弦张量与奇异粒子产生

用一种参数化的形式研究了弦上具有硬胶子纽结所导致的有效弦张量的增长.在此种形式下强子一强子碰撞中有效弦张量随碰撞能量的增加而增加是来自碰撞中微喷注的产生.这种机制既能解释强子一强子碰撞奇异夸克压低因子随碰撞能量之增而增的行为,又能解释e⁺e^－湮没中该因子与能量近似无关的现象.     

ee0A GeV S+Au反应中光子产生的低横动量增强

用建立在LUND弦模型特别是PYTHIA事件产生器基础上的描写极端相对论性核一核碰撞的强子和弦级联模型—JPCIAE研究了入射能量为200A GeV的S+ Au中心碰撞中光子产生.模型同时考虑了部分子QCD散射过程、强子末态相互作用以及强子衰变等光子产生反应道并作了协调处理.JPCIAE模型计算结果很好再现了WA93实验数据所呈现的低横动量增强效应.     

200A GeV S+Au反应中光子产生的低横动量增强

用建立在LUND弦模型特别是PYTHIA事件产生器基础上的描写极端相对论性核-核碰撞的强子和弦级联模型--JPCIAE研究了入射能量为200A GeV的S+Au中心碰撞中光子产生.模型同时考虑了部分子QCD散射过程、强子末态相互作用以及强子衰变等光子产生反应道并作了协调处理.JPCIAE模型计算结果很好再现了WA93实验数据所呈现的低横动量增强效应.     

相对论性核碰撞中φ介子增强

用强子和弦级联模型LUCIAE系统地研究了相对论性p+p和Pb+Pb碰撞中φ介子产生.在模型框架内实验测得的158AGeV/cPb+Pb碰撞中φ介子产额相对于同能量p+p碰撞的增强现象,依赖于弦发射胶子的集体效应和奇异夸克压低约化两机制,在一定程度内能得到解释.     

高能强子–强子碰撞中喷注性质的蒙特卡洛研究

在喷注“圆锥判定法”的基础上,对高能强子–强子碰撞中产生的喷注(微喷注)的性质进行了蒙特卡洛研究.采用以喷注动量为z轴的“喷注坐标系”,给出了表征喷注性质的各物理量在新坐标系中的分布情况.结果表明,圆锥判定法能够作为一种有效手段来对高能强子–强子碰撞和相对论重离子碰撞中发生的硬和半硬过程开展实验研究.由有喷注事件和无喷注事件的多重数分布可以看到,E_t=2GeV是用圆锥法确定喷注的合理的横能截断值.     

400GeV/c pp碰撞间歇指数的测量

利用CERNNA27合作组提供的LEBC泡室照片对400GeV/c pp碰撞产生的带电粒子赝快度分布进行了测量.计算了标度阶乘矩.得出间歇指数随矩阶数的增加而增加,随平均多重数的增加而变小;反常分形维数dq随q的增加而增加.这表明在强子-强子碰撞中多粒子产生具有自相似级联的性质.     

强子碰撞中平均横动量的多重数分布随截断的变化

利用三火球模型,对s=22GeV情况下,强子碰撞中平均横动量的多重数分布曲线斜率、随横动量截断值的增加由负转变为正进行研究的结果表明,是侧边火球产生强子的平均横动量,随横动量截断值的增加的增幅大于中心火球产生强子的平均横动量的增幅而出现的.     

核–核碰撞中喷注淬火导致的双强子产生压低

通过考虑喷注淬火效应,分析了相对论性高能重离子碰撞中双强子的产生.结果表明,喷注淬火压低了大不变质量谱和大横动量的双强子的产生.与质子–质子碰撞的情形类似,核–核的擦边碰撞(碰撞参数很大)产生的强子有很强的背靠背的关联.在核–核对心碰撞(碰撞参数很小)中,由于喷注穿过强作用物质导致的喷注淬火介质效应,产生的强子的背靠背的关联几乎消失.     

相对论重离子碰撞中Λ粒子产生和Λ/p比率

利用强子输运–弦碎裂模型研究了200A GeV重离子碰撞中Λ多重产生和Λ/p比率.研究了它们随反应系统大小和碰撞中心度的变化.研究表明Λ和p多重数都随反应系统增大和随碰撞中心度提高而增大,但它们的比率基本保持不变.Λ的湮没截面大小变化对Λ的多重数产生有一定影响,但对Λ/p比率影响不大.结果很好地再现了pp反应中Λ/p比率.并获得这一比率在AA反应中是pp反应中的3—5倍.     

几何模型中强子散射的算子理论

用算子形式的S矩阵,在几何模型下导出二个强子散射ab→cd过程的弹性散射,绕射分解,一次非弹性碰撞,双交换碰撞过程的微分截面公式。并计算了双交换碰撞事例,与实验比较能初步相符合。     

String Tension and Production of Strange Particles

The increase of effective string tension as a result of the hard gluon kinks on a string is investigated using a parametrization form. In this form the effective string tension increasing with energies in hadron-hadron collisions is due to the mini-jet (gluon) production in the collisions. The data for the energy dependence of the strange quark suppression factor in hadron-hadron collisions are very well reproduced with this mechanism. Meanwhile, the experimental phenomena of approximate energy independence of the strange quark suppression factor in e⁺e^－ -annihilations are discussed.     

