RHIC高能核—核碰撞实验中的喷注淬火效应
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摘要
研究高温高密的核物质以及寻找夸克胶子等离子体(QGP)，实验上最有效的方法是通过极端相对论性重离子碰撞。近二十年来，从美国布鲁海汶国家实验室(BNL)质心系能量(SNN)~(1/2)=5GeV的AGS实验，欧洲核子研究中心(CERN)(SNN)~(1/2)=17.2GeV的SPS实验，到2000年开始并且目前正在运行的BNL(SNN)~(1/2)=130GeV，200GeV的相对论重离子对撞机(RHIC)实验，人们已经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考虑，有迹象表明夸克胶子等离子体这一新物质形态已在RHIC实验中形成。美国哥伦比亚大学著名学者M.Gyulassy教授撰文(arXiv:nucl-th／0403032)认为“QGP已在RHIC实验中发现”。预计在2007年，将在CERN建成的大型强子对撞机(LHC)，其质心系能量大约是RHIC的30倍，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，人们相继提出大横动量光子和双轻子对的直接产生，奇异粒子相对产额的增长，J／ψ压低，Bose-Einstein关联等。在RHIC能区，由于碰撞能量很高，硬或半硬过程是重要的，与碰撞硬过程相关的硬探针信号，特别是喷注淬火(Jet Quenching)或喷注能量损失成为目前QGP信号研究的热点。
M.Gyulassy和Xin-Nian Wang在QCD框架下提出了一个模型(G-W模型)，率先讨论了在重离子碰撞中非弹性碰撞产生的部分子喷注穿过强作用介质时由多重散射诱导胶子辐射导致的能量损失，指出和弹性碰撞的能量损失相比，
画
博士学位论文
DO〔1’ORALD【55五RTAI’ION
非弹性碰撞的能量损失的贡献是主要的。R.Baier，Yl;.L.Dokshitzer，A·H.
Mueller，s.Peigne和D，schiff(BDMPS)等人关于非弹性碰撞的能量损失的
细致计算表明:由胶子辐射产生的非阿贝尔LPM效应导致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关系。M.Gyulassy，P.Levai
和1.Vitev(G口)在G一W模型的基础上发展了oPacit}’展开技术计算喷注的能
量损失。王恩科和王新年讨论了在热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，其热胶子的吸收对喷注能量损失有重要影
口向。
部分子喷注穿过强作用介质时，多重散射不仅会诱导胶子辐射，也会诱导
实光子或虚光子辐射。本文研究的第一部分是利用GD厂发展起来的oPacity展开
技术，讨论了具有热质量的部分子喷注穿过强作用介质由多重散射诱导的光子
辐射和双轻子产生。结果表明，辐射光子的能量损失与介质靶的厚度成线性依
赖关系，反映出阿贝尔LPM效应和非阿贝尔LPM效应本质上的不同。同时我们
还得到了辐射光子的横动量谱，发现光子的产生率随横动量的减小而增加。多
重散射诱发的辐射光子的贡献集中在小横动量区域。对于在喷注方向附近发射
小横动量的光子而言，多重散射扮演了一个重要角色。结合其他过程的光子产
生，将使我们对QGP的电磁信号的认识更全面。利用对光子辐射的研究方法和
结果，推广到介质诱导的双轻子产生的研究中，得到了双轻子的不变质量谱，
发现双轻子的产生率随不变质量的减小而增加。在双轻子的不变质量的较小区
域或在虚光子的横动量的较小区域，多重散射诱发的双轻子产生占主要地位。
由于大横动量的强子谱主要是喷注的碎裂产生的，因此，和同一能量下
的p一p碰撞比较，高能重离子碰撞中的部分子能量损失将压低大横动量的强子
谱。部分子能量损失对末态粒子谱、非对心碰撞形成的椭圆流以及对心碰撞中
形成的单喷注(monojet)的影响是目前QGP形成的硬探针信号的重要研究热
点。考虑到核介质效应，特别是QGP介质的喷注淬火效应，本文研究的第二部
⑧羔糕鑫一
分和第三部分内容是RHIC高1毙核一核碰撞实验中的单强子和双强子产生。在近
期现有的高能核一核碰撞产生单强子和双强子的研究中，人们只计算到核子一核
子碰撞中产生强子的领头阶(LO)，通过人为地引入一个因子K计及高阶修正
的贡献。本文通过直接计算核子一核子碰撞产生强子的次领头阶(NLO)的贡
献，考察了办面万=200GeV的Au一Au核碰撞的单强子和双强子产生，避免引入
因子K计及高阶修正的贡献的人为的不确定性。计算结果表明，考虑喷注淬火
效应后，对于单强子产生过程，不同对心度碰撞产生的带电强子谱和中性二介子
谱分别与最近的STAR实验组和PHENIX实验组数据符合。在大横动量区域，喷
注淬火效应导致的非对心碰撞产生的带电单强子椭圆流参数。2与最近的STAR实
验组数据符合。对于不同对心度碰撞带电双强子的产生的理论计算表明，对
心碰撞中，大横动量的强子谱背靠背的关联消失，产生单喷注(MOnojet)事
件。而擦边碰撞中仍然产生双喷注事件。双强子喷注的方位角关联与对心度的
依赖关系和STAR实验组的数据符合。我们还对双强子产生的横动量谱和不变
质量谱，以及横动量谱和不变质量谱对应的椭圆流参数vZ给出了理论预言，有
待RHIC实验的证实。
Hard process are considered as a good tool to study the properties of the quark matter produced in ultra-relativistic heavy-ion collision because it can probe the early stage of the evolution of dense system, during which a quark-gluon plasma (QGP) could exist for a short period of time. One important aspect of hard processes is jet's energy loss or jet quenching due to gluon radiation induced by multiple rescattering for a energetic parton jet going through the strong interacting medium.
