喷注淬火的集体流与演化效应和强作用介质的热化
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摘要
大自然纷繁复杂的物理现象中存在着四种基本的相互作用力—万有引力,弱相互作用力,电磁相互作用力和强相互作用力,每一种力都以交换玻色传播子作为媒介。量子色动力学(QCD)是描述强相互作用的非阿贝尔规范理论。它有两个显著的基本特性:渐进自由和夸克禁闭。渐进自由表明横动量交换越大或夸克之间的距离越小,夸克之间的相互作用越弱。而夸克禁闭则表明在一般情况下,夸克被禁闭在强子中,实验上没有观察到孤立的夸克。
李政道等人曾于上世纪七十年代预言高能重离子碰撞有可能形成高温高密的极端环境,从而导致强子物质的状态发生改变,生成由大量解除禁闭的夸克,反夸克和胶子组成的一种在夸克层次上的新物质形态—夸克胶子等离子体(Quark Gluon Plasma-QGP),夸克解除禁闭的过程同时也伴随着真空的改变。这一预言极大得推动了高能核物理学理论和实验的发展。近二十年来,从美国布鲁海汶国家实验室(BNL)的AGS实验和欧洲核子研究中心(CERN)的SPS实验,到2000年开始并且目前正在运行的BNL的相对论重离子对撞机(RHIC)实验,人们积累了大量丰富的实验数据。通过对实验数据的全面仔细分析和理论计算表明,在RHIC实验中可能生成了一种强耦合的夸克物质(strongly coupled Quark Gluon Plasma-sQGP)。预计,目前在CERN运行的大型强子对撞机(LHC)将产生寿命更长的QGP物质,有助于进一步了解QGP的基本特性。
在夸克胶子等离子体是否形成的信号研究中,人们相继提出了一些能够区分夸克胶子等离子相和普通强子相的物理观察量。例如:J/ψ压低,大横动量光子产生,奇异粒子相对产额的增大,Bose-Einstein关联等等。在RHIC和LHC能区,由于碰撞能量很高,硬或半硬过程非常重要。因此,把硬过程产生的喷注作为硬探针,通过硬探针与夸克胶子物质相互作用所携带的信息作为硬探针信号的喷注淬火(Jet Quenching)效应引起了人们的极大关注。喷注淬火效应描述的是在相对论性重离子碰撞中,由硬过程而产生的部分子喷注在穿过强相互作用介质时由于多重散射而损失能量。由于部分子喷注的能量损失与它穿过的介质紧密相关,能量损失对末态粒子谱,非对心碰撞而形成的椭圆流以及对心碰撞中形成的单喷注(monojet)的影响有可能成为QGP形成的信号。
M.Gyulassy和王新年在QCD框架下提出了G-W模型,讨论了多次散射诱导胶子辐射导致的部分子喷注穿过强作用介质的能量损失,BDMPS(Baier,Dokshitzer,Mueller,Peigue,Schiff)和GLV(Gyulassy,Levai,Vitev)指出和弹性碰撞的能量损失相比,非弹性碰撞的能量损失的贡献是主要的,喷注的能量损失与发生多次散射的强作用介质靶的厚度的平方成正比。此后,其它作者采用不同的方法和不同的近似,也得到了类似的平方依赖关系的结果。王恩科和王新年接着讨论了在静态热QGP介质中喷注能量损失的细致平衡效应,指出对于中等能量大小的喷注,其热胶子的吸收对唢注能量损失有重要影响。M.Gyulassy和王新年在接下来的工作中考虑了强作用介质的横向膨胀对能量损失的影响。在这些研究中,考虑到了由于介质膨胀导致的介质密度变化而引起的对部分子能量损失的影响。但忽略了高能重离子碰撞中由于温度,描述化学非平衡的逃逸度的时空演化过程。而这些都会影响到德拜屏蔽质量,平均自由程的大小,继而影响喷注在强相互作用介质中的散射次数(opacity),因而对喷注能量损失产生重要影响。
本文的研究工作主要分成两个部分,第一部分研究流和部分子演化对喷注淬火效应的影响。对于部分子演化对喷注淬火的影响,我们利用微扰QCD推导出一组几率方程来研究高能碰撞早期夸克和胶子的化学非平衡的演化过程。我们选择不同的初始条件放入几率方程中来研究演化后横动量随快度的分布和粒子数随快度的分布,并将结果与实验结果进行对照。我们确定了在RHIC中的金—金对心碰撞中,初始温度为550MeV,初始胶子逸度为0.3。我们还得到了温度和逸度随时间的演化关系。我们发现考虑化学非平衡后,与Bjorken预计的1+1维膨胀相比,温度下降的更快。利用这个演化关系,我们分析了德拜屏蔽质量,平均自由程随固有时的变化关系,继而得到了opacity与喷注穿过的介质厚度之间的依赖关系。结果表明,在考虑膨胀介质中化学非平衡的过程后,由胶子辐射产生的喷注的能量损失与喷注穿过强作用介质的距离成线性关系,而不是在静态强作用介质中预言的平方关系。