Generalized parton distributions of the photon

We present a first calculation of the generalized parton distributions of the photon (both polarized and unpolarized) using overlaps of light-front wave functions at leading order in α and zeroth order in αs; for non-zero transverse momentum transfer and zero skewness. We present the novel parton content of the photon in transverse position space.     

Helicity-dependent generalized parton distributions for nonzero skewness

We investigate the helicity-dependent generalized parton distributions (GPDs) in momentum as well as transverse position (impact) spaces for the u and d quarks in a proton when the momentum transfer in both the transverse and the longitudinal directions are nonzero. The GPDs are evaluated using the light-front wave functions of a quark–diquark model for nucleon where the wave functions are constructed by the soft-wall AdS/QCD correspondence. We also express the GPDs in the boost-invariant longitudinal position space.     

Measurement of dijet cross-sections in low {\rm Q}^2 ep collisions and the extraction of an effective parton density for the virtual photon

The triple-differential dijet cross-section, , is measured with the H1 detector at HERA as a function of the photon virtuality , the fraction of the photon's momentum carried by the parton entering the hard scattering, , and the square of the mean transverse energy, , of the two highest jets. Jets are found using a longitudinal boost-invariant clustering algorithm in the center of mass frame. The measurements cover the ranges GeV in virtuality and in inelasticity y. The results are well described by leading order QCD models which include the effects of a resolved component to the virtual photon. Models which treat the photon as point-like fail to describe the data. An effective leading order parton density for the virtual photon is extracted as a function of the photon virtuality, the probing scale and the parton momentum fraction. The and probing scale dependences of the parton density show characteristic features of photon structure, and a suppression of this structure with increasing is seen. Received: 18 December 1998 / Published online: 25 February 2000     

Photoproduction of large transverse momentum mesons and production of large-pT photons and leptons in proton-proton collisions

The photoproduction of large transverse momentum mesons is examined in a leading particle model using the techniques of the covariant parton model. In the leading particle model the large-p_T meson is produced in a high-energy, wide-angle scattering: γ + q → M + q. Numerical results such as inclusive cross sectioons, angular distributions and particle ratios are presented for the pseudoscalar mesons. The Landshoff-Polking-horne quark-fusion model is then applied to the production of large transverse momentum photons and leptons in proton-proton collisions. In the lepton case this model differs from the usual Drell-Yan mechanism in that the lepton pair has a small invariant mass but comes from a large-p_T virtual photon.     

Some numerical studies of the evolution of generalized parton distributions

We study the evolution behavior of generalized parton distributions at small longitudinal momentum fraction. Particular attention is paid to the ratio of a generalized parton distribution and its forward limit, to the mixing between quarks and gluons, and to the dependence on the squared momentum transfer t.     

Study of scalar meson f 0(980) from B→f 0(980)K * decays

We show how parton distributions unintegrated over the parton transverse momentum, k _t , may be generated, at NLO accuracy, from the known integrated (DGLAP-evolved) parton densities determined from global data analyses. A few numerical examples are given, which demonstrate that sufficient accuracy is obtained by keeping only the LO splitting functions together with the NLO integrated parton densities. However, it is important to keep the precise kinematics of the process, by taking the scale to be the virtuality rather than the transverse momentum, in order to be consistent with the calculation of the NLO splitting functions.     

Why breakup of photons and pions into forward dijets is so different: Predictions from nonlinear nuclear <Emphasis Type="Italic">k</Emphasis><Subscript>⊥</Subscript> factorization

Based on an approach to non-Abelian propagation of color dipoles in a nuclear medium, we formulate a nonlinear k_⊥ factorization for the breakup of photons and pions into forward hard dijets in terms of the collective Weizsäcker-Williams glue of nuclei. We find quite distinct practical consequences of nonlinear nuclear k_⊥ factorization for interactions of pointlike photons and nonpointlike pions. In the former case, the large transverse momentum p_⊥ of jets comes from the intrinsic momentum of quarks and antiquarks in the photon, and nuclear effects manifest themselves as an azimuthal decorrelation with an acoplanarity momentum of the order of the nuclear saturation momentum Q _A . In the breakup of pions off free nucleons to the leading order in pQCD, the spectator parton has a small transverse momentum and the hard dijet cross section is suppressed. In the breakup of pions off heavy nuclei, the forward hard jets are predicted to be entirely decorrelated. We comment on the sensitivity of the pionic dijet cross section to the pion distribution amplitude. The predicted distinction between the breakup of photons and pions can be tested by the sphericity and thrust analysis of the forward hadronic system in the COMPASS experiment at CERN.     

The transverse momentum of partons

It is shown that when the parton model is formulated in proper, covariant fashion the average transverse momentum of the partons in a hadron varies with their fractional longitudinal momentum x. It is small near x = 0, but could rise to a large as x → 1.     

QCD jets ine + e −-annihilation and the transition into hadrons

A model has been developed describing QCD jets in the leading logarithmic approximation (LLA) and the subsequent transition into hadrons via a chain decay model. Besides of the production of mesons, the model is also able to describe baryon production. Agreement with recent data frome ⁺ e ^?-annihilation is found. This includes average multiplicities, average transverse momenta and transverse momentum distributions, longitudinal momentum distributions as well as particle production ratios π^α:K ^π : p(0070), charge compensation probabilities and energy flux correlations. The model is compared to similar models developed by other authors.     

