1,575 research outputs found

    Does the hard pomeron obey Regge factorisation?

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    While data for the proton structure function demand the presence of a hard-pomeron contribution even at quite small Q^2, previous fits to the pp and p\bar p total cross sections have found that in these there is little or no room for such a contribution. We re-analyse the data and show that itmay indeed be present and that, further, it probably obeys Regge factorisation: sigma^{gamma p}(s,Q_1^2)sigma^{gamma p}(s,Q_2^2)= sigma^{pp}(s)sigma^{gamma gamma}(s,Q_1^2,Q_2^2) for all values of Q_1^2 and Q_2^2.Comment: 11 pages, 10 figure

    Charm production at HERA

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    The ZEUS data on the charm structure function F_2^c at small x fit well to a single power of x, corresponding to the exchange of a hard pomeron that is flavour-blind. When combined with the contribution from the exchange of a soft pomeron, the hard pomeron gives a good description of elastic J/ŌąJ/\psi photoproduction.Comment: 6 pages, plain tex, with 3 figures embedded using epsf. Typos corrected in equations (10) and (11

    Exclusive Vector Photoproduction: Confirmation of Regge Theory

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    Recent small-t ZEUS data for exclusive rho photoproduction are in excellent agreement with exchange of the classical soft pomeron with slope alpha'=0.25 GeV^{-2}. Adding in a flavour-blind hard-pomeron contribution, whose magnitude is calculated from the data for exclusive J/psi photoproduction, gives a good fit also to the ZEUS data for rho photoproduction at larger values of t, and to phi photoproduction.Comment: 6 pages, plain tex, with 5 figures embedded using eps

    The Interest of Large-t Elastic Scattering

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    Existing data for large-tt pppp elastic-scattering differential cross-sections are energy-independent and behave as t‚ąí8t^{-8}. This has been explained in terms of triple-gluon exchange, or alternatively through triple-singlet exchange. A discussion is given of the problems raised by each of these explanations, and of the possibility that at RHIC or LHC energies the exchange of three BFKL pomerons might result in a rapid rise with energy.Comment: 6 pages, plain tex, 3 figures embedded with eps

    Unusual high-pT jet events at HERA

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    We calculate the cross-section for events at HERA where the proton loses only a minute fraction of its initial energy, all of which goes into producing a single pair of transverse jets.Comment: 4 pages plus figure

    Unitarisation of the hard pomeron and black-disk limit at the LHC

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    Recent models of soft diffraction include a hard pomeron pole besides the usual soft term. Such models violate the black-disk limit around Tevatron energies, so that they need to be supplemented by a unitarisation scheme. Several such schemes are considered in this letter, where we show that they lead to a large uncertainty at the LHC. We also examine the impact of unitarisation on various small-t observables, the slope in t of the elastic cross section, or the ratio of the real to imaginary parts of the scattering amplitude, leading to the conclusion that the existence of a hard pomeron in soft scattering may be confirmed by LHC data.Comment: 15 pages, 12 figures, late

    Pomeron physics: an update

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    Key issues in pomeron physics include whether the hard and soft pomerons are distinct objects, and whether the hard pomeron is already present in amplitudes at Q2=0Q^2=0. It is urgent to learn how to combine perturbative and nonperturbative concepts, and to construct a sound theory of perturbative evolution at small xx. Other questions are whether screening corrections are small, and gap survival probabilities large. Finally, do diffractive processes present a good way to discover the Higgs?Comment: 12 pages, 23 figures embedded with epsf Summary talk at Diffraction2000, Calabria, September 200

    EFFECTS OF SHADOWING IN DOUBLE POMERON EXCHANGE PROCESSES

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    The effects of shadowing in double Pomeron exchange processes are investigated within an eikonal approach with a Gaussian input. Damping factors due to screening are calculated for this process and compared with the factors obtained for total, elastic and single diffraction cross sections. Our main conclusion is that counting rate calculations, of various double Pomeron exchange processes (without screening corrections) such as heavy quark and Higgs production are reduced by a factor of 5 in the LHC energy range, when screening corrections are applied.Comment: 9 pages, latex, 5 figures obtainable from author

    Perturbative Evolution and Regge Behaviour

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    The known analytic properties of the Compton amplitude at small Q2Q^2 place significant constraints on its behaviour at large Q2Q^2. This calls for a re-evaluation of the role of perturbative evolution in past fits to data.Comment: Revised figures 4 and
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