我们同时还考虑了细致平衡对能量损失的影响,对于中等能量大小的喷注,只有当喷注穿过热QGP介质的距离很短时,其热胶子的吸收能量所占的总喷注能量损失的比例较大,而当喷注能量较大或者喷注穿过热QGP介质距离较大时,吸收能量的贡献并不大。对于流效应对喷注淬火的影响,在Yukawa势的基础上,我们导出新的势模型来描述喷注和存在集体流的介质中靶的相互作用,这一新的势模型可用来研究存在集体流的夸克物质介质中的喷注淬火现象。利用这一新的势模型,我们考察了流对具有细致平衡效应的喷注能量损失的影响。结果表明,沿着喷注正方向的集体流减小了喷注在强相互作用介质中的散射次数(opacity),增大了LPM压低因子。设集体流沿喷注方向的速度大小为ν_z,那么喷注在考虑集体流的强相互作用介质中的能量损失是静态系统中的1-ν_z倍。这个结果将对椭圆流的大小及高横动量区强子谱的压低产生影响。
本文研究的第二部分利用两种方法,WN和CGC考察了不同的部分子产生机制。我们首先利用WN计算出由于领头阶硬散射而产生的部分子分布,这个结果与Eskola预计的部分子分布完全一致。我们还考察了初态和末态的胶子辐射和多重散射过程,与利用CGC方法得到的部分子分布进行比较,发现在RHIC能区的Au-Au对心碰撞中,利用CGC产生的部分子数目和横动量都略大于利用WN得到的结果。而在LHC能区,利用CGC产生的部分子随快度分布基本与利用WN的所得结果相同,而横动量随快度分布略大于WM所得结果。将两种不同的部分子分布放入BAMPS模型中分析QGP介质的热化过程,考察了中心快度区横能量随快度分布dE_T/dy丨y=0,捕述动量各向同性的物理量〈p_z~2/E~2〉和逸度等物理量。结果表明,在RHIC能区,利用WN所得的dE_T/dy丨y=0基本与实验结果一致,而CGC所得结果略高于实验数据。两种方法都得到同样的趋于动力学平衡的演化过程,而化学平衡的演化过程略有不同。利用WN所得的部分子分布情况,介质一直处于化学平衡中,而CGC告诉我们,在1.5fm/c后,系统才达到化学平衡。同时,我们还预言了LHC的Pb—Pb对心碰撞中,中心快度区横能量随快度分布的大小应介于1620GeV到2150GeV之间。
In relativistic heavy ion collisions jet quenching has been proposed as a good probe of the hot and dense medium in high - energy nuclear collisions.Jet quenching is caused by gluon radiation induced by multiple scattering during jet propagates the hot QCD medium.As discovered in high-energy heavy-ion collisions at RHIC,jet quenching is manifested in both the suppression of single inclusive hadron spectrum at high transverse momentum PT region and the disappearance of the typical back-to-back jet structure in dihadron correlations.It is found that the analytic approximation of the energy loss by stimulated emission in a static medium is proportional to square of the propagation distance in a static medium.Later the detailed balance effect of jet energy loss in hot QGP medium is investigated.With partial cancellation by stimulated emission, the net result is an energy gain via absorption which reduces the effective patton energy loss. Such a reduction is found to be important for intermediate parton energy but can be neglected at very high energies.
