Photon-jet correlations in pppp and ppˉp \bar p collisions

We compare results of the $k_t$-factorization approach and the next-to-leading order collinear-factorization approach for photon-jet correlations in $pp$ and $p \bar p$ collisions at RHIC and Tevatron energies. We discuss correlations in the azimuthal angle as well as in the two-dimensional space of transverse momentum of photon and jet. Different unintegrated parton distributions (UPDF) are included in the $k_t$-factorization approach. The results depend on UPDFs used. The standard collinear approach gives cross section comparable to the $k_t$-factorization approach. For correlations of the photon and any jet the NLO contributions dominate at relatively small azimuthal angles as well as for asymmetric transverse momenta. For correlations of the photon with the leading jet (the one having the biggest transverse momentum) the NLO approach gives zero contribution at $\phi_{-} < \pi/2$ which opens a possibility to study higher-order terms and/or UPDFs in this region.Comment: 14 pages, 17 figure

Pion and Kaon Production in Nucleon - Nucleon Collisions

Inclusive cross section for pion production in proton - proton collisions are calculated based on unintegrated parton distribution functions (uPDFs). In addition to purely gluonic terms the present approach includes also quark degrees of freedom. Phenomenological fragmentation functions from the literature are used. The new mechanisms are responsible for $\pi^+$ - $\pi^-$ asymmetry. In contrast to standard collinear approach, application of 2 $\to$ 1 $k_t$ - factorization approach can be extended towards much lower transverse momenta, both at mid and forward rapidity region. The results of the calculation are compared with SPS and RHIC data.Comment: a talk presented by Marta Tichoruk at the international conference MESON2006, Cracow, June 2006, 5 pages, 3 figure

On the analytical approximation to the GLAP evolution at small x and moderate Q^2

Comparing the numerically evaluated solution to the leading order GLAP equations with its analytical small-x approximation we have found that in the domain covered by a large fraction of the HERA data the analytic approximation has to be augmented by the formally non-leading term which has been usually neglected. The corrected formula fits the data much better and provides a natural explanation of some of the deviations from the $\sigma$ scaling observed in the HERA kinematical range.Comment: LaTeX, 13 pages including 6 figures, figure 5 replaced, to appear in Phys. Lett.

A unified BFKL and GLAP description of F2F_2 data

We argue that the use of the universal unintegrated gluon distribution and the $k_T$ (or high energy) factorization theorem provides the natural framework for describing observables at small x. We introduce a coupled pair of evolution equations for the unintegrated gluon distribution and the sea quark distribution which incorporate both the resummed leading $ln (1/x)$ BFKL contributions and the resummed leading $ln (Q^2)$ GLAP contributions. We solve these unified equations in the perturbative QCD domain using simple parametic forms of the nonperturbative part of the integrated distributions. With only two (physically motivated) input parameters we find that this $k_T$ factorization approach gives an excellent description of the measurements of $F_2 (x,Q^2)$ at HERA. In this way the unified evolution equations allow us to determine the gluon and sea quark distributions and, moreover, to see the x domain where the resummed $ln (1/x)$ effects become significant. We use $k_T$ factorization to predict the longitudinal structure function $F_L (x,Q^2)$ and the charm component of $F_2 (x,Q^2)$.Comment: 25 pages, LaTeX, 9 figure

Dijet Production at Hadron--Hadron Colliders in the BFKL Approach

The production in high-energy hadron collisions of a pair of jets with large rapidity separation is studied in an improved BFKL formalism. By recasting the analytic solution of the BFKL equation as an explicit order-by-order sum over emitted gluons, the effects of phase space constraints and the running coupling are studied. Particular attention is paid to the azimuthal angle decorrelation of the jet pair. The inclusion of sub-leading effects significantly improves the agreement between the theoretical predictions and recent preliminary measurements from the Dzero collaboration.Comment: 19 pages LaTeX; one figure corrected; conclusions unchange

Unintegrated CCFM parton distributions and transverse momentum of gauge bosons

Transverse momentum distribution of gauge bosons $W^{\pm}$ and $Z^0$ is calculated using unintegrated parton distributions obtained by solving leading order CCFM equation(s) in the impact parameter space. For illustration we compare the results of the fixed-order collinear, soft-gluon resummation and CCFM approaches. A parameter of a nonperturbative form factor is adjusted to the D0 data. In contrast to the collinear approach, the leading order calculation based on the unintegrated CCFM parton distributions generates transverse momentum distributions of gauge bosons, which are almost consistent with experimental data provided the parameter of the nonperturbative form factor is appropriately adjusted. The $W^+$-$W^-$ asymmetry is calculated as a function of the boson rapidity and transverse momentum. Predictions are given for RHIC and LHC.Comment: 19 pages, 8 figure

