夸克喷注和胶子喷注硬度特性的研究

用蒙特卡洛模拟方法研究了Z⁰强子衰变中的三喷注事件.用b标记方法辨别,得到夸克喷注和胶子喷注.定义了一个新的物理量—硬度.分别计算了发射了胶子的夸克喷注和胶子喷注的硬度分布.通过胶子和夸克喷注的平均多重数和平均横动量对硬度的依赖性以及他们的多重数和横动量在相同硬度区间内的分布情况的计算与比较,发现夸克喷注和胶子喷注的硬度特性有显著的差别.夸克喷注的特性几乎与硬度无关,而胶子喷注的特性与硬度之间存在较强的关联.当Hd>10GeV时,胶子喷注的平均多重数和平均横动量都明显地大于夸克喷注的平均多重数和平均横动量;而当H_d<6GeV时,结果正好相反.这表明,只是对于较硬的胶子,胶子喷注比夸克喷注“胖”的结论才能成立.     

胶子喷注和夸克喷注性质的蒙特卡洛研究

用蒙特卡洛方法研究了91.2GeV e⁺e^－碰撞产生的3喷注事件.用3个喷注之间的夹角来标识各个喷注,分别计算了3个喷注的能量及能量分布,并在相同能量下计算了3个喷注的多重数,横动量及其分布.通过与能量相同的2喷注事件中单夸克喷注的上述性质的比较,得到了从3喷注事件中挑选胶子喷注和夸克喷注的一种简便方法.这样挑选出来的胶子和夸克喷注在性质上与QCD的理论预言一致,并且胶子和夸克喷注的平均多重数比值的计算结果与实验观测值符合     

在s=10—178GeV的e+e－湮没中粲介子与美介子的产额

根据电弱统一标准模型,本文研究了e⁺e^－湮没中各种味道i(i=u,d,s,c,b)的夸克喷注产生几率P_iP_i是能量s的函数.计算的5种夸克喷注产生的几率之比P_u:P_d:P_s:P_c:P_b当s<30GeV时为4:1:1:4:1,当s=m_z=91.16GeV时为7:9:9:7:9,而当s=78与117GeV时为1:1:1:1:1.利用P_c与P_b,我们计算了s=10—178GeV的e⁺e^－湮没中粲介子与美介子的产额     

对Z0三喷注事例中色联接敏感可观测量的研究

将末态部分子的色分离联接引入JETSET事例产生器,通过用喷注的DURHAM判据y_cut及用喷注夹角限制事例形状,对Z⁰衰变进行三喷注事例挑选,发现可以得到对色联接敏感的三喷注事例.对于这类事例,末态强子的平均带电多重数及胶子喷注中的一系列可观测量对色联接的敏感性随y_cut值的减小而增大;在小y_cut值时,色中性流和色分离两种联接导致了显著差别.使通过实验测量鉴别部分子的色联接成为可能.     

双喷注事件中单个喷注内部动力学起伏的蒙特卡洛研究

用蒙特卡洛方法研究了91.2GeV e⁺e^－碰撞产生的双喷注事件中单个喷注内部的动力学起伏.结果表明,喷注内部的动力学起伏的各向异性随着截断参数y_cut的变化而明显地改变.存在一个转变点,在该点处动力学起伏在纵横平面是各向异性的,而在横平面是各向同性的.由此得到了产生喷注的一个标度.     

HERA硬光生过程中的光子产生

在领头级近似下计算了HERA硬光生过程中直接光子和分解光子与质子作用的单、双光子产生截面.结果表明:当x_T≤0.15时,对单光子产生的主要贡献来自于分解光子过程;当x_T≥0.15时,主要贡献来自于直接光子过程.双光子的主要产生源则是分解光子的两个子过程:qq→γγ和gg→γγ,且双光子的产生截面与同等条件下,h₁h₂→γγX的截面在同一量级.研究双光子产生有助于探明分解光子的结构.     

夸克喷注与胶子喷注角分布特性的研究

用蒙特卡洛模拟Jetset 7.4产生质心能量为91.2GeV的正负电子对撞事件. 在引入喷注的圆锥角后, 对不同味的夸克喷注和由不同味夸克发射的胶子产生的喷注的角分布特性进行了仔细研究. 结果发现, 在相同的能量下, 胶子喷注的角分布范围比夸克喷注的角分布范围要明显宽一些. 在带电粒子数、横动量相同的条件下, 胶子喷注的平均圆锥角比夸克喷注的平均圆锥角大很多. 喷注的平均圆锥角随带电多重数以及喷注的横动量的分布都呈线性正关联. 由不同味的夸克发射的胶子产生的喷注的圆锥角分布特性相同；而不同味的夸克喷注的圆锥角分布存在明显的差异.     