In this dissertation we studied parton equilibration effect on jet quenching with detailed balance.We first studied the parton evolution process from chemical non-equilibrium towards a fully equilibrated parton plasma.We derive a set of rate equations based on perturbative QCD to describe the chemical equilibration of gluons and quarks,and then put different initial conditions in to get the rapidity distribution of transverse energy dE_T/dy and particle multiplicites dN/dy. We compare the data with that from experiment and get that at the initial time 0.3fm/c,the initial temperature is 550MeV,gluon fugacity is 0.3.We also find that the parton gas cools faster than that predicted by Bjorken's scaling solution.With the parton equilibration,it is shown that the Debye screening mass and the inverse mean free path for a gluon reduce with the increasing proper time in the QGP medium.The parton equilibration has effect on patton energy loss with detailed balance,the patton energy loss from stimulated emission is linear dependence on the propagating distance rather than square dependence in the static medium. The energy gain is found to be important not only for intermediate parton energy,but also for the small propagating distance of the energetic parton.This will increase the energy dependence and propagating distance dependence of the parton energy loss and will affect the shape of suppression of moderately high PT hadron spectra.
Moreover,we studied the flow effects on jet energy loss with detailed balance.We derived a new potential for the interaction of a hard jet with the parton target.It can be used to study the jet quenching phenomena in the presence of collective flow of the quark-gluon medium.With this new potential,we have investigated the effect of collective flow on jet energy loss with detailed balance.Collective flow along the jet direction,with the velocity V_z leads to a reduced opacity, (1 - v_z)~2 times that in static medium,and an increased LPM gluon formation factor,(1 + v_z) times that in static medium.The energy gain without rescattering is the same as in the static medium,but the total energy loss to the first order of opacity is(1 - v_z) times that in the static medium.Compared to calculations for a static medium,our results will affect the suppression of high p_T hadron spectrum and anisotropy parameter v_2 in high-energy heavy-ion collisions.
At RHIC,one of the important signals of QGP is elliptic flow,which reflects the transverse anisotropy of the particle momentum distribution.Parton cascade model,which is proposed to provide a detailed description of the evolution of nuclear collisions at high energy from the onset of hard interactions among the partons,is used to study the elliptic flow.Recently,a new parton cascade model,BAMPS model(Boltzmann approach of multiparton scatterings),is developed with considering the important motivation for developing a consistent algorithm to handle inelastic process like gg←→ggg besides gg←→gg scatterings to study the QCD matter thermalization.In order to study thermalization of the Quark Gluon Plasma at RHIC and LHC in any kinetic approach,one needs suitable initial conditions for the parton production.Here,we use two different methods WN and CGC to study parton production and studied the difference of the rapidity distributions of parton number dN/dy and transverse energy dE_T/dy between the two methods.First,we present the lowest order minijet production from WN and find that the result is consistent with that from Eskola.We then study parton shower and multiple interaction and compare the parton production with that from CGC.It is shown that dN/dy and dE_T/dy from CGC at midrapidity at RHIC are larger than that from WN by using Pythia.We then put the two different parton production into BAMPS model to study QCD matter thermalization and get the same kinetic equilibration,but different chemical equilibration,the parton system stays in chemical equilibrium if using wounded nucleons,but if using CGC,chemical equilibrium is achieved at the same timesacle,1.5 fm/c.We also find the transverse energy dE_T/dy|_y=0 at time 4fm/c using the initial conditions from wounded nucleons is consistent with the experiment data 620±33GeV in central Au-Au collisions at RHIC,but the transverse energy using parton production from CGC is a bit larger.We also study the QCD matter thermalization at LHC and predict that the rapidity distribution of transverse energy dE_T/dy|_y=0 is from 1620 to 2150 GeV.
