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Register a new userModel (In)dependent Features of the Hard Pomeron
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We discuss the small-x behaviour of the next-to-leading BFKL equation, depending on various smoothing out procedures of the running coupling constant at low momenta. While scaling violations (with resummed and calculable anomalous dimensions) turn out to be always consistent with the renormalization group, we argue that the nature and the location of the so-called hard Pomeron are dependent on the smoothing out procedure, and thus really on soft hadronic interactions.
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We ask the question whether the quark and gluon distributions in the Pomeron obtained from QCD fits to hard diffraction processes at HERA can be dynamically generated from a state made of ``valence-like gluons and sea quarks as input. By a method combining backward Q^2-evolution for data exploration and forward Q^2-evolution for a best fit determination, we find that the diffractive structure functions published by the H1 collaboration at HERA can be described by a simple ``valence-like input at an initial scale of order mu^2 ~ 2.3-2.7 GeV^2. The parton number sum rules at the initial scale mu^2 for the H1 fit gives 2.1pm .1pm .1 and .13pm .01 pm .02 for gluon and sea quarks respectively, corresponding to an initial Pomeron state made of (almost) only two gluons. It has flat gluon density leading to a plausible interpretation in terms of a gluonium state.
We test the hypothesis that diffractive scattering in the perturbative and non-perturbative domain is determined by the exchange of a single pomeron with a scale dependent trajectory. Present data on diffractive vector meson production are well compatible with this model and recent results for $J/psi$ photoproduction at LHC strongly support it. The model is inspired by concepts of gauge/string duality applied to the pomeron.
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.
A Regge pole model for Pomeron-Pomeron total cross section in the resonance region $sqrt{M^2}le$ 5 GeV is presented. The cross section is saturated by direct-channel contributions from the Pomeron as well as from two different $f$ trajectories, accompanied by the isolated f$_0(500)$ resonance which dominates the $sqrt{M^{2}}lesssim 1$ GeV region. A slowly varying background is taken into account. The calculated Pomeron-Pomeron total cross section cannot be measured directly, but is an essential part of central diffractive processes. In preparation of future calculations of central resonance production at the hadron level, and corresponding measurements at the LHC, we normalize the Pomeron-Pomeron cross section at large masses $sigma_{t}^{PP} (sqrt{M^2}rightarrow infty) approx$ 1 mb as suggested by QCD-motivated estimates.
A model for Pomeron-Pomeron total cross section in the resonance region $sqrt{M^{2}} le$ 5 GeV is presented. This model is based on Regge poles from the Pomeron and two different $f$ trajectories, and includes the isolated f$_{0}(500)$ resonance in the region $sqrt{M^{2}}lesssim 1$ GeV. A slowly varying background is included. The presented Pomeron-Pomeron cross section is not directly measurable, but is an essential ingredient for calculating exclusive resonance production at the LHC.
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