Δr的实验测量与唯象分析

本文根据LEP和CDF等实验组的最新实验结果,导出了辐射修正因子Δr的实验值,并把该结果和根据标准模型理论计算得到的Δr理论值进行了比较.本文计算了全单圈和主要双圈的贡献,根据几率分布得到了对M_t比较严格的上限限制;当取Higgs粒子质量M_H=100GeV时M_t<175GeV(95%C.L.).本文还分析计算了其它实验误差和理论误差对M_t的影响.     

pp→ΛΛ过程的胶子交换势与pp→ΛΛ过程的反应截面

本文给出了夸克-反夸克对 湮没和夸克-反夸克对 产生过程的单胶子和双胶子交换势准至P²/m²级近似的形式,据此推出了pp→ΛΛ过程的等效位势.应用这个位势在平面波玻恩近似(PWBA)下计算pp→ΛΛ反应的微分截面和总截面,得到了与实验符合得较好的结果.     

R值测量及相关物理

工作在北京正负电子对撞机(BEPC)上的北京谱仪(BES)对2—5GeV能区的R值进行了精确测量.这一成果对寻找新物理和新粒子Higgs在国际高能物理界带来巨大冲击.本文对R值测量及相关物理,如μ子反常磁矩α_μ,QED耦合常数α(M_z²),μ子偶素的基态超精细劈裂(HFS)以及确定强耦合常数α_s和重夸克质量等的最新进展和未来展望在实验和理论两方面均作了较详尽的介绍.     

Dependence of Jet Photoproduction on Scales in Higher Order QCD Corrections at HERA

The dependencies of two-jet cross section on renormalization scale and factorization scales of resolved photon and proton are investigated up to next-to-leading order (NLO) QCD. It is shown that the NLO cross section is quite sensitive to the choice of the renormalization scale and the factorization scales of resolved photon and proton in large invariant mass and large negative. rapidity of the two-jet system.     

Resolved Photoproduction of J/ψ at HERA

The J/ψ photoproduction via resolved photon process at HERA is studied. It turns out that the J/ψ production is dominated by the leading order production process in low pt range (p_t < 4 GeV), while in the high pt range (p_t > 4 GeV), it is dominated by the fragmentation processes. The difference of J/ψ cross section caused by using GRV and DO photon structure functions is large, especially in the large negative rapidity range. Hence it is possible to use the J/ψ photoproduction process to probe the parton distribution of photon and proton, as well as to test the gluon and heavy quark fragmentation of J/ψ from the perturbative QCD.     

Measurement of inclusive D^{\ast\pm} and associated dijet cross sections in photoproduction at HERA

Inclusive photoproduction of mesons has been measured for photon-proton centre-of-mass energies in the range GeV and photon virtuality 1 GeV. The data sample used corresponds to an integrated luminosity of 37 pb. Total and differential cross sections as functions of the transverse momentum and pseudorapidity are presented in restricted kinematical regions and the data are compared with next-to-leading order (NLO) perturbative QCD calculations using the “massive charm” and “massless charm” schemes. The measured cross sections are generally above the NLO calculations, in particular in the forward (proton) direction. The large data sample also allows the study of dijet production associated with charm. A significant resolved as well as a direct photon component contribute to the cross section. Leading order QCD Monte Carlo calculations indicate that the resolved contribution arises from a significant charm component in the photon. A massive charm NLO parton level calculation yields lower cross sections compared to the measured results in a kinematic region where the resolved photon contribution is significant. Received: 9 July 1998 / Published online: 22 October 1998     

NLO electroweak corrections to Higgs boson production at hadron colliders

Results for the complete NLO electroweak corrections to Standard Model Higgs production via gluon fusion are included in the total cross section for hadronic collisions. Artificially large threshold effects are avoided working in the complex-mass scheme. The numerical impact at LHC (Tevatron) energies is explored for Higgs mass values up to 500 GeV (200 GeV). Assuming a complete factorization of the electroweak corrections, one finds a +5% shift with respect to the NNLO QCD cross section for a Higgs mass of 120 GeV both at the LHC and the Tevatron. Adopting two different factorization schemes for the electroweak effects, an estimate of the corresponding total theoretical uncertainty is computed.     

QCD and electro-weak corrections to the production of Higgs+2 jets at the LHC

We present complete next-to-leading-order (NLO) QCD correction and electro-weak (EW) correction to the production cross section of Higgs-Boson in association with two hard quark jets by using Monte-Carlo numerical calculation program, HAWK, for various photon parton distribution functions at the Large Hadron Collider with center of mass energy 7, 8, 13, and 14 TeV without and with typical vector-Boson-fusion cuts on the tagging jets. In our calculation we include complete contribution from the full set of t-channel, u-channel and s-channel Feynman diagrams, and corresponding interferences as well as NLO QCD and EW corrections.     

QCD corrections to double J/psi production in e+e- annihilation at sqrt[s]=10.6 GeV

Next-to-leading-order (NLO) QCD corrections to double J/psi production in e+e- annihilation at sqrt[s]=10.6 GeV are calculated. We find that they greatly decrease the cross section, with a K factor (NLO/LO) ranging from -0.31 to 0.25 depending on the renormalization scale. Although the renormalization scale dependence indicates a large uncertainty, when combined with the NLO QCD corrections to J/psi+etac production, it can explain why the double J/psi production could not be found at B factories while the J/psi+etac production could, despite the fact that cross section of the former is larger than that of the latter at LO by a factor of 1.8.     

