Production Of Heavy Flavor Jets In High-energy Nuclear Collisions

In the Standard Model(SM),the protons and neutrons in the nucleus are also com-posed of the more basic particles,quarks and gluons,which described by the Quantum Chromodynamics(QCD).However,due to the natural property of color confinement in QCD,we can not observe isolated free quarks or gluons in nature.High-energy heavy-ion collisions at the Large Hadron Collider(LHC)at CERN and the Relativistic Heavy Ion Collider(RHIC)at the Brookhaven National Laboratory in the United States have opened up new avenues for studying the deconfined QCD matter.High-energy nucleus-nucleus collisions deposit a large amount of energy into a small collision region then create an ex-tremely hot and dense environment in which quarks and gluons can be deconfined under such extreme conditions,thus forming a new state of matter-quark-gluon plasma(QGP).Studying the properties of QGP helps us understand the basic theory of QCD.However,since the QGP lifetime is very short(?10fm)in experiment,its properties need to be in-vestigated with specific probes.The high pT parton produced in the initial hard scattering will strongly interact with the medium when passing through the QGP matter,resulting in energy loss,referred as "jet quenching".Jet quenching effect is a powerful probe to study the properties of QGP,which has been extensively studied both at experiment and theory.In this paper,we will use the theoretical calculations based on the next-to-leading order QCD matrix element matched with parton shower(NLO+PS),simulating the QGP time-space evolution by a 2+1D viscous hydrodynamic model,considering the elastic and inelastic in-medium parton energy loss as well as the Cold Nuclear Matter(CNM)effects,to study the jet quenching effect of heavy flavor jet in high-energy nuclear collisions.In high-energy nuclear collisions,due to the large mass(MQ>>TQGP),heavy quarks are almost all produced in the initial hard scattering in very early stage before the QGP formed,then interact with the medium during the QGP evolution,hence be regarded as effective probes to the properties of the QCD medium.During the past decade,the exper-imental measurements including the nuclear modification factor RAA and the azimuthal anisotropy v2 for heavy flavor hadrons both at the RHIC and the LHC have attracted much attention in the field of high energy physics.A lot of theoretical calculations have been made to confront with these measurements,which greatly improve our understanding of the in-medium heavy quarks evolution.It's generally believed that heavy quarks lose energy by the elastic scattering with the thermal parton and the medium-induced gluon radiation,but their relative contributions are still unclear.Besides,the mass effects of jet quenching has become one of the highlights in heavy-ion collision physics.Due to the large mass of heavy quarks,the medium-induced gluon radiation at small angle are significantly suppressed by the dead-cone effect result in less energy loss of heavy quarks relative to light quarks and gluon in the medium,but these differences have not been observed in experiment.In addition,the current experimental measurement of heavy quarks is gener-ally through the reconstruction of heavy taste mesons.Due to the lack of understanding of the non-perturbative hadronization processes,the current study on heavy flavor meson is still quite dependent on the phenomenological hadronization model,which also poses a challenge to the theoretical calculation.In recent years,the heavy flavor jets measure-ments at the LHC have made great strides in high-energy nuclear collisions.The heavy flavor jets are defined as the jets containing heavy quarks or heavy flavor mesons inside the jet cone.The advantage of heavy flavor jets is that their final-state observables are hardly affected by the hadronization processes.The study on the nuclear modification of heavy flavor jets is of great significance to reveal the in-medium energy loss mechanism of heavy quarks,to explore the properties of QGP and to understand the mass effect of "jet quenching".Heavy quarks interact with the medium result in energy loss and transverse momen-tum broadening during their propagation in QGP,and this transverse momentum broad-ening can be regarded as a diffusion in momentum space.The recent CMS measurements on the radial distribution of heavy quarks inside jets makes it possible to reveal this diffu-sion effect in experiment.In this paper,we present the first theoretical calculation of the D0 meson radial distribution in jet both in p+p and Pb+Pb collisions at 5.02 TeV which provide a quite decent description on the CMS data.At low D0 meson pT(4-20 GeV),a visible shift towards large r is found in the normalized distribution 1/NJD DNJD/dr in Pb+Pb collisions relative to the p+p baseline.Whereas,for high pT D0 mesons,no significant modification is observed for the D0-jet correlation in the Pb+Pb collisions.To further understand the diffusion of charm quarks in jets,we estimate the net effect on the charm diffusion from collisional and radiative mechanism and demonstrate the pT dependence of this diffusion effect.We find that collisional process has significant effects at low pT,especially dominates at 0-5 GeV,and the radiative process has a non-zero effect even at high pT?50 GeV.The total diffusion effect decreases with D0 meson pT which explains the significant modification at low D0 meson pT measured in experiment.Besides,for unit energy loss,the collisional interaction shows much stronger angular deviation than radia-tive interaction at pT<5 GeV.The strong diffusion effects of low pT charm quarks relative to jets may act as a sensitive probe to the distinction between collisional and radiative interaction,and