Effects of the formation time of parton shower on jet quenching in heavy-ion collisions

Jet quenching has successfully served as a hard probe to study the properties of Quark-Gluon Plasma(QGP).As a multi-particle system,jets require time to develop from a highly virtual parton to a group of partons close to mass shells.In this study,we present a systematical analysis on the effects of this formation time on jet quenching in relativistic nuclear collisions.Jets from initial hard scatterings were simulated with Pythia,and their interactions with QGP were described using a Linear Boltzmann Transport(LBT) model that incorporates both elastic and inelastic scatterings between jet partons and the thermal medium.Three different estimations of the jet formation time were implemented and compared,including instantaneous formation,formation from single splitting,and formation from sequential splittings,before which no jet-medium interaction was assumed.We found that deferring the jet-medium interaction with a longer formation time not only affects the overall magnitude of the nuclear modification factor of jets but also its dependence on the jet transverse momentum.

The medium-temperature dependence of jet transport coefficient in high-energy nucleus-nucleus collisions

The medium-temperature T dependence of the jet transport coefficient̂q was studied via the nuclear modification factor RAA(p_(T))and elliptical flow parameter v_(2)(p_(T))for large transverse momentum p_(T) hadrons in high-energy nucleus-nucleus collisions.Within a next-to-leading-order perturbative QCD parton model for hard scatterings with modified fragmentation functions due to jet quenching controlled by q,we check the suppression and azimuthal anisotropy for large p_(T) hadrons,and extract q by global fits to RAA(pT)and v_(2)(pT)data in A+A collisions at RHIC and LHC,respectively.The numerical results from the best fits show that q∕T^(3) goes down with local medium-temperature T in the parton jet trajectory.Compared with the case of a constant q∕T^(3),the going-down T dependence of q∕T^(3) makes a hard parton jet to lose more energy near T_(c) and therefore strengthens the azimuthal anisotropy for large pT hadrons.As a result,v_(2)(p_(T))for large pT hadrons was enhanced by approximately 10%to better fit the data at RHIC/LHC.Considering the first-order phase transition from QGP to the hadron phase and the additional energy loss in the hadron phase,v_(2)(p_(T))is again enhanced by 5-10%at RHIC/LHC.

Transverse momentum balance of dijets in Xe+Xe collisions at the LHC

We present a theoretical study of the medium modifications of the p_(T)balance (x_(J)) of dijets in Xe+Xe collisions at■.The initial production of dijets was carried out using the POWHEG+PYTHIA8 prescription,which matches the next-toleading-order (NLO) QCD matrix elements with the parton shower (PS) effect.The SHELL model described the in-medium evolution of nucleus–nucleus collisions using a transport approach.The theoretical results of the dijet xJin the Xe+Xe collisions exhibit more imbalanced distributions than those in the p+p collisions,consistent with recently reported ATLAS data.By utilizing the Interleaved Flavor Neutralisation,an infrared-and-collinear-safe jet flavor algorithm,to identify the flavor of the reconstructed jets,we classify dijets processes into three categories:gluon–gluon (gg),quark–gluon (qg),and quark–quark (qq),and investigated the respective medium modification patterns and fraction changes of the gg,qg,and qq components of the dijet sample in Xe+Xe collisions.It is shown that the increased fraction of qg component at a small x_(J)contributes to the imbalance of the dijet;in particular,the q_(1)g_(2)(quark-jet-leading) dijets experience more significant asymmetric energy loss than the g_(1)q_(2)(gluon-jet-leading) dijets traversing the QGP.By comparing the■of inclusive,■ dijets in Xe+Xe collisions,we observe■.Moreover,ρ_(Xe),P_(b),the ratios of the nuclear modification factors of dijets in Xe+Xe to those in Pb+Pb,were calculated,which indicates that the yield suppression of dijets in Pb+Pb is more pronounced than that in Xe+Xe owing to the larger radius of the lead nucleus.

