Angular Dependence Of Radiative Energy Loss Of Heavy Quarks

Lattice QCD predicts a phase transition from strongly interacting matter to quark-gluon plasma(QGP) formed at sufficiently high temperatures and densities. The aim of relativistic heavy ion experiments—Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider(LHC) is to explore such state of matter and study its properties.Experiment at RHIC have observed the suppression of high-p_T light hadrons in nucleus-nucleus collisions compared to those in p+p and d+Au collision.It has been referred to as jet quenching which is due to the energy loss of hard partons produced at the initial stage of nucleus-nucleus that propagate through the medium formed in ultra-relativistic heavy ion collisions.It provide an efficient probe to map the evolution of the QGP.The energy loss may come from the induced gluon bremsstrahlung and the collisional energy loss.The first mechanisms is dominate and has been the subject of intensive interest.At RHIC and LHC,heavy flavors will be abundantly produced.This will offer a good opportunity to characterize the medium formed in the collisions as heavy quarks are mainly produced by hard processes at the very beginning of the collisions.Heavy quarks lose less energy compared to light quarks due to the mass effect of heavy quarks which lead to the suppression of gluon radiation at small anglesθ＜m/E(dead cone effect).However,a significant suppression of electrons from heavy quarks in mid-rapidity region at high pt has been observed in central Au-Au collision at RHIC,indicating substantial energy loss of heavy quarks.The medium induced energy loss of a parton has been the subject of intensive interest.Currently there are different scenarios on the energy loss of heavy quarks. Magdalena Djordjevic and Miklos Gyulassy(DG) follow from the reaction operator method and generalize the GLV opacity expansion for light quarks to heavy quarks. Another scenario on energy loss of heavy quarks is derived by U.A.Wiedemann.The angular distribution of medium-modified radiative gluons off light quarks had been studied.It is very important as it allows to predict the energy loss outside a jet cone and discuss medium modification of jet shape and jet multiplicity distributions. However,it was not studied for heavy quarks.In this thesis,we study it.We recall the gluon radiation spectrum of heavy quarks derived by Magdalena Djordjevic,Miklos Gyulassy and Wiedemann,and study the angular dependence of gluon radiation spectrum off heavy quarks.We compare these angular distributions to light quarks and dead cone approximation schemes and simply analyse how the mass effect shows up in the angular distribution of gluons.We find that the fraction of the medium-induced radiative energy loss outside an angle with respect to energy of heavy quarks△E/E(θ) is found to have a minimum and a maximum at small angle for small path length and for large path length the minimum and maximum disappear.△E/E(θ) calculated from the dead cone approximation has a maximum at small angleθ.Next,we will study the angular dependence of medium-induced gluon radiation for an expanding medium,and discuss medium modifications of jet shape and jet multiplicity off heavy quarks.