Electroproduction of vector mesons: factorization of end-point contributions

The end-point contributions in the quark longitudinal momentum fraction of the virtual photon ( g^*\gamma^* ) to vector meson (V) impact factor to the diffractive electroproduction amplitude can be factorized in terms of a generalized parton evolution of the target parton distribution. This result is used to model the helicity amplitudes g^* p ? V p\gamma^* p \rightarrow V p in terms of small x generalized parton distributions.     

The physical structure of non-scaling parton models

We discuss the nature of the constituent system seen by probe of momentum Q in non-scaling parton models. The dependence at fixed energy of the parton transverse momentum upon x, the longitudinal momentum fraction, is also investigated. On rather general grounds it is shown that it will increase with x at large x and in a particular model it is found to decrease with x at small x.     

Measurement of dijet cross sections in photoproduction at HERA

Dijet cross sections as functions of several jet observables are measured in photoproduction using the H1 detector at HERA. The data sample comprises data with an integrated luminosity of 34.9 pb. Jets are selected using the inclusive algorithm with a minimum transverse energy of 25 GeV for the leading jet. The phase space covers longitudinal proton momentum fraction and photon longitudinal momentum fraction in the ranges and . The predictions of next-to-leading order perturbative QCD, including recent photon and proton parton densities, are found to be compatible with the data in a wide kinematical range. Received: 4 January 2002 / Published online: 12 July 2002     

Wigner Distributions in Light-Front Quark Models

We discuss the quark Wigner distributions which represent the quantum-mechanical analogues of the classical phase-space distributions. These functions can be obtained through a Fourier transform in the transverse space of the generalized transverse momentum dependent parton distributions, which encode the most general one-body information of partons in momentum space. In particular, we present a study within light-front quark models. The quark orbital angular momentum is also obtained from the phase-space average of the orbital angular momentum operator weighted with the Wigner distribution of unpolarized quark in a longitudinally polarized nucleon. The corresponding results calculated within different light-front quark models are compared with alternative definitions of the quark orbital angular momentum as given in terms of generalized parton distributions and transverse momentum dependent parton distributions.     

强作用介质诱导的光子辐射和双轻子产生

考察了相对论性高能重离子碰撞中产生的硬部分子喷注穿过强作用介质时,喷注与介质中的部分子多次散射诱导的光子辐射与双轻子产生,得到了对应于opacity展开第一阶的光子横动量谱,辐射光子导致的喷注的能量损失以及双轻子的不变质量谱,结果表明,光子的产生率随横动量的增加而降低,双轻子的产生率随其不变质量的增加而减小,辐射光子导致的能量损失线性依赖强作用介质靶的厚度。     

Quark-antiquark recombination in the fragmentation region

Meson production at low transverse momentum in the fragmentation region (x>0.5) in pp collisions is considered in the context of the parton model. It is shown that the quark decay process is unimportant compared to the quark-antiquark recombination process. The calculated results of the model agree very well with the experimental data on the longitudinal momentum distributions of π^± and K^± in both the shapes and the normalizations.     

Extent of sensitivity of single photon production to parton distribution functions

We have studied the production of single isolated prompt photons in high-energy proton–proton collisions at the RHIC ( \(\sqrt {s}\) = 200 GeV) and the LHC ( \(\sqrt {s}\) = 7 TeV) energies within the framework of perturbative QCD upto next-to leading order of strong coupling (α _s). We have used five different parametrizations of parton distribution function (PDF) starting from the old CTEQ4M to the new CT10 distributions and compared our results with the recent single-prompt photon data from the PHENIX and the CMS Collaborations. The prompt photon cross-section is found to be described equally well by all the PDFs within the experimental errors at the RHIC and the LHC energies. The deviation in the single-prompt photon yield for different PDF sets is within ±20% when compared to CTEQ4M, indicating the upper bound of uncertainty in determining the gluon density. The diphoton measurement could be a potential candidate to constrain the gluon distribution inside the proton.     

Neutral kaon production inK + p and π+ p interactions at 250 GeV/c

The inclusive production of neutral kaons is studied inK ⁺ p and π⁺ p interactions at 250 GeV/c. Total and semi-inclusive cross sections and average kaon multiplicities are presented and compared with the data at lower energies. The longitudinal and transverse momentum distributions and their energy dependence are analyzed. The results are interpreted in the framework of recent parton models.     

Perturbative and non-perturbative QCD corrections to wide-angle Compton scattering

We investigate corrections to the handbag approach for wide-angle Compton scattering off protons at moderately large momentum transfer: the photon–parton subprocess is calculated to next-to-leading order in and contributions from the generalized parton distribution E are taken into account. Photon and proton helicity flip amplitudes are non-zero due to these corrections, which leads to a wealth of polarization phenomena in Compton scattering. Thus, for instance, the incoming photon asymmetry or the transverse polarization of the proton is non-zero, although small. Received: 31 October 2001 / Revised version: 11 December 2001 / Published online: 8 February 2002