Unintegrated gluon distributions and Higgs boson production in proton-proton collisions

Inclusive cross sections for Higgs boson production in proton-proton collisions are calculated in the formalism of unintegrated gluon distributions (UGDF). Different UGDF from the literature are used. Although they were constructed in order to describe the HERA deep-inelastic scattering $F_2$ data, they lead to surprisingly different results for Higgs production. We present both two-dimensional invariant cross section as a function of Higgs rapidity and transverse momentum, as well as corresponding projections on rapidity or transverse momentum. We quantify the differences between different UGD's by applying different cuts on interrelations between transverse momentum of Higgs and transverse momenta of both fusing gluons. We focus on large rapidity region. The interplay of the gluon-gluon fusion and weak-boson fusion in rapidity and transverse momentum is discussed. We find that above $p_T \sim$ 50-100 GeV the weak-gauge-boson fusion dominates over gluon-gluon fusion.Comment: 32 pages, 18 figures, corrected version, restructured, misprints removed, discussion added, new figure added, in print in EPJ

QCD Predictions for the Transverse Energy Flow in Deep-Inelastic Scattering in the Small x HERA Regime

The distribution of transverse energy, $E_T$, which accompanies deep-inelastic electron-proton scattering at small $x$, is predicted in the central region away from the current jet and proton remnants. We use BFKL dynamics, which arises from the summation of multiple gluon emissions at small $x$, to derive an analytic expression for the $E_T$ flow. One interesting feature is an $x^{-\epsilon}$ increase of the $E_T$ distribution with decreasing $x$, where $\epsilon = (3\alpha_s/\pi)2\log 2$. We perform a numerical study to examine the possibility of using characteristics of the $E_T$ distribution as a means of identifying BFKL dynamics at HERA.Comment: 16 pages, REVTEX 3.0, no figures. (Hardcopies of figures available on request from Professor A.D. Martin, Department of Physics, University of Durham, DH1 3LE, England.) Durham preprint : DTP/94/0

Inclusive production of J/ψJ/\psi meson in proton-proton collisions at BNL RHIC

Inclusive cross sections for $J/\psi$ production in proton-proton collisions were calculated in the $k_t$-factorization approach for the RHIC energy. Several mechanisms were considered, including direct color-singlet mechanism, radiative decays of $\chi_c$ mesons, decays of $\psi'$, open-charm associated production of $J/\psi$ as well as weak decays of B mesons. Different unintegrated gluon distributions from the literature were used. We find that radiative $\chi_c$ decays and direct color-singlet contributions constitute the dominant mechanism of $J/\psi$ production. These process cannot be consistently treated within collinear-factorization approach. The results are compared with recent RHIC data. The new precise data at small transverse momenta impose stringent constraints on UGDFs. Some UGDFs are inconsistent with the new data. The Kwieci\'nski UGDFs give the best description of the data. In order to verify the mechanism suggested here we propose $J/\psi$ -- jet correlation measurement and an independent measurement of $\chi_c$ meson production in $\pi^+ \pi^-$ and/or $K^+ K^-$ decay channels. Finally, we address the issue of \J spin alignment.Comment: 26 pages, 20 figures, the text was slightly modified, the title was modified, more discussion was added, one figure was removed, one was adde

Dijet correlations at RHIC, leading-order ktk_t-factorization approach versus next-to-leading order collinear approach

We compare results of $k_t$-factorization approach and next-to-leading order collinear-factorization approach for dijet correlations in proton-proton collisions at RHIC energies. We discuss correlations in azimuthal angle as well as correlations in two-dimensional space of transverse momenta of two jets. 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. For collinear NLO case the situation depends significantly on whether we consider correlations of any two jets or correlations of leading jets only. In the first case the $2 \to 2$ contributions associated with soft radiations summed up in the $k_t$-factorization approach dominate at $\phi \sim \pi$ and at equal moduli of jet transverse momenta. The collinear NLO $2 \to 3$ contributions dominate over $k_t$-factorization cross section at small relative azimuthal angles as well as for asymmetric transverse momentum configurations. In the second case the NLO contributions vanish at small relative azimuthal angles and/or large jet transverse-momentum disbalance due to simple kinematical constraints. There are no such limitations for the $k_t$-factorization approach. All this makes the two approaches rather complementary. The role of several cuts is discussed and quantified.Comment: 26 pages, 17 figure