Medium Modification Of Jet Substructures In High Energy Nuclear Collisions

A large amount of energy may be deposited in the center of high-energy nuclear collisions,resulting in an extremely hot and dense environment where the quark and gluon confined in the nucleon may be dissolved to form a new state of matter,quark gluon plasma(QGP).The QGP cannot be observed directly under current experimental conditions since QGP exist only for very short time(about 10 fm/c).To study the properties of QGP,researchers have proposed a series of specific probes,among them is jet quenching,the process in which high pT partons produced in the initial hard scattering would suffer energy loss since partons strongly interact with the QGP medium.Jet quenching has been shown a very powerful tool to study the formation and the properties of the QGP.With the continuous developments of experimental measurements and theoretical advances,research on jet quenching has been extended from productions of single hadrons,double-hadrons and photon-hadron correlations,towards full jet observable,especially recently to medium modifications of internal structures of jets(jet substructure).In this paper,we have made a series of detailed theoretical studies on the medium modification of jet substructures in high-energy nuclear collisions.Our framework of theoretical simulation calculation is as follows:The partonic spectra in the initial hard scattering of elementary collisions are obtained by the NLO+PS event generator such as POWHEG+PYTHIA or SHERPA,which match the next-to-leading order(NLO)matrix elements with parton showering(PS),and the energy loss of a fast parton traversing the hot/dense QCD medium is calculated by Monte Carlo simulation within the higher-twist formalism of jet quenching.The space-time evolution information of QGP is obtained by a 2+1D viscous hydrodynamics model,and the cold nuclear matter(CNM)effects has been included.The research work in this thesis mainly includes the following contents:1)By using the theoretical calculation framework mentioned above,we present the model calculations of event normalized girth distributions for inclusive jets in p-p and PbPb collisions at(?)=5.02 TeV,which give good descriptions of ALICE measurements.It is shown that the girth distributions of inclusive jets in Pb+Pb are shifted to lower girth regions relative to those in p-p collisions.Thus,the nuclear modification factor of girth distributions for inclusive jets is larger than unity at small girth regions and smaller than unity at large girth regions.This behavior results from softer fragments inside a jet as well as the fraction alteration of gluon/quark initiated jets in heavy-ion collisions.We further predict the girth distributions for Z0 tagged jets in Pb+Pb collisions at(?)=5.02 TeV and demonstrate that the medium modification on girth distributions for Z0 tagged jets is less pronounced compared to that for inclusive jets because the dominant components of Z0 tagged jets are quark-initiated jets.2)We have systematically calculated the distribution of transverse momentum dispersion and its medium modification,another observable of jet substructure in heavy ion collisions.Compared with the p-p collision,the distribution of jet transverse momentum dispersion in Pb-Pb is shifted to larger dispersion region for inclusive jet which is quite different from jet girth.The momentum dispersion is analyzed as the sum of two terms,standard deviation of momentum(δ)and particle number of jet(1/n),in order to understand deeper the dispersion shift.Medium modification of momentum dispersion enhanced by both parton redistribution(both δ2 and 1/n enhanced)and the alteration of overall quark/gluon fraction.We also calculate dispersion of groomed jets,find that the grooming process may reduce medium modification of the dispersion since grooming will enhance the value of 1/n and meanwhile lead to lower δ,whereas the correction of 1/n is more pronounced.3)The observable of "jet girth”and "jet transverse momentum dispersion”studied in the previous two works are only related to the transverse momentum of the particles inside jet and the position of jet particles.Other observables of this kind of jet substructures,including LHA,thrust,and multiplicity,together with girth and dispersion,are called the generalized jet angularities.We have also calculated the distributions and medium modifications of these three jet angularities(LHA,thrust,and multiplicity)for inclusive jets and Z0 tagged jets in Pb-Pb collisions and have compared jet angularities before and after grooming with a unified framework.In addition,the dependence of distributions and medium modifications for jet angularities on parameter α has also been calculated,as well as the preliminary calculation results obtained by using LBT model.4)Interactions between heavy quark and medium lead to energy loss and broadening of transverse momentum when they pass through QGP,the broadening of transverse momentum can be regarded as a diffusion in momentum space.The CMS measurements of heavy quarks on the radial distribution inside jets make it possible to reveal this diffusion effect experimentally.In this work,we present our theoretical results of the D0 meson radial distributions in p-p and Pb-Pb collisions at(?)=5.02 TeV.A visible shift towards large r is found at low D0 m1son(5<pT<25 GeV)in the normalized distribution 1/NJD/dNJD/dr,in Pb-Pb collisions relative to the p-p baseline.Whereas,no significant modification is observed for the D0-jet correlation in the Pb+Pb collisions for high D0 mesons(pT>25 GeV).