Gauge Boson Tagged Jets Production In Relativistic Heavy Ion Collisions

One of the main goals of ultra-relativistic nucleus-nucleus collisions,such as those performed at the Relativistic Heavy-Ion Collider(RHIC)at Brookhaven National Labora-tory(BNL)and the Large Hadron Collider(LHC)at European Organization for Nuclear Research(CERN),is to create the hot and dense quark-gluon plasma(QGP)composed of deconfined quarks and gluons and study its various novel properties.These quarks and gluons,known as partons,created prior to the formation of the QGP,traverse the hot-dense medium and experience elastic and inelastic scatterings with the constituents of the medium.The interactions between the hard jets and the soft medium usually reduce the energy of the incoming jet partons.This process,called jet quenching,plays an es-sential role in studying transport properties of the QGP.There have been many striking experimental signatures of jet energy loss observed at RHIC and the LHC,such as the suppression of high momentum hadrons,the shift of transverse momentum asymmetry of back-to-back dijet and Z/?-jet pairs,the modification of jet substructures,as compared to elementary proton-proton collisions.Gauge-boson-tagged jet production is regarded as a "golden channel" to study the jet quenching.The gauge boson will not participate in the strong-interactions directly and can be considered as the proxy of the initial energy of the parton before it propagates through the QGP medium and loses energy.In this thesis,the Monte Carlo program SHERPA which combines the NLO pQCD with resummation of a matched parton shower(PS),is used for the initial gauge boson-tagged jet production;the parton propagation in QGP medium is simulated by the Linear Boltzmann Transport(LBT)model with bulk medium evolution provided by the Berkeley-Wuhan CLVisc 3+1D hydrodynamics.Based on this framework,we will carry out detailed calculations of medium modification on the correlations between gauge boson and the recoiled jets.And than,with a modified cluster hadronization model,which translates the coloured quarks and gluons into colourless pri-mary hadrons,we show a detailed investigation of medium modification on Z/?+hadron correlations as well as the jet substructures.Finally,we present the first theoretical study of medium modifications of the global geometrical pattern,i.e.,transverse sphericity(S?)distribution of jet events with parton energy loss in relativistic heavy-ion collisions.With both higher order corrections and matched soft/collinear radiation as well as a sophisticated treatment of parton energy loss and medium response,our numerical cal-culations can provide the best description so far of all available observables of Z0+jet simultaneously in both p+p and Pb+Pb collisions,in particular,the modification of az-imuthal angle correlation in ??jZ=|?jet-?Z|,the shift of the distribution in transverse momentum asymmetry xjZ=pTjet/pTZ,the reduction of pTZ dependence of the mean value(xjz),and the overall suppression of averaged Z0-tagged jets per boson Rjz at(?)=5.02 TeV as measured by the CMS experiment,We sh ow that higher-order corrections to Z0+jet production play al indispensable role in understanding Z0+jet azimuthal angle correlation at small and intermediate ??jz,and momentum imbalance at small xjz.There is no transverse momentum broadening observed for Z-jet azimuthal correlations at LHC energy scale.Jet quenching of the sub-leading jets is shown to lead to suppression of Z0-jet azimuthal angle difference ??jz.The transverse momentum asymmetry xjz=pTjet/pTZ is shifted to smaller value in PbPb collisions relative to pp collisions due to jet-medium interactions.The detailed calculation of the overall suppression of<xjz)indicates that jets tagged by higher energy gauge boson will lose larger fraction of its energy when propagat-ing thought the QGP.The reduction of absolute jet yields above the kinematic thresholds lead to the overall suppression of Rjz,detailed analysis of Rjz shows that higher energy gauge boson will lose smaller fraction of its jets partners.As a result the steep falling of the pTjet spectrums,the nuclear modification factor for Z-jet transverse momentum spec-tra in different PTZ intervals are enhanced in small region and suppressed in large pTjet region.The calculation of jet shape indicates that a larger fraction of jet energy is carried by soft particles at large distances from the jet axis in PbPb collisions due to the strong interactions between jet partons and medium constitutes.We also carried out similar calculations for W/?/H tagged jets process which have similar modification behaviors as Z-jets.The comparison between Z+jets and H+jets show that gluon jets will lose larger fraction of its energy both in vacuum and hot-dense medium.In order to provide further insight into the structure of jets in the QGP,a modified cluster hadronization model is used to simulate the non-perturbative transition of coloured quarks and gluons into colourless primary hadrons.Hadronization occurs via preconfine-ment of colour singlet clusters which decay according to a simple phase-space scheme.Our model can well describe the experimental data on Z+chareged-hadrons azimuthal angle correlations ??Z,ch,charged hadron transverse momentum spectrums pTch as well as the fragmentation functions ?TZ,trk both in pp and PbPb collisions.An excess of low pT hadrons and a depletion of high pT hadrons are observed in central PbPb collisions,with a transition around 3 GeV.A detailed calculation show that hadrons with large transverse momentum are mainly distributed in the opposite direction of the recoiling Z boson,while softer hadrons are more widely distributed in the whole phase space.However,hadrons from negative partons are uniformly distributed in the whole phase space and are essential to low pT hadrons,especially for ??Z,ch distributions.In addition to Z-hadron correlations,we also looked at how the structure of jets in PbPb collisions is modified relative to that in pp collisions.Medium modification on jet profile and jet fragmentation functions indicate that particles carrying a large fraction of the jet momentum are generally closely aligned with the jet axis,whereas low-momentum particles are observed to have a much broader angular distribution relative to jet axis in PbPb collisions due to jet-medium interactions.Furthermore,we present the first theoretical study of medium modifications of the global geometrical pattern,i.e.,transverse sphericity(S?)distribution of jet events with parton energy loss in relativistic heavy-ion collisions at 5.02 TeV.In our investigation,POWHEG+PYTHIA is employed to make an accurate description of transverse sphericity in the p+p baseline,which combines the next-to-leading order(NLO)pQCD calculations with the matched parton shower(PS).The Linear Boltzmann Transport(LBT)model of the parton energy loss is implemented to simulate the in-medium evolution of jets.We calculate the event normalized transverse sphericity distribution in central Pb+Pb collisions at the LHC,and give its medium modifications.An enhancement of transverse sphericity distribution at small S? region but a suppression at large S? region are observed in A+A collisions as compared to their p+p references,which indicates that in overall the geometry of jet events in Pb+Pb becomes more asymmetry.We demonstrate that for events with 2 jets in the final-state of heavy-ion collisions,the jet quenching makes the event more asymmetry with medium-induced gluon radiation,which leads to larger S? in PbPb collisions.However,for events with? 3 jets,parton energy loss in the QCD medium leads to the events more imbalance,which leads to smaller S? in PbPb collisions.Besides,small S? region is dominated by 2 jets events,while large S? region is dominated by multi-jets event.As a results of which,S? is enhanced in small S? region,and suppressed in large S? region.