the prospective measurements would provide more precise estimations on the heavy quarks diffusion coefficient and also give additional constrains for the current theoretical models on heavy quark energy loss.At the same time,we also estimate the ra-dial distribution of bottom quarks in jets,and find that,in p+p collisions,bottom quarks more likely distribute at the peripheral region of jet,but charm quark trend to center.These differences would lead different nuclear modification on the radial distribution.At last,we also predict the nuclear modification factor for charm jet in 0-10%Au+Au col-lisions at(?)GeV at the RHIC and 0-10%Pb+Pb collisions at(?)TeV at the LHC as predictions for the future measurements.We find that the RAA at LHC is slightly dependent on pT due to the steeper cross section spectrum at higher pT,and the RAA at RHIC has no pT dependence.The perturbative QCD predicted a "dead-cone" effect for the medium-induced gluon radiation of heavy quarks,which decreases the energy loss of heavy flavor jets relative to the light quark jets,but no clear evidence has been found in experiment.The recent CMS measurements on the nuclear modification factor RAA inclusive jet and b-jet,as well as the transverse momentum imbalance xj of inclusive dijets and bb dijets,provide evidence for the mass effect of heavy quark jet energy loss to some extent.In this paper,based on our parton energy loss framework,we estimate the nuclear modification factor of inclusive jet and b-jet at 2.76 TeV,and we actually find a higher RAA of b-jet relative to inclusive jet at lower pT,but at higher pT the mass effects disappear.In addition,we also calculate the transverse momentum imbalance xj of the inclusive dijets and bb dijets based on the back-to-back condition of CMS measurements at 5.02 TeV,because the back-to-back requirements would significantly suppress the contribution from GSP processes which are still not well understood in nuclear-nuclear collisions.Our calculations show the nuclear modification on xj of bb dijets is smaller than that of inclusive dijets which may indicate bottom quark energy loss is smaller than light quark.Beyond to that,we also present the predictions of the nuclear modification on the azimuthal angular correlations ??bb between the bb dijets.We find stronger modification on the bb dijets with smaller ??bb,which lead to a suppression at smaller ??bb region and enhancement at larger ??bb in the normalized distribution.And by estimate the averaged transverse momentum(pT)of leading and sub-leading as a function of ??bb,we find the(pT)is increasing with??bb especially for sub-leading b-jet,which can well explain the stronger suppression at smaller ??bb.Furthermore,based on the nice description on the nuclear modification of inclusive jet shape,we calculate and compare this modification of inclusive jet,charm-jet and bottom-jet in Pb+Pb collisions at(?)TeV.We find stronger modification in b-jet than that in inclusive jet and c-jet,especially the enhancement at small r region and suppression at larger r region.These results directly reflect the mass efffect of the bottom quark,and the future experimental measurement of the bottom jet shape in the heavy ion collision will hopefully solve the puzzle of the mass hierarchy of jet quenching.Vector bosons tagged jets(V+jet)are regarded as "golden channel" to study the prop-erties of QGP,since,on the one hand,jets interact strongly with the medium and lose energy,on the other hand,vector bosons do not participate in strong interactions but gauge the initial energy of jets,which make it available to estimate the strength of in-medium jet interaction and the properties of QGP.The vector boson tagged heavy flavor jets will be new tools to study the properties of QGP and the mass effect jet quenching,but still no research was made on this topic.Based on the good pp-baseline provided by SHERPA,we extend our study to the medium modification of Z0 boson tagged b-jets.Firstly,our calculations give a good description on the modification of ??jz and xjz of Z0+jet in Pb+Pb collisions at 5.02 TeV by comparing with the available experimental data.And on this basis,we present the first calculation of the azimuthal angular corre-lation ??bZ of Z0+(b)jet both in p+p and 0-10%Pb+Pb collisions at(?)TeV.We observed an constant suppression factor versus ??bZ,unlike the case in Z0+jet,the requirement of b-tagging excludes the contribution from multiple jets.In addition,for Z0+(2b)jet,we also calculated the azimuthal angle ??bZ distribution between the two b-jets and observed stronger suppression at small ?bb where dominated by gluon splitting processes relative to at large ?bb We estimate the(pT)distribution and demonstrate the modification patterns on these azimuthal angle are very sensitive to the initial b-jet pT distribution versus ??bZ(or ??bb).In fact,we find the angular deviation of high pT jet due to the in-medium interactions is negligible compared to the influence from the(pT)distribution.On the other hand,benefited from the high purity of light-quark jet in Z0+jet events,we expect to address the mass depende1ce of the jet quenching effects between Z0+jet and Z0+(b)jet.We calculate xjz of Z0+jet and xbZ of Z0+(b)jet,and by comparing their shift in Pb+Pb collisions relative to pp-baseline,we actually find a significant difference in the energy loss of light-quark jet and b-jet.We also calculate another observable,the nuclear modificatio,factor IAA,of Z0+jet and Z0+(b)jet,and find obvious stronger suppression in Z0+jet production in A+A collisions relative to that of Z0+(b)jet.In general,we believe these comparison can also be implemented at the LHC base on the current experimental technologies to find the key that unlocks the puzzle of mass hierarchy in jet quenching.