李政道等人曾于上世纪七十年代预言高能重离子碰撞有可能形成高温高密的极端环境,从而导致强子物质的状态发生改变,生成由大量解除禁闭的夸克,反夸克和胶子组成的一种在夸克层次上的新物质形态—夸克胶子等离子体(Quark Gluon Plasma-QGP),夸克解除禁闭的过程同时也伴随着真空的改变。这一预言极大得推动了高能核物理学理论和实验的发展。近二十年来,从美国布鲁海汶国家实验室(BNL)的AGS实验和欧洲核子研究中心(CERN)的SPS实验,到2000年开始并且目前正在运行的BNL的相对论重离子对撞机(RHIC)实验,人们积累了大量丰富的实验数据。通过对实验数据的全面仔细分析和理论计算表明,在RHIC实验中可能生成了一种强耦合的夸克物质(strongly coupled Quark Gluon Plasma-sQGP)。预计,目前在CERN运行的大型强子对撞机(LHC)将产生寿命更长的QGP物质,有助于进一步了解QGP的基本特性。
在夸克胶子等离子体是否形成的信号研究中,人们相继提出了一些能够区分夸克胶子等离子相和普通强子相的物理观察量。例如:J/ψ压低,大横动量光子产生,奇异粒子相对产额的增大,Bose-Einstein关联等等。在RHIC和LHC能区,由于碰撞能量很高,硬或半硬过程非常重要。因此,把硬过程产生的喷注作为硬探针,通过硬探针与夸克胶子物质相互作用所携带的信息作为硬探针信号的喷注淬火(Jet Quenching)效应引起了人们的极大关注。喷注淬火效应描述的是在相对论性重离子碰撞中,由硬过程而产生的部分子喷注在穿过强相互作用介质时由于多重散射而损失能量。由于部分子喷注的能量损失与它穿过的介质紧密相关,能量损失对末态粒子谱,非对心碰撞而形成的椭圆流以及对心碰撞中形成的单喷注(monojet)的影响有可能成为QGP形成的信号。
M.Gyulassy和王新年在QCD框架下提出了G-W模型,讨论了多次散射诱导胶子辐射导致的部分子喷注穿过强作用介质的能量损失,BDMPS(Baier,Dokshitzer,Mueller,Peigue,Schiff)和GLV(Gyulassy,Levai,Vitev)指出和弹性碰撞的能量损失相比,非弹性碰撞的能量损失的贡献是主要的,喷注的能量损失与发生多次散射的强作用介质靶的厚度的平方成正比。此后,其它作者采用不同的方法和不同的近似,也得到了类似的平方依赖关系的结果。王恩科和王新年接着讨论了在静态热QGP介质中喷注能量损失的细致平衡效应,指出对于中等能量大小的喷注,其热胶子的吸收对唢注能量损失有重要影响。M.Gyulassy和王新年在接下来的工作中考虑了强作用介质的横向膨胀对能量损失的影响。在这些研究中,考虑到了由于介质膨胀导致的介质密度变化而引起的对部分子能量损失的影响。但忽略了高能重离子碰撞中由于温度,描述化学非平衡的逃逸度的时空演化过程。而这些都会影响到德拜屏蔽质量,平均自由程的大小,继而影响喷注在强相互作用介质中的散射次数(opacity),因而对喷注能量损失产生重要影响。
本文的研究工作主要分成两个部分,第一部分研究流和部分子演化对喷注淬火效应的影响。对于部分子演化对喷注淬火的影响,我们利用微扰QCD推导出一组几率方程来研究高能碰撞早期夸克和胶子的化学非平衡的演化过程。我们选择不同的初始条件放入几率方程中来研究演化后横动量随快度的分布和粒子数随快度的分布,并将结果与实验结果进行对照。我们确定了在RHIC中的金—金对心碰撞中,初始温度为550MeV,初始胶子逸度为0.3。我们还得到了温度和逸度随时间的演化关系。我们发现考虑化学非平衡后,与Bjorken预计的1+1维膨胀相比,温度下降的更快。利用这个演化关系,我们分析了德拜屏蔽质量,平均自由程随固有时的变化关系,继而得到了opacity与喷注穿过的介质厚度之间的依赖关系。结果表明,在考虑膨胀介质中化学非平衡的过程后,由胶子辐射产生的喷注的能量损失与喷注穿过强作用介质的距离成线性关系,而不是在静态强作用介质中预言的平方关系。我们同时还考虑了细致平衡对能量损失的影响,对于中等能量大小的喷注,只有当喷注穿过热QGP介质的距离很短时,其热胶子的吸收能量所占的总喷注能量损失的比例较大,而当喷注能量较大或者喷注穿过热QGP介质距离较大时,吸收能量的贡献并不大。对于流效应对喷注淬火的影响,在Yukawa势的基础上,我们导出新的势模型来描述喷注和存在集体流的介质中靶的相互作用,这一新的势模型可用来研究存在集体流的夸克物质介质中的喷注淬火现象。利用这一新的势模型,我们考察了流对具有细致平衡效应的喷注能量损失的影响。结果表明,沿着喷注正方向的集体流减小了喷注在强相互作用介质中的散射次数(opacity),增大了LPM压低因子。设集体流沿喷注方向的速度大小为ν_z,那么喷注在考虑集体流的强相互作用介质中的能量损失是静态系统中的1-ν_z倍。这个结果将对椭圆流的大小及高横动量区强子谱的压低产生影响。
