318 research outputs found

    A Gribov equation for the photon Green's function

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    We present a derivation of the Gribov equation for the gluon/photon Green's function D(q). Our derivation is based on the second derivative of the gauge-invariant quantity Tr ln D(q), which we interpret as the gauge-boson `self-loop'. By considering the higher-order corrections to this quantity, we are able to obtain a Gribov equation which sums the logarithmically enhanced corrections. By solving this equation, we obtain the non-perturbative running coupling in both QCD and QED. In the case of QCD, alpha_S has a singularity in the space-like region corresponding to super-criticality, which is argued to be resolved in Gribov's light-quark confinement scenario. For the QED coupling in the UV limit, we obtain a \propto Q^2 behaviour for space-like Q^2=-q^2. This implies the decoupling of the photon and an NJLVL-type effective theory in the UV limit.Comment: 12 pages, 5 figures; version to be published in Eur. Phys. J.

    Production of electroweak bosons in e+e- annihilation at high energies

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    Production of electroweak bosons in e+e- annihilation into quarks and into leptons at energies much greater than 100 Gev is considered. We account for double-logarithmic contributions to all orders in electroweak couplings. It is assumed that the bosons are emitted in the multi-Regge kinematics. The explicit expressions for the scattering amplitudes of the process are obtained. It is shown that the cross sections of the photon and Z production have the identical energy dependence and asymptotically their ratio depends only on the Weinberg angle whereas the energy dependence of the cross section of the W production is suppressed by factor s^{-0.4} compared to them.Comment: Revtex4, 16 pages, 7 figures, 2 table

    Nuclear effects in g1A(x,Q2)g_{1A}(x,Q^2) at small xx in deep inelastic scattering on 7^7Li and 3^3He

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    We suggest to use polarized nuclear targets of 7^7Li and 3^3He to study nuclear effects in the spin dependent structure functions g1A(x,Q2)g_{1A}(x,Q^2). These effects are expected to be enhanced by a factor of two as compared to the unpolarized targets. We predict a significant xx dependence at 104÷103x0.210^{-4} \div 10^{-3} \leq x \leq 0.2 of g1A(x,Q2)/g1N(x,Q2)g_{1A}(x,Q^2)/g_{1N}(x,Q^2) due to nuclear shadowing and nuclear enhancement. The effect of nuclear shadowing at x103x \approx 10^{-3} is of an order of 16% for g1A=7n.s.3/2(x,Q2)/g1Nn.s.(x,Q2)g_{1A=7}^{n.s. 3/2}(x,Q^2)/g_{1N}^{n.s.}(x,Q^2) and 10% for g1A=3n.s(x,Q2)/g1Nn.s.(x,Q2)g_{1A=3}^{n.s}(x,Q^2)/g_{1N}^{n.s.}(x,Q^2). By imposing the requirement that the Bjorken sum rule is satisfied we model the effect of enhancement. We find the effect of enhancement at x0.125(0.15)x \approx 0.125 (0.15) to be of an order of 20(55)20 (55)% for g1A=7n.s.3/2(x,Q2)/g1Nn.s.(x,Q2)g_{1A=7}^{n.s. 3/2}(x,Q^2)/g_{1N}^{n.s.}(x,Q^2) and 14(40)14 (40)% for g1A=3n.s(x,Q2)/g1Nn.s.(x,Q2)g_{1A=3}^{n.s}(x,Q^2)/g_{1N}^{n.s.}(x,Q^2), if enhancement occupies the region 0.05x0.20.05 \leq x \leq 0.2 (0.1x0.20.1 \leq x \leq 0.2). We predict a 2% effect in the difference of the scattering cross sections of deep inelastic scattering of an unpolarized projectile off 7^7Li with MJM_{J}=3/2 and MJM_{J}=1/2. We also show explicitly that the many-nucleon description of deep inelastic scattering off 7^7Li becomes invalid in the enhancement region 0.05<x0.20.05 < x \leq 0.2.Comment: 29 pages, 5 figures, RevTe

    Diffractive Processes at the LHC

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    We consider diffractive processes which can be measured at the LHC. Analysis of diffractive events will give unique information about the high energy asymptotics of hadron scattering. In semihard diffraction one may study the partonic structure of the Pomeron. Central Exclusive Diffractive production provides a possibility to investigate the new particles (Higgs bosons, SUSY particles,...) in an exceptionally clean environment.Comment: 12 pages, To be published in the Proc. of the Gribov-75 Memorial Workshop, Budapest, May 200

