Dijet and photon-jet correlations in proton-proton collisions at RHIC

We discuss correlations in azimuthal angle as well as correlations in two-dimensional space of transverse momenta of two jets as well as photon and jet. Some k_t-factorization subprocesses are included for the first time in the literature. Different unintegrated gluon/parton distributions are used in the k_t-factorization approach. The results depend on UGDF/UPDF used. The collinear NLO 2 ↦3 contributions dominate over k_t-factorization cross section at small relative azimuthal angles as well as for asymmetric transverse momentum configurations.     

Angular distribution of particle flows and perturbative QCD predictions

We discuss the predictions of perturbative QCD for angular flows of final state particles in two and three jet events including their cms energy and jet resolution (y_cut) dependence. The simple analytical formulae for gluon bremsstrahlung from primary partons, modified for gluon cascading, reproduce the main features of the experimental data well. For y_cut-selected events, the particle flow is derived from a superposition of colour dipoles in much the same way that photon radiation is derived from electric dipoles. Received: 16 December 2002 / Revised version: 12 February 2003 / Published online: 24 March 2003     

Spin-dependent processes involving a real photon

We calculate the helicity difference quark and gluon distributions in a polarized real photon using the Altarelli-Parisi equations. The corresponding helicity difference fragmentation functions to produce a real photon are dealt with in the same way. Both sets of results are presented in the form of simple parametrizations and the former used to investigate spin-spin asymmetries in the photoproduction of two high transverse momentum jets. These asymmetries are classed as “four jet” or “three jet” depending on whether any photon fragments are present in the final state along the beam axis. We find that the four-jet asymmetry should provide an experimental test for the presence of the anomalous polarized photon structure function while the three-jet asymmetry should provide new information about the spin structure of a polarized proton and in particular its gluon component.     

Jet–jet and hadron–jet correlations in hadro- and electroproduction

We discuss, in the framework of perturbative QCD at next to leading order, two related observables which are usually considered to provide tests of the BFKL dynamics: jet–jet correlations at Tevatron energies and forward particle–jet correlations at HERA. In the first case we study the rapidity gap dependence of the azimuthal correlations and find slightly too strong correlations at large gap. In the second case we discuss the cross section as well as the azimuthal correlations over a rapidity gap range of 5 units. We find that the requirement of a forward particle imposes strong kinematical constraints which distort the distributions, notably at small rapidity gaps. We also show that the decorrelation is stronger in electroproduction than in hadron–hadron collisions. Unfortunately data are not yet available for comparison.     

Multiple collisions and induced gluon bremsstrahlung in QCD

Induced soft gluon bremsstrahlung associated with multiple collisions is calculated via perturbative QCD. We derive the non-abelian analog of the Landau-Pomeranchuk effect that suppresses induced soft radiation with formation times exceeding the mean free path. The dependence of the suppression effect on the SU(N) representation of the jet parton as well as the kinematic variables is expressed through a radiation formation factor. The soft radiation with k < μ, where μ is the infrared screening scale in the medium, is shown to lead to an approximately constant radiative energy loss per unit length.     

Fluctuations and correlations of soft gluons at the nonperturbative stage of evolution of QCD jets

At the nonperturbative stage of jet evolution, fluctuations of soft gluons are less than those for coherent states under specific conditions. This fact suggests that there can arise squeezed gluon states. The angular and rapidity dependences of the gluon correlation function are investigated at this stage of jet evolution. It is shown that these new states of soft gluons can display sub-Poissonian or super-Poissonian statistics corresponding to, respectively, antibunching and bunching of gluons, by analogy with squeezed photon states.     

The photon structure and exclusive production of vector mesons in \gamma\gamma collisions

The process of exclusive vector meson production, , is studied for almost real photons. This process may be reduced to the photoproduction of off the meson. We discuss the possibility of extracting the gluon distribution of and of the photon from such a measurement. Predictions are also given for the reaction for various cms energies typical for LEP and for the future linear colliders. Received: 13 December 2000 / Revised version: 6 January 2001/ Published online: 6 April 2001     

NLL Monte-Carlo approach in 1 or 2 jets photoproduction

We present a method for calculating the photoproduction of jets at HERA based on Next to Leading Logarithm QCD calculations. It is implemented in a Monte-Carlo generator which allows us to easily compute any infra-red safe cross sections for 1 or 2 jet observables using various jet reconstruction algorithms. We focus on the possibility of extracting the gluon contents of the photon and of the proton from present and future H1 and ZEUS data. Received: 1 June 2000 / Revised version: 19 July 2000 / Published online: 25 September 2000     

