5,099 research outputs found

    Breaking of k_\perp-factorization for Single Jet Production off Nuclei

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    The linear k_\perp-factorization is part and parcel of the pQCD description of high energy hard processes off free nucleons. In the case of heavy nuclear targets the very concept of nuclear parton density becomes ill-defined as exemplified by the recent derivation [2] of nonlinear nuclear k_\perp-factorization for forward dijet production in DIS off nuclei. Here we report a derivation of the related breaking of k_\perp-factorization for single-jet processes. We present a general formalism and apply it to several cases of practical interest: open charm and quark and gluon jet production in the central to beam fragmentation region of \gamma^*p,\gamma^*A, pp and pA collisions. We show how the pattern of k_\perp-factorization breaking and the nature and number of exchanged nuclear pomerons do change within the phase space of produced quark and gluon jets. As an application of the nonlinear k_\perp-factorization we discuss the Cronin effect. Our results are also applicable to the p_\perp-dependence of the Landau-Pomeranchuk-Migdal effect for, and nuclear quenching of, jets produced in the proton hemisphere of pA collisions.Comment: 55 pages, 9 eps figures, presentation shortened, a number of typos removed, to appear in Phys. Rev.

    Production of two ccˉc \bar c pairs in gluon-gluon scattering in high energy proton-proton collisions

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    We calculate cross sections for ggQQˉQQˉg g \to Q \bar Q Q \bar Q in the high-energy approximation in the mixed (longitudinal momentum fraction, impact parameter) and momentum space representations. Besides the total cross section as a function of subsystem energy also differential distributions (in quark rapidity, transverse momentum, QQQ Q, QQˉQ \bar Q invariant mass) are presented. The elementary cross section is used to calculate production of (ccˉ)(ccˉ)(c \bar c) (c \bar c) in single-parton scattering (SPS) in proton-proton collisions. We present integrated cross section as a function of proton-proton center of mass energy as well as differential distribution in M(ccˉ)(ccˉ)M_{(c \bar c)(c \bar c)}. The results are compared with corresponding results for double-parton scattering (DPS) discussed recently in the literature. We find that the considered SPS contribution to (ccˉ)(ccˉ)(c \bar c)(c \bar c) production is at high energy (s>\sqrt{s} > 5 TeV) much smaller than that for DPS contribution.Comment: 17 pages, 11 figure

    Unitarity cutting rules for the nucleus excitation and topological cross sections in hard production off nuclei from nonlinear k_t-factorization

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    At the partonic level, a typical final state in small-x deep inelastic scattering off nuclei and hard proton-nucleus collisions can be characterized by the multiplicity of color-excited nucleons. Within reggeon field theory, each color-excited nucleon is associated with the unitarity cut of the pomeron exchanged between the projectile and nucleus. In this communication we derive the unitarity rules for the multiplicity of excited nucleons, alias cut pomerons, alias topological cross sections, for typical hard dijet production processes. We demonstrate how the coupled-channel non-Abelian intranuclear evolution of color dipoles, inherent to pQCD, gives rise to the reggeon field theory diagrams for final states in terms of the uncut, and two kinds of cut, pomerons. Upon the proper identification of the uncut and cut pomeron exchanges, the topological cross sections for dijet production follow in a straightforward way from the earlier derived nonlinear k_t - factorization quadratures for the inclusive dijet cross sections. The concept of a coherent (collective) nuclear glue proves extremely useful for the formulation of reggeon field theory vertices of multipomeron - cut and uncut - couplings to particles and between themselves. A departure of our unitarity cutting rules from the ones suggested by the pre-QCD Abramovsky-Kancheli-Gribov rules, stems from the coupled-channel features of intranuclear pQCD. We propose a multiplicity re-summation as a tool for the isolation of topological cross sections for single-jet production.Comment: 53 pages, 16 eps-figures, to appear in Phys. Rev.

    Quenching of Leading Jets and Particles: the p_t Dependent Landau-Pomeranchuk-Migdal effect from Nonlinear k_t Factorization

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    We report the first derivation of the Landau-Pomeranchuk-Migdal effect for leading jets at fixed values of the transverse momentum p_t in the beam fragmentation region of hadron-nucleus collisions from RHIC (Relativistic Heavy Ion Collider) to LHC (Large Hadron Collider). The major novelty of this work is a derivation of the missing virtual radiative pQCD correction to these processes - the real-emission radiative corrections are already available in the literature. We manifestly implement the unitarity relation, which in the simplest form requires that upon summing over the virtual and real-emission corrections the total number of scattered quarks must exactly equal unity. For the free-nucleon target, the leading jet spectrum is shown to satisfy the familiar linear Balitsky-Fadin-Kuraev-Lipatov leading log(1/x) (LL-1/x) evolution. For nuclear targets, the nonlinear k_t-factorization for the LL-1/x evolution of the leading jet sepctrum is shown to exactly match the equally nonlinear LL-1/x evolution of the collective nuclear glue - there emerges a unique linear k_t-factorization relation between the two nonlinear evolving nuclear observables. We argue that within the standard dilute uncorrelated nucleonic gas treatment of heavy nuclei, in the finite energy range from RHIC to LHC, the leading jet spectrum can be evolved in the LL-1/x Balitsky-Kovchegov approximation. We comment on the extension of these results to, and their possible reggeon field theory interpretation for, mid-rapidity jets at LHC.Comment: 36 pages, 8 eps figs, revised, discussion on reggeon interpretation and refs. adde