Particle production at large p_(T) in Xe+Xe collisions with jet quenching using the higher twist approach

The production of π^(0),η,andφin the most central(0%-5%)Xe+Xe collisions at √s_(NN)=5.44 TeV is investigated in the framework of the perturbative QCD(pQCD)improved parton model at an accuracy of next-toleading order(NLO).The jet quenching effect is effectively incorporated by medium-modified fragmentation functions via the higher-twist approach.Predictions of the nuclear modification factors of π^(0),η,and φ as functions of the transverse momentum p_(T) are made with the jet transport parameter q_(0),which is extracted from the available experimental data of charged hadrons provided by ALICE and CMS.The particle ratios η/π^(0),φ/π^(0) as functions of p_(T) in Xe+Xe collisions at √s_(NN)=5.44 TeV as well as in 0%-5% Pb+Pb collisions at √s_(NN)=5.02 TeV are also presented.The numerical simulations of the scaled ratios of charged hadron production in the Xe+Xe 5.44 TeV system over those in the Pb+Pb 5.02 TeV system give a good description of the CMS data,and the scaled ratios of π^(0),η,and φ production coincide with the curve of charged hadron production.

Jet quenching parameter from a soft wall AdS/QCD model

We study the effect of chemical potential and nonconformality on the jet quenching parameter in a holographic QCD model with conformal invariance broken by background dilaton.The presence of chemical potential and nonconformality both increase the jet quenching parameter,thus enhancing the energy loss,consistently with the findings of the drag force.

Quenching and flow of charm and bottom quarks via semi-leptonic decay of D and B mesons in Pb+Pb collisions at the LHC

Heavy flavor particles provide important probes of the microscopic structure and thermodynamic properties of the quark-gluon plasma(QGP)produced in high-energy nucleus-nucleus collisions.We studied the energy loss and flow of charm and bottom quarks inside the QGP via the nuclear modification factor(R_(AA))and elliptic flow coefficient(v_(2))of their decayed leptons in heavy-ion collisions at the LHC.The dynamical evolution of the QGP was performed using the CLVisc(3+1)-dimensional viscous hydrodynamics model;the evolution of heavy quarks inside the QGP was simulated with our improved Langevin model that considers both collisional and radiative energy loss of heavy quarks;the hadronization of heavy quarks was simulated via our hybrid coalescence-fragmentation model;and the semi-leptonic decay of D and B mesons was simulated via PYTHIA.Using the same spatial diffusion coefficient for charm and bottom quarks,we obtained smaller R_(AA) and larger v_(2) of charm decayed leptons than bottom decayed leptons,indicating stronger energy loss of charm quarks than bottom quarks inside the QGP within our current model setup.

Hadron productions and jet substructures associated with Z^(0)/γ in Pb+Pb collisions at the LHC

We carry out a detailed study of medium modifications on Z^(0)/γ+hadron correlations as well as jet substructures in association with Z^(0)/γ in Pb+Pb collisions with √S_(NN)=5.02TeV at the LHC.We utilize the linear Boltzmann transport(LBT)model to simulate the jet-medium interactions and medium response,and an extended cluster hadronization model to investigate the nonperturbative transition of quarks and gluons into final hadrons in heavy-ion collisions.Including hadronization effect,we can well describe Z^(0)/γ+hadrons correlations and Z^(0)/γ-tagged jet substructures in both p+p and Pb+Pb collisions simultaneously.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 Pb+Pb collisions due to jet-medium interactions.In particular,we find that Z^(0)/γ-tagged hadron correlations are sensitive to the soft particles from the dense medium and medium response,while jet-substructures show weak dependence on those soft hadrons with only a fraction of them falling inside the jet area.

Radial profile of bottom quarks in jets in high-energy nuclear collisions

Angular correlations between a heavy quark(HQ)and its tagged jet are potentially new tools to gain insight into the in-medium partonic interactions in relativistic heavy-ion collisions.In this work,we present the first theoretical study on the radial profiles of B mesons in jets in Pb+Pb collisions at the Large Hadron Collider(LHC).The initial production of a bottom quark tagged jet in p+p is computed by SHERPA,which matches the next-to-leading order matrix elements with contributions of parton showers,whereas the massive quark traversing the quark-gluon plasma is described by a Monte Carlo model,SHELL,which can simultaneously simulate light and heavy flavor in-medium energy loss within the framework of Langevin evolution.In p+p collisions,we find that at lower PQ/T the radial profiles of heavy flavors in jets are sensitive to the heavy quark mass.In 0-10%Pb+Pb collisions at√SNN=5.02 TeV,we observe an inverse modification pattern of the B meson radial profiles in jets at4PQ/T<20 GeV compared to those of D mesons:the jet quenching effects narrow the jet radial profiles of B mesons in jets while broadening those of D mesons in jets.We find that in A+A collisions,the contribution dissipated from the higher PQ/T>20 GeV region naturally has a narrower initial distribution and consequently leads to a narrower modification pattern of the radial profile;however the diffusion nature of the heavy flavor in-medium interactions will give rise to a broader modification pattern of the radial profile.These two effects consequently compete and offset with each other,and the b quarks in jets benefit more from the former and suffer less diffusion effect compared to that of c quarks in jets.These findings can be tested in the future experimental measurements at the LHC to gain better understanding of the mass effect of jet quenching.

