Plasma (QGP).To tell whether there is QGP formation we should have some measure-ments which are able to distinguish between the QGP phase and normal hadronic phase. We call these measurements as QGP signature. One of the QGP signature found in high energy heavy-ion collisions is the Jet Quench-ing.Jet quenching or the suppression of large transverse momentum spec-tra can be used as an effective probe of the properties of dense medium created in high energy heavy-ion collisions. Because of multiple scattering and induced gluon bremsstrahlung, an energetic parton propagating in dense medium will lose a significant amount of energy and therefore soften its final fragmentation functions. These modified fragmentation functions will lead to the suppression of large transverse momentum single hadron spectra, photon-hadron correlations both away-side and same-side dihadron correlations in high-energy heavy-ion collisions.Such proposed phenomena have indeed been observed in experiments at the Relativistic Heavy-ion Collider (RHIC). Phenomenological studies of the observed jet quenching phenomena at RHIC indicate a scenario of strong interaction between energetic partons and the hot medium with an extremely high initial parton density. The same phenomena are also predicted in deeply inelastic scattering (DIS) off large nuclei when the struck quark propagates through the target nuclei though the extracted parton density in cold nuclei is much smaller than that in the hot matter produced in the central Au+Au collisions at RHIC.In the first part of this dissertation, within the framework of generalized factorization of higher-twist contributions to semi-inclusive cross section of deeply inelastic scattering off a large nucleus, multiple parton scattering leads to an effective medium-modified fragmentation function and the correspond-ing medium-modified DGLAP evolution equations. We extend the study to include gluon multiple scattering and induced quark-antiquark production via gluon fusion.In the second part of this dissertation, we numerically solve these medium-modified DGLAP (mDGLAP) evolution equations and study the scale (Q2), energy (E), length (L) and jet transport parameter (q) dependence of the modified fragmentation functions for a jet propagating in a uniform medium with finite length (a "brick" problem). We also discuss the concept of parton energy loss within such mDGLAP evolution equations and its connection to the modified fragmentation functions.In the third part of this dissertation, we concentrate on the jet evolution in realistic nuclear medium. With a realistic Wood-Saxon nuclear geometry, we calculate the modified fragmentation functions and compare to exper-imental data of DIS off large nuclei. And we extracted the jet transport parameter at the center of a large nucleus via compared our calculations with the HERMES experimental data for three different nucleus. Further-more, we calculate the modification factor RAA for QGP produced in high energy nuclear collisions. We extracted the jet transport parameter from comparing with RHIC data also.