    Anatomy of the differential gluon structure function of the proton from the experimental data on F_2p

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    The use of the differential gluon structure function of the proton F(x,Q2){\cal F}(x,Q^{2}) introduced by Fadin, Kuraev and Lipatov in 1975 is called upon in many applications of small-x QCD. We report here the first determination of F(x,Q2){\cal F}(x,Q^{2}) from the experimental data on the small-x proton structure function F2p(x,Q2)F_{2p}(x,Q^{2}). We give convenient parameterizations for F(x,Q2){\cal F}(x,Q^{2}) based partly on the available DGLAP evolution fits (GRV, CTEQ & MRS) to parton distribution functions and on realistic extrapolations into soft region. We discuss an impact of soft gluons on various observables. The x-dependence of the so-determined F(x,Q2){\cal F}(x,Q^{2}) varies strongly with Q^2 and does not exhibit simple Regge properties. None the less the hard-to-soft diffusion is found to give rise to a viable approximation of the proton structure function F_{2p}(x,Q^2) by the soft and hard Regge components with intercepts \Delta_{soft}=0 and \Delta_{hard}\sim 0.4.Comment: 37 pages, 25 figure

    Glue in the pomeron from nonlinear k_\perp-factorization

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    We derive the nonlinear k_\perp-factorization for the spectrum of jets in high-mass diffractive deep inelastic scattering as a function of three hard scales - the virtuality of the photon Q^2, the transverse momentum of the jet and the saturation scale Q_A. In contrast to all other hard reactions studied so far, we encounter a clash between the two definitions of the glue in the pomeron -- from the inclusive spectrum of leading quarks and the small-\beta evolution of the diffractive cross section. This clash casts a further shadow on customary applications of the familiar collinear factorization to a pQCD analysis of diffractive deep inelastic scattering.Comment: 9 page

    The BFKL-Regge factorization and F2bF_2^b, F2cF_2^c, FLF_L at HERA: physics implications of nodal properties of the BFKL eigenfunctions

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    The asymptotic freedom is known to split the leading-log\log BFKL pomeron into a series of isolated poles in the complex angular momentum plane. One of our earlier findings was that the subleading hard BFKL exchanges decouple from such experimentally important observables as small-xx charm, F2cF_2^c, and the longitudinal, FLF_L, structure functions of the proton at moderately large Q2Q^2. For instance, we predicted precocious BFKL asymptotics of F2c(x,Q2)F_2^c(x,Q^2) with intercept of the rightmost BFKL pole \alpha_{\Pom}(0)-1=\Delta_{\Pom}\approx 0.4. On the other hand, the small-xx open beauty photo- and electro-production probes the vacuum exchange for much smaller color dipoles which entails significant subleading vacuum pole corrections to the small-xx behavior. In view of the accumulation of the experimental data on small-xx F2cF_{2}^{c}, F2bF_{2}^{b} and FLF_{L} we extend our early predictions to the kinematical domain covered by new HERA measurements. Our parameter-free results agree well with the determination of F2cF_2^c, FLF_L and published H1 results on F2bF_2^b but slightly overshoot the very recent (2008, preliminary) H1 results on F2bF_2^b.Comment: 13 pages, 4 figures, JETP Letters, Fig.5 remove

    The Wave Function of 2S Radially Excited Vector Mesons from Data for Diffraction Slope

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    In the color dipole gBFKL dynamics we predict a strikingly different Q^2 and energy dependence of the diffraction slope for the elastic production of ground state V(1S) and radially excited V'(2S) light vector mesons. The color dipole model predictions for the diffraction slope for \rho^0 and \phi^0 production are in a good agreement with the data from the fixed target and collider HERA experiments. We present how a different form of anomalous energy and Q^2 dependence of the diffraction slope for V'(2S) production leads to a different position of the node in radial wave function and discuss a possibility how to determine this position from the fixed target and HERA data.Comment: 20 pages and 6 figures. Title change

    Asymptotic behaviour of the total cross section of p-p scattering and the Akeno cosmic ray data

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    I present a new determination of the total cross section for proton-proton collisions from the recent Akeno results on absorption of the cosmic ray protons in the p-Air collisions. Extrapolation to the SSC energy suggests σtot(pp)(160170)mb\sigma_{tot}(p-p) \approx (160-170) mb. I also comment on a possible sensitivity of the p-Air cross section determinations to assumptions on the inelasticity of nuclear collisions at high energy.Comment: . 6 pages, 0 figure