p_(T) dispersion of inclusive jets in high-energy nuclear collisions

In this study,we investigate the impact of jet quenching on the p_(T) dispersion(p_(T)D)of inclusive jets(R=0.2)in central Pb+Pb(0%-10%)collisions at √s=2.76 TeV.The partonic spectrum in the initial hard scattering of elementary collisions is obtained by an event generator POWHEG+PYTHIA,which matches the next-to-leading order(NLO)matrix elements with parton showering,and the energy loss of a fast parton traversing through hot/dense QCD medium is calculated using Monte Carlo simulation within the Higher-Twist formalism of jet quenching in heavy-ion collisions.We present model calculations of the normalized p_(T)D distributions of inclusive jets in p+p and central Pb+Pb collisions at √s=2.76 TeV,which offer good descriptions of ALICE measurements.It is shown that the p_(T)D distributions of inclusive jets in central Pb+Pb collisions shift significantly to a higher p_(T)D region relative to those in p+p collisions.Thus,the nuclear modification ratio of the p_(T)D distributions of inclusive jets is smaller than unity in the small p_(T)D region and larger than one in the large p_(T)D region.This behavior is caused by a more uneven p_(T) distribution for jet constituents as well as the fraction alteration of quark/gluon initiated jets in heavy-ion collisions.The difference in p_(T)D distribution between groomed and ungroomed jets in Pb+Pb collisions is also discussed.

Large angle radiation effect on jet measurement in pp collisions at s^(1/2)=7 TeV at the LHC

Jet measurement is an ideal probe to explore the properties of the hot dense matter created in ultra- relativistic heavy-ion collisions. Recent results at the LHC show that large angle radiation is non-negligible, but the mechanisms and phenomenology of large angle radiation are still unclear and hotly debated. Considering the coexistence and competition of different physics mechanisms qualitatively, it is assumed that the radiation angle is enhanced randomly over a wide range based on the collinear approximation. Its effects on di-jet momentum imbalance, jet fragmentation function and jet shape are studied in pp collisions at 7 TeV. The results show that di-jet asymmetry is insensitive to large angle radiation, while jet shape and jet fragmentation functions are more sensitive and could explain experimental data well. We conclude that de-collimated radiation cannot be ignored for soft jets, and there is a contribution from large angle radiation （Ф 〉 0.7） of about 8%, which is significant for jet intrinsic structure measurement at PT,jeT 〈80 GeV/c.

Medium modifications of girth distributions for inclusive jets and Z0+jets in relativistic heavy-ion collisions at the LHC

In this paper,we investigate the medium modifications of girth distributions for inclusive jets and Z^0 tagged jets with a small radius( R=0.2 )in Pb+Pb collisions with √s==2.76 TeV at the LHC.The partonic spectra in the initial hard scattering of elementary collisions are obtained by an event generator POWHEG+PYTHIA,which matches the next-to-leading order(NLO)matrix elements with parton showering,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 in heavy-ion collisions.We present the model calculations of event normalized girth distributions for inclusive jets in p+p and Pb+Pb collisions at √s=2.76 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.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 Z^0 boson tagged jets in Pb+Pb collisions at √s=2.76 TeV and demonstrate that the medium modification on girth distributions for Z^0 tagged jets is less pronounced compared to that for inclusive jets because the dominant components of Z^0 tagged jets are quark-initiated jets.

Heavy flavor quenching and flow:the roles of initial condition,pre-equilibrium evolution,and in-medium interaction

Within an advanced Langevin-hydrodynamics framework coupled to a hybrid fragmentation-coalescence hadronization model,we study heavy flavor quenching and flow in relativistic heavy-ion collisions.We investigate how the initial heavy quark spectrum,the in-medium energy loss and hadronization mechanisms of heavy quarks,the evolution profile of the pre-equilibrium stage,the medium flow,and the temperature dependence of heavy quark diffusion coefficients influence the suppression and elliptic flow of heavy mesons at the RHIC and the LHC.Our results show that the different modeling of initial conditions,pre-equilibrium evolution,and in-medium interactions can individually yield uncertainties of approximately 10-40% in D meson suppression and flow at a low transverse momentum.We also find that proper combinations of collisional versus radiative energy loss,coalescence versus fragmentation in hadronization,and the inclusion of medium flow are the most important factors for describing the suppression and elliptic flow of heavy mesons.