Nonperturbative and perturbative aspects of photo- and electroproduction of vector mesons

We discuss various aspects of vector meson production, first analysing the interplay between perturbative and nonperturbative aspects of the QCD calculation. Using a general method adapted to incorporate both perturbative and nonperturbative aspects, we show that nonperturbative effects are important for all experimentally available values of the photon virtuality Q². We compare the huge amount of experimental information now available with our theoretical results obtained using a specific nonperturbative model without free parameters, showing that quite simple features are able to explain the data.     

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     

k(t) factorization for hard processes in nuclei

Two widely proposed k(t)-dependent gluon distributions in the small-x saturation regime are investigated using two-particle back-to-back correlations in high energy scattering processes. The Weizs?cker-Williams gluon distribution, interpreted as the number density of gluon inside nucleus, is studied in the quark-antiquark jet correlation in deep inelastic scattering. On the other hand, the unintegrated gluon distribution, defined as the Fourier transform of the color-dipole cross section, is probed in the direct photon-jet correlation in pA collisions.     

An investigation of a possible hard contribution to φ photoproduction

We investigate the possibility that the process of φ photoproduction may have a significant hard perturbative QCD component. This suggestion is based on a study of the energy dependence of the forward φ photoproduction cross section followed by a calculation where we show that a coherent sum of the pQCD and conventional soft Pomeron contributions provides an excellent reproduction of the experimental data. Our results suggest that the transition from the predominantly soft photoproduction of light ρ and ω vector mesons to the predominantly hard photoproduction of heavy J/Ψ and is smooth and gradual, similar to the transition observed in deep inelastic scattering studies of the proton structure function in the small x limit. Our predictions for higher HERA energies are presented.     

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.     

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     

Inclusive Higgs Boson Production at the LHC within the QCD k t -Factorization Approach

Within the QCD kt-factorization approach, the cross sections for the processes of Higgs boson inclusive production and decay at LHC energies are derived. The calculus relies on transverse-momentum-dependent unintegrated gluon distribution functions in the proton obtained by numerically solving the Catani-Ciafaloni-Fiorani-Marchesini (CCFM) equation. On the whole, the ATLAS and CMS measurements of differential cross sections for the Higgs boson production at collision energies of 8 and 13 TeV are adequately reproduced. Our results demonstrate that, with the QCD kt-factorization approach, higher order perturbative corrections are effectively taken into account. The dependence of the results obtained on the form of the unintegrated gluon distribution is investigated.     

The Feynman-Schwinger Representation in QCD

The proper time path integral representation is derived explicitly for Green's functions in QCD. After an introductory analysis of perturbative properties, the total gluonic field is separated in a rigorous way into a nonperturbative background and valence gluon part. For nonperturbative contributions the background perturbation theory is used systematically, yielding two types of expansions, illustrated by direct physical applications. As an application, we discuss the collinear singularities in the Feynman-Schwinger representation formalism. Moreover, the generalization to nonzero temperature is made and expressions for partition functions in perturbation theory and nonperturbative background are explicitly written down.     

Small x phenomenology on gluon evolution through the BFKL equation in light of a constraint in multi-Regge kinematics

We investigate the impact of so-called kinematic constraint on gluon evolution at small x.Implanting the constraint on the real emission term of the gluon ladder diagram, we obtain an integro-differential form of the Balitsky-Fadin-Kuraev-Lipatov(BFKL) equation. Later we solve the equation analytically using the method of characteristics. We sketch the Bjorken x and transverse momentum k_t~2 dependence of our solution of unintegrated gluon distributions f (x,k_t~2) in the kinematic constraint supplemented BFKL equation and contrasted the same with the original BFKL equation. Then we extract the integrated gluon density xg(x, Q~2) from unintegrated gluon distributions f (x,k_t~2) and compared our theoretical prediction with that of global data fits, namely NNPDF3.1 sx and CT14. Finally we illustrate the phenomenological implication of our solution for unintegrated gluon distribution f (x,k_T~2) towards exploring high precision HERA DIS data by the theoretical prediction of proton structure functions(F_2 and F_L).