In addition to the gluon radiation, the bremsstrahl ung processes induced by multiple rescattering include also the photon radiation. Based on the GW model, we apply the opacity expansion technique to investigate the induced photon radiation and dilepton production in strong interacting medium. The real photon radiation and the dilepton invariant-mass spectrums are given up to the first order in opacity expansion. It is shown that, both the real photon radiation and dilepton invariant spectrums decrease with increasing the transverse momentum: In the low transverse momentum region the rescattering effect plays an important role; At fixed transverse momentum the dilepton production induced by multiple rescattering decrease with increasing dilepton's invariant-mass. For real photon radiation the Abelian LPM effect lead to that the parton jet's energy loss has
linear dependence on the thickness of the targets instead of quadratic dependence arising from non-Abelian LPM effect for gluon radiation.
Because parton jet energy loss gives rise to modification of fragmentation functions, jet quenching in heavy ion collision leads to the suppression of large transverse momentum hadrons spectra as compared to pp collision at the same energy. Taking into account the jet quenching, we calculate numerically single hadron spectra at next-to-leading order (NLO) and the ellipse flow parameter v2 caused by the asymmetric energy loss in transverse plane in Au+Au collisions (s~(1/2)=200GeV). It turns out that the jet quenching can describe the large pr pion and charged hadron spectra in heavy-ion collisions at the RHIC energies very well by different centrality in Au+Au. We also calculate dihadron production in Au+Au collisions (s~(1/2)=200GeV) and the corresponding parameter v2. It turns out that the invariable mass spectrum and large transverse momentum dihadron are all suppressed for the central nucleus-nucleus collision because of jet quenching effect. It is similar to the p-p collision, the produced hadron back-t
o-back correlation is strong for the peripheral nucleus-nucleus collision. This correlation is much weaker with decreasing the compact parameter. For central collision, the back-to-back correlation disappears. We show that w2 is approximately a constant in large transverse momentum region both in single hadron and dihadron production.
M.Gyulassy和Xin-Nian Wang在QCD框架下提出了一个模型(G-W模型)，率先讨论了在重离子碰撞中非弹性碰撞产生的部分子喷注穿过强作用介质时由多重散射诱导胶子辐射导致的能量损失，指出和弹性碰撞的能量损失相比，
画
博士学位论文
DO〔1’ORALD【55五RTAI’ION
非弹性碰撞的能量损失的贡献是主要的。R.Baier，Yl;.L.Dokshitzer，A·H.