Holographic energy loss near critical temperature in an anisotropic background

We study the energy loss of a quark moving in a strongly coupled quark gluon plasma under the influence of anisotropy.The heavy quark drag force,diffusion coefficient,and jet quenching parameter are calculated using the Einstein–Maxwell-dilaton model,where the anisotropic background is characterized by an arbitrary dynamical parameter A.Our findings indicate that as the anisotropic factor A increases,the drag force and jet quenching parameter both increase,while the diffusion coefficient decreases.Additionally,we observe that the energy loss becomes more significant when the quark moves perpendicular to the anisotropy direction in the transverse plane.The enhancement of the rescaled jet quenching parameters near critical temperature Tc,as well as drag forces for a fast-moving heavy quark is observed,which presents one of the typical features of quantum chromodynamics phase transition.

通过相对论重粒子碰撞中的J/ψ产生揭秘胶子喷注淬火机制

长久以来,喷注淬火被认为是探测重离子碰撞中形成的夺克胶子等离子体的关键信号之一,尽管过去在喷注火研究方面已经付出了巨大的努力、但是区分夸克和胶子的喷注淬火仍然是一个非常大的挑战本文发现大横动量J/主要由胶子碎裂产生,因此为探测胶子在热密介质中的能量损失提供了一个独特的敏感探针,作者首先基于领头幂次的NRQCD因子化理论计算了质子-质子碰撞中的大横动量J/产生,发现胶子碎裂是大横动量J/产生的主要机制,然后通过线性玻尔兹曼输运模型和相对论流体力学模拟了核-核碰撞中喷注与热密介质的相互作用,所得数值结果很好地描述了大横动量J/的核修正因子和椭圆流,揭示出胶子能量损失是大横动量J/产额压低的主要机制。作者进一步通过数据驱动的贝叶斯分析方法证实了这个新颖的结论,并定量提取出夸克胶子等离子体中的胶子能量损失分布。

High pt and photon physics with ALICE at LHC

ALICE,A Large Ion Collider Experiment,is dedicated to study the QCD matter at extreme high temperature and density to understand the Quark Gluon Plasma （QGP） and phase transition.High-transversemomentum photons and neutral mesons from the initial hard scattering of partons can be measured with ALICE calorimeters,PHOS （PHOton Spectrometer） and EMCAL （ElectroMagnetic CALorimeter）.Combing the additional central tracking detectors,the γ-jet and π 0-jet measurements thus can be accessed.These measurements offer us a sensitive tomography probe of the hot-dense medium generated in the heavy ion collisions.In this paper,high p T and photon physics is discussed and the ALICE calorimeters capabilities of high-transverse-momentum neutral mesons and γ-jet measurements are presented.

Probing the color structure of the perfect QCD fluids via soft-hard-event-by-event azimuthal correlations

We develop a comprehensive dynamical framework, CIBJET, to calculate on an event-by-event basis the dependence of correlations between soft （PT 〈2 GeV） and hard （PT 〉 10 GeV） azimuthal flow angle harmonics on the color composition of near-perfect QCD fluids produced in high energy nuclear collisions at RHIC and LHC. CIBJET combines consistently predictions of event-by-event VISHNU2＋I viscous hydrodynamic fluid fields with CUJET3.1 predictions of event-by-event jet quenching. We find that recent correlation data favor a temperature dependent color composition including bleached chromo-electric q（T）＋g（T） components and an emergent chromo-magnetic degrees of freedom re（T） consistent with non-perturbative lattice QCD information in the confinement/deconfinement temperature range.

Effect of parton cascade to medium

Jet quenching has been proposed as a probe of the properties of the strongly interacting quarkgluon-plasma in high energy heavy ion collisions.At the meantime,it is also important to study the excitation of medium by propagating jets.Based on Boltzmann equation,a Monte Carlo was constructed to simulate the elastic collisions with thermal partons by energetic jets.Medium modification due to jet-medium interaction has been studied within this model in a uniform medium.