本文研究的第二部分利用两种方法,WN和CGC考察了不同的部分子产生机制。我们首先利用WN计算出由于领头阶硬散射而产生的部分子分布,这个结果与Eskola预计的部分子分布完全一致。我们还考察了初态和末态的胶子辐射和多重散射过程,与利用CGC方法得到的部分子分布进行比较,发现在RHIC能区的Au-Au对心碰撞中,利用CGC产生的部分子数目和横动量都略大于利用WN得到的结果。而在LHC能区,利用CGC产生的部分子随快度分布基本与利用WN的所得结果相同,而横动量随快度分布略大于WM所得结果。将两种不同的部分子分布放入BAMPS模型中分析QGP介质的热化过程,考察了中心快度区横能量随快度分布dE_T/dy丨y=0,捕述动量各向同性的物理量〈p_z~2/E~2〉和逸度等物理量。结果表明,在RHIC能区,利用WN所得的dE_T/dy丨y=0基本与实验结果一致,而CGC所得结果略高于实验数据。两种方法都得到同样的趋于动力学平衡的演化过程,而化学平衡的演化过程略有不同。利用WN所得的部分子分布情况,介质一直处于化学平衡中,而CGC告诉我们,在1.5fm/c后,系统才达到化学平衡。同时,我们还预言了LHC的Pb—Pb对心碰撞中,中心快度区横能量随快度分布的大小应介于1620GeV到2150GeV之间。
In relativistic heavy ion collisions jet quenching has been proposed as a good probe of the hot and dense medium in high - energy nuclear collisions.Jet quenching is caused by gluon radiation induced by multiple scattering during jet propagates the hot QCD medium.As discovered in high-energy heavy-ion collisions at RHIC,jet quenching is manifested in both the suppression of single inclusive hadron spectrum at high transverse momentum PT region and the disappearance of the typical back-to-back jet structure in dihadron correlations.It is found that the analytic approximation of the energy loss by stimulated emission in a static medium is proportional to square of the propagation distance in a static medium.Later the detailed balance effect of jet energy loss in hot QGP medium is investigated.With partial cancellation by stimulated emission, the net result is an energy gain via absorption which reduces the effective patton energy loss. Such a reduction is found to be important for intermediate parton energy but can be neglected at very high energies.