Analytic coupling and Sudakov effects in exclusive processes: pion and \gamma ^*\gamma \to \pi ^0 form factors

We develop and discuss in technical detail an infrared-finite factorization and optimized renormalization scheme for calculating exclusive processes, which enables the inclusion of transverse degrees of freedom without entailing suppression of calculated observables, like form factors. This is achieved by employing an analytic, i.e., infrared stable, running strong-coupling which removes the Landau singularity at by a minimum power-behaved correction. The ensuing contributions to the cusp anomalous dimension – related to the Sudakov form factor – and to the quark anomalous dimension – which controls evolution – lead to an enhancement at high of the hard part of exclusive amplitudes, calculated in perturbative QCD, while simultaneously improving its scaling behavior. The phenomenological implications of this framework are analyzed by applying it to the pion's electromagnetic form factor, including the NLO contribution to the hard-scattering amplitude, and also to the pion–photon transition at LO. For the pion wave function, an improved ansatz of the Brodsky–Huang–Lepage type is employed, which includes an effective (constituent-like) quark mass, GeV. Predictions for both form factors are presented and compared to the experimental data, applying Brodsky–Lepage–Mackenzie commensurate scale setting. We find that the perturbative hard part prevails at momentum transfers above about 20 GeV, while at lower values the pion form factor is dominated by Feynman-type contributions. The theoretical prediction for the form factor indicates that the true pion distribution amplitude may be somewhat broader than the asymptotic one. Received: 19 May 2000 / Revised version: 16 August 2000 / Published online: 13 November 2000     

Diffractive upsilon photo-production off the proton in the QCD inspired model

Based on the generalized vector meson dominance model in QCD, we study the photoproduction of vector meson Υ off the proton by use of the QCD-inspired model in which the contributions from quark–quark, gluon–gluon and quark–gluon interference term to observable are taken into consideration. Calculations are performed for total cross section σ_tot, differential cross section dσ/dt, ratio of the real part to imaginary part of forward scattering amplitude ρ and nuclear slop parameter function β. We analyze the individual contributions from quark gluon degrees of freedom and the QCD Odderon to the total cross section σ_tot (s), differential cross section dσ/dt, ratio of the real part to imaginary part of the forward scattering amplitude ρ, and nuclear slop parameter function β. The mediators of interactions between projectiles (the quark and antiquark pair fluctuated from the real photon) and the proton target (three quark system) are the tensor Glueball and Odderon instead of using the usual Pomeron exchange. The theoretical predictions for σ_tot(s) are consistent with the experimental data within error bars of the data. The data for dσ/dt, β and ρ are urgently needed. The gluon–gluon interaction makes a significant contribution to the observables while the Odderon contribution is negligibly small. Therefore, we may conclude that it is impossible to find the QCD Odderon in the γ+p→Υ+p process as suggested before.     

Next-to-leading-order QCD correction to e(+)e(-)-->J/psi+eta(c) at sqrt[s]=10.6 GeV

One of the most challenging open problems in heavy quarkonium physics is the double charm production in e+e- annihilation at B factories. The measured cross section of e+e- --> J/psi + eta(c) is much larger than leading order (LO) theoretical predictions. With the nonrelativistic QCD factorization formalism, we calculate the next-to-leading order (NLO) QCD correction to this process. Taking all one loop self-energy, triangle, box, and pentagon diagrams into account, and factoring the Coulomb-singular term into the cc bound state wave function, we get an ultraviolet and infrared finite correction to the cross section of e+e- --> J/psi + eta(c) at sqrt[s] = 10:6 GeV. We find that the NLO QCD correction can substantially enhance the cross section with a K factor (the ratio of NLO to LO) of about 1.8-2.1; hence, it greatly reduces the large discrepancy between theory and experiment.     

Factorization scheme and parton distributions in the polarized virtual photon target

We investigate spin-dependent parton distributions in the polarized virtual photon target in perturbative QCD up to the next-to-leading order (NLO). In the case , where is the mass squared of the probe (target) photon, the parton distributions can be predicted completely up to NLO, but they are factorization-scheme dependent. We analyze the parton distributions in six different factorization schemes and discuss their scheme dependence. We study, in particular, the QCD and QED axial anomaly effects on the first moments of the parton distributions to see the interplay between the axial anomalies and factorization schemes. We also show that the factorization-scheme dependence is characterized by the large-x behaviors of the quark distributions in the virtual photon. The gluon distribution is predicted to be the same up to NLO among the six factorization schemes examined. In particular, the first moment of the gluon distribution is found to be factorization-scheme independent up to NLO. Received: 24 January 2001 / Published online: 18 May 2001