    Electroweak 2 -> 2 amplitudes for electron-positron annihilation at TeV energies

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    The non-radiative scattering amplitudes for electron-positron annihilation into quark and lepton pairs in the TeV energy range are calculated in the double-logarithmic approximation. The expressions for the amplitudes are obtained using infrared evolution equations with different cut-offs for virtual photons and for W and Z bosons, and compared with previous results obtained with an universal cut-off.Comment: Revtex4, 17 pages, 7 figures. Some minor changes made, more refs adde

    Nuclear shadowing in deep inelastic scattering on nuclei: leading twist versus eikonal approaches

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    We use several diverse parameterizations of diffractive parton distributions, extracted in leading twist QCD analyses of the HERA diffractive deep inelastic scattering (DIS) data, to make predictions for leading twist nuclear shadowing of nuclear quark and gluon distributions in DIS on nuclei. We find that the HERA diffractive data are sufficiently precise to allow us to predict large nuclear shadowing for gluons and quarks, unambiguously. We performed detailed studies of nuclear shadowing for up and charm sea quarks and gluons within several scenarios of shadowing and diffractive slopes, as well as at central impact parameters. We compare these leading twist results with those obtained from the eikonal approach to nuclear shadowing (which is based on a very different space-time picture) and observe sharply contrasting predictions for the size and Q^2-dependence of nuclear shadowing. The most striking differences arise for the interaction of small dipoles with nuclei, in particular for the longitudinal structure function F_{L}^{A}.Comment: 43 pages, 16 figures, requires JHEP style fil

    A global reanalysis of nuclear parton distribution functions

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    We determine the nuclear modifications of parton distribution functions of bound protons at scales Q21.69Q^2\ge 1.69 GeV2^2 and momentum fractions 105x110^{-5}\le x\le 1 in a global analysis which utilizes nuclear hard process data, sum rules and leading-order DGLAP scale evolution. The main improvements over our earlier work {\em EKS98} are the automated χ2\chi^2 minimization, simplified and better controllable fit functions, and most importantly, the possibility for error estimates. The resulting 16-parameter fit to the N=514 datapoints is good, χ2/d.o.f=0.82\chi^2/{\rm d.o.f}=0.82. Within the error estimates obtained, the old {\em EKS98} parametrization is found to be fully consistent with the present analysis, with no essential difference in terms of χ2\chi^2 either. We also determine separate uncertainty bands for the nuclear gluon and sea quark modifications in the large-xx region where they are not stringently constrained by the available data. Comparison with other global analyses is shown and uncertainties demonstrated. Finally, we show that RHIC-BRAHMS data for inclusive hadron production in d+Au collisions lend support for a stronger gluon shadowing at x<0.01x<0.01 and also that fairly large changes in the gluon modifications do not rapidly deteriorate the goodness of the overall fits, as long as the initial gluon modifications in the region x0.020.04x\sim 0.02-0.04 remain small.Comment: 33 pages, 14 figure

    Quarkonium from the Fifth Dimension

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    Adding fundamental matter of mass m_Q to N=4 Yang Mills theory, we study quarkonium, and "generalized quarkonium" containing light adjoint particles. At large 't Hooft coupling the states of spin<=1 are anomalously light (Kruczenski et al., hep-th/0304032). We examine their form factors, and show these hadrons are unlike any known in QCD. By a traditional yardstick they appear infinite in size (as with strings in flat space) but we show that this is a failure of the yardstick. All of the hadrons are actually of finite size ~ \sqrt{g^2N}/m_Q, regardless of their radial excitation level and of how many valence adjoint particles they contain. Certain form factors for spin-1 quarkonia vanish in the large-g^2N limit; thus these hadrons resemble neither the observed J/Psi quarkonium states nor rho mesons.Comment: 57 pages, LaTeX, 5 figure

    Soft-Gluon Resummation for Bottom Fragmentation in Top Quark Decay

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    We study soft-gluon radiation in top quark decay within the framework of perturbative fragmentation functions. We present results for the b-quark energy distribution, accounting for soft-gluon resummation in both the MSbar coefficient function and in the initial condition of the perturbative fragmentation function. The results show remarkable improvement and the b-quark energy spectrum in top quark decay exhibits very little dependence on factorization and renormalization scales. We present some hadron-level results in both x_B and moment space by including non-perturbative information determined from e+e- data.Comment: 19 pages, 6 figures, JHEP style. Few changes after referee report, one reference added, numerical results unchange
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