Mueller，s.Peigne和D，schiff(BDMPS)等人关于非弹性碰撞的能量损失的
细致计算表明:由胶子辐射产生的非阿贝尔LPM效应导致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关系。M.Gyulassy，P.Levai
和1.Vitev(G口)在G一W模型的基础上发展了oPacit}’展开技术计算喷注的能
量损失。王恩科和王新年讨论了在热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，其热胶子的吸收对喷注能量损失有重要影
口向。
部分子喷注穿过强作用介质时，多重散射不仅会诱导胶子辐射，也会诱导
实光子或虚光子辐射。本文研究的第一部分是利用GD厂发展起来的oPacity展开
技术，讨论了具有热质量的部分子喷注穿过强作用介质由多重散射诱导的光子
辐射和双轻子产生。结果表明，辐射光子的能量损失与介质靶的厚度成线性依
赖关系，反映出阿贝尔LPM效应和非阿贝尔LPM效应本质上的不同。同时我们
还得到了辐射光子的横动量谱，发现光子的产生率随横动量的减小而增加。多
重散射诱发的辐射光子的贡献集中在小横动量区域。对于在喷注方向附近发射
小横动量的光子而言，多重散射扮演了一个重要角色。结合其他过程的光子产
生，将使我们对QGP的电磁信号的认识更全面。利用对光子辐射的研究方法和
结果，推广到介质诱导的双轻子产生的研究中，得到了双轻子的不变质量谱，
发现双轻子的产生率随不变质量的减小而增加。在双轻子的不变质量的较小区
域或在虚光子的横动量的较小区域，多重散射诱发的双轻子产生占主要地位。
由于大横动量的强子谱主要是喷注的碎裂产生的，因此，和同一能量下
的p一p碰撞比较，高能重离子碰撞中的部分子能量损失将压低大横动量的强子
谱。部分子能量损失对末态粒子谱、非对心碰撞形成的椭圆流以及对心碰撞中
形成的单喷注(monojet)的影响是目前QGP形成的硬探针信号的重要研究热
点。考虑到核介质效应，特别是QGP介质的喷注淬火效应，本文研究的第二部
⑧羔糕鑫一
分和第三部分内容是RHIC高1毙核一核碰撞实验中的单强子和双强子产生。在近
期现有的高能核一核碰撞产生单强子和双强子的研究中，人们只计算到核子一核
子碰撞中产生强子的领头阶(LO)，通过人为地引入一个因子K计及高阶修正
的贡献。本文通过直接计算核子一核子碰撞产生强子的次领头阶(NLO)的贡
献，考察了办面万=200GeV的Au一Au核碰撞的单强子和双强子产生，避免引入
因子K计及高阶修正的贡献的人为的不确定性。计算结果表明，考虑喷注淬火
效应后，对于单强子产生过程，不同对心度碰撞产生的带电强子谱和中性二介子
谱分别与最近的STAR实验组和PHENIX实验组数据符合。在大横动量区域，喷
注淬火效应导致的非对心碰撞产生的带电单强子椭圆流参数。2与最近的STAR实
验组数据符合。对于不同对心度碰撞带电双强子的产生的理论计算表明，对
心碰撞中，大横动量的强子谱背靠背的关联消失，产生单喷注(MOnojet)事
件。而擦边碰撞中仍然产生双喷注事件。双强子喷注的方位角关联与对心度的
依赖关系和STAR实验组的数据符合。我们还对双强子产生的横动量谱和不变
质量谱，以及横动量谱和不变质量谱对应的椭圆流参数vZ给出了理论预言，有
待RHIC实验的证实。
Hard process are considered as a good tool to study the properties of the quark matter produced in ultra-relativistic heavy-ion collision because it can probe the early stage of the evolution of dense system, during which a quark-gluon plasma (QGP) could exist for a short period of time. One important aspect of hard processes is jet's energy loss or jet quenching due to gluon radiation induced by multiple rescattering for a energetic parton jet going through the strong interacting medium.
In addition to the gluon radiation, the bremsstrahl ung processes induced by multiple rescattering include also the photon radiation. Based on the GW model, we apply the opacity expansion technique to investigate the induced photon radiation and dilepton production in strong interacting medium. The real photon radiation and the dilepton invariant-mass spectrums are given up to the first order in opacity expansion. It is shown that, both the real photon radiation and dilepton invariant spectrums decrease with increasing the transverse momentum: In the low transverse momentum region the rescattering effect plays an important role; At fixed transverse momentum the dilepton production induced by multiple rescattering decrease with increasing dilepton's invariant-mass. For real photon radiation the Abelian LPM effect lead to that the parton jet's energy loss has
linear dependence on the thickness of the targets instead of quadratic dependence arising from non-Abelian LPM effect for gluon radiation.
Because parton jet energy loss gives rise to modification of fragmentation functions, jet quenching in heavy ion collision leads to the suppression of large transverse momentum hadrons spectra as compared to pp collision at the same energy. Taking into account the jet quenching, we calculate numerically single hadron spectra at next-to-leading order (NLO) and the ellipse flow parameter v2 caused by the asymmetric energy loss in transverse plane in Au+Au collisions (s~(1/2)=200GeV). It turns out that the jet quenching can describe the large pr pion and charged hadron spectra in heavy-ion collisions at the RHIC energies very well by different centrality in Au+Au. We also calculate dihadron production in Au+Au collisions (s~(1/2)=200GeV) and the corresponding parameter v2. It turns out that the invariable mass spectrum and large transverse momentum dihadron are all suppressed for the central nucleus-nucleus collision because of jet quenching effect. It is similar to the p-p collision, the produced hadron back-t
o-back correlation is strong for the peripheral nucleus-nucleus collision. This correlation is much weaker with decreasing the compact parameter. For central collision, the back-to-back correlation disappears. We show that w2 is approximately a constant in large transverse momentum region both in single hadron and dihadron production.
引文
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