In this dissertation we studied parton equilibration effect on jet quenching with detailed balance.We first studied the parton evolution process from chemical non-equilibrium towards a fully equilibrated parton plasma.We derive a set of rate equations based on perturbative QCD to describe the chemical equilibration of gluons and quarks,and then put different initial conditions in to get the rapidity distribution of transverse energy dE_T/dy and particle multiplicites dN/dy. We compare the data with that from experiment and get that at the initial time 0.3fm/c,the initial temperature is 550MeV,gluon fugacity is 0.3.We also find that the parton gas cools faster than that predicted by Bjorken's scaling solution.With the parton equilibration,it is shown that the Debye screening mass and the inverse mean free path for a gluon reduce with the increasing proper time in the QGP medium.The parton equilibration has effect on patton energy loss with detailed balance,the patton energy loss from stimulated emission is linear dependence on the propagating distance rather than square dependence in the static medium. The energy gain is found to be important not only for intermediate parton energy,but also for the small propagating distance of the energetic parton.This will increase the energy dependence and propagating distance dependence of the parton energy loss and will affect the shape of suppression of moderately high PT hadron spectra.
Moreover,we studied the flow effects on jet energy loss with detailed balance.We derived a new potential for the interaction of a hard jet with the parton target.It can be used to study the jet quenching phenomena in the presence of collective flow of the quark-gluon medium.With this new potential,we have investigated the effect of collective flow on jet energy loss with detailed balance.Collective flow along the jet direction,with the velocity V_z leads to a reduced opacity, (1 - v_z)~2 times that in static medium,and an increased LPM gluon formation factor,(1 + v_z) times that in static medium.The energy gain without rescattering is the same as in the static medium,but the total energy loss to the first order of opacity is(1 - v_z) times that in the static medium.Compared to calculations for a static medium,our results will affect the suppression of high p_T hadron spectrum and anisotropy parameter v_2 in high-energy heavy-ion collisions.
At RHIC,one of the important signals of QGP is elliptic flow,which reflects the transverse anisotropy of the particle momentum distribution.Parton cascade model,which is proposed to provide a detailed description of the evolution of nuclear collisions at high energy from the onset of hard interactions among the partons,is used to study the elliptic flow.Recently,a new parton cascade model,BAMPS model(Boltzmann approach of multiparton scatterings),is developed with considering the important motivation for developing a consistent algorithm to handle inelastic process like gg←→ggg besides gg←→gg scatterings to study the QCD matter thermalization.In order to study thermalization of the Quark Gluon Plasma at RHIC and LHC in any kinetic approach,one needs suitable initial conditions for the parton production.Here,we use two different methods WN and CGC to study parton production and studied the difference of the rapidity distributions of parton number dN/dy and transverse energy dE_T/dy between the two methods.First,we present the lowest order minijet production from WN and find that the result is consistent with that from Eskola.We then study parton shower and multiple interaction and compare the parton production with that from CGC.It is shown that dN/dy and dE_T/dy from CGC at midrapidity at RHIC are larger than that from WN by using Pythia.We then put the two different parton production into BAMPS model to study QCD matter thermalization and get the same kinetic equilibration,but different chemical equilibration,the parton system stays in chemical equilibrium if using wounded nucleons,but if using CGC,chemical equilibrium is achieved at the same timesacle,1.5 fm/c.We also find the transverse energy dE_T/dy|_y=0 at time 4fm/c using the initial conditions from wounded nucleons is consistent with the experiment data 620±33GeV in central Au-Au collisions at RHIC,but the transverse energy using parton production from CGC is a bit larger.We also study the QCD matter thermalization at LHC and predict that the rapidity distribution of transverse energy dE_T/dy|_y=0 is from 1620 to 2150 GeV.
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