相对论重离子碰撞中的重味产生

重味粒子是新的物质形态——夸克胶子等离子体的敏感探针。 利用相对论流体力学描述夸克胶子等离子体的时空演化, 采用输运方程模拟重味粒子在夸克胶子等离子体中的运动, 既考虑重味粒子的热胶子离解, 也通过细致平衡原理包含重味粒子在热密媒质中的重产生。 正是由于离解与重产生之间的竞争以及竞争对于碰撞能量、 横动量和快度的依赖性, 自然解释了在RHIC能区的J/ψ疑难, 预言了在LHC能区由于重产生取得主导地位, J/ψ的核修正因子在中心和半中心碰撞中将随着参与反应核子数的增大而升高, 同时其平均横动量会受到强烈的压低。 Heavy quarkonium is a sensitive signature of the new state of matter-quark gluon plasma produced in high energy nuclear collisions. We describe the space time evolution of the quark gluon plasma by relativistic hydrodynamic equations and the quarkonium motion by transport equation. We found that the competition between the gluon dissociation and regeneration can explain naturally the J/ψ puzzles at RHIC energy. We predict the increase of the nuclear modification factor in semi central and central collisions and the related transverse momentum suppression at LHC energy.     

Multiplicity distribution in proton-proton and proton-antiproton collisions at high energies

Multiplicity distributions of secondary hadrons produced in p $ \bar p $ \bar p and collisions are very different. There are three types of inelastic processes in p $ \bar p $ \bar p scattering. The first one is the production of a shower of secondary hadrons in gluon string decay. The second one is the shower produced from the decay of two quark strings, and the third one is the shower produced from the decay of three quark strings. At the same time, there are just two types of inelastic processes for pp scattering: the shower from the gluon string and the shower from two quark strings. The multiplicity distribution and the average multiplicity of charged hadrons for an energy of 14 TeV are predicted.     

Heavy quark energy loss in a nuclear medium

Multiple scattering, modified fragmentation functions, and radiative energy loss of a heavy-quark propagating in a nuclear medium are investigated in perturbative QCD. Because of the quark mass dependence of the gluon formation time, the medium size dependence of heavy-quark energy loss is found to change from a linear to a quadratic form when the initial energy and momentum scale are increased relative to the quark mass. The radiative energy loss is also significantly suppressed relative to a light quark due to the suppression of collinear gluon emission by a heavy quark.     

Direct probes of linearly polarized gluons inside unpolarized hadrons

We show that linearly polarized gluons inside unpolarized hadrons can be directly probed in jet or heavy quark pair production in electron-hadron collisions. We discuss the simplest cos2? asymmetries and estimate their maximal value, concluding that measurements of the unknown linearly polarized gluon distribution in the proton should be feasible in future Electron-Ion Collider or Large Hadron electron Collider experiments. Analogous asymmetries in hadron-hadron collisions suffer from factorization breaking contributions and would allow us to quantify the importance of initial- and final-state interactions.     

Jet charge in high-energy nuclear collisions

The averaged jet charge characterizes the electric charge of the initiating parton and provides a powerful tool to distinguish quark jets from gluon jets.We predict,for the first time,the medium modification of the averaged jet charge in the heavy-ion collisions at the LHC,where jet productions in p+p collisions are simulated by PYTHIA6,and the parton energy loss in QGP is calculated with two Monte Carlo models of jet quenching:PYQUEN and JEWEL.We found that the distribution of averaged jet charge is significantly suppressed by initial state isospin effects due to the participation of neutrons with zero electric charge during nuclear collisions.The considerable enhancement of the averaged jet charge in central Pb+Pb collisions is observed relative to peripheral collisions,since the jet quenching effect is more pronounced in central collisions.The distinct feature of the averaged jet charge between quark and gluon jets,along with the sensitivity of medium modifications on the jet charge to flavor dependence of the parton energy loss,could be very useful to discriminate the energy loss pattern between quark and gluon jets in heavy-ion collisions.     

Heavy quark energy loss in high multiplicity proton-proton collisions at the LHC

One of the most promising probes to study deconfined matter created in high energy nuclear collisions is the energy loss of (heavy) quarks. It has been shown in experiments at the Relativistic Heavy Ion Collider that even charm and bottom quarks, despite their high mass, experience a remarkable medium suppression in the quark gluon plasma. In this exploratory investigation we study the energy loss of heavy quarks in high multiplicity proton-proton collisions at LHC energies. Although the colliding systems are smaller than compared to those at the Relativistic Heavy Ion Collider (p+p vs Au+Au), the higher energy might lead to multiplicities comparable to Cu+Cu collisions at the Relativistic Heavy Ion Collider. The interaction of charm quarks with this environment gives rise to a non-negligible suppression of high momentum heavy quarks in elementary collisions.     

Spin-dependent parton distributions from polarized structure function data

In the past year, polarized deep inelastic scattering experiments at CERN and SLAC have obtained structure function measurements off proton, neutron and deuteron targets at a level of precision never before achieved. The measurements can be used to test the Bjorken and Ellis-Jaffe sum rules, and also to obtain information on the parton distributions in polarized nucleons. We perform a global leading-order QCD fit to the proton deep inelastic data in order to extract the spin-dependent parton distributions. By using parametric forms which are consistent with theoretical expectations at large and smallx, we find that the quark distributions are now rather well constrained. We assume that there is no significant intrinsic polarization of the strange quark sea. The data are then consistent with a modest amount of the proton's spin carried by the gluon, although the shape of the gluon distribution is not well constrained, and several qualitatively different shapes are suggested. The spin-dependent distributions we obtain can be used as input to phenomenological studies for future polarized hadron-hadron and lepton-hadron colliders.