655 research outputs found
Extracting the Distribution Amplitudes of the rho meson from the Color Glass Condensate
We extract the leading twist-2 and subleading twist-3 Distribution Amplitudes
(DAs) of the rho meson using the HERA data on diffractive rho photoproduction.
We do so using several Colour Glass Condensate (CGC) inspired and a Regge
inspired dipole models. We find that our extracted twist-2 DA is not much model
dependent and is consistent with QCD Sum Rules and lattice predictions. The
extracted twist-3 DA is more model dependent but is still consistent with the
Sum Rules prediction.Comment: 21 pages, 10 figures, 3 tables. Section 6 revised, figures 8 and 9
and table 3 updated. Conclusions essentially unchange
What if the Higgs couplings to W and Z bosons are larger than in the Standard Model?
We derive a general sum rule relating the Higgs coupling to W and Z bosons to
the total cross section of longitudinal gauge boson scattering in I=0,1,2
isospin channels. The Higgs coupling larger than in the Standard Model implies
enhancement of the I=2 cross section. Such an enhancement could arise if the
Higgs sector is extended by an isospin-2 scalar multiplet including a doubly
charged, singly charged, and another neutral Higgs.Comment: 11 pages, no figures. v2: comments and references added. v3: early
QCD references adde
The colour dipole approach to small-x processes
We explain why it is possible to formulate a wide variety of high energy
(small-x) photon-proton processes in terms of a universal dipole cross section
and compare and contrast various parameterizations of this function that exist
in the literature.Comment: 6 pages, latex, 2 figures. Contribution to Durham Collider Workshop
(Sept 99) proceeding
Testing the dynamics of high energy scattering using vector meson production
I review work on diffractive vector meson production in photon-proton
collisions at high energy and large momentum transfer, accompanied by proton
dissociation and a large rapidity gap. This process provides a test of the high
energy scattering dynamics, but is also sensitive to the details of the
treatment of the vector meson vertex.
The emphasis is on the description of the process by a solution of the
non-forward BFKL equation, i.e. the equation describing the evolution of
scattering amplitudes in the high-energy limit of QCD. The formation of the
vector meson and the non-perturbative modeling needed is also briefly
discussed.Comment: 17 pages, 8 figures. Brief review to appear in Mod. Phys. Lett.
Saturation and geometric scaling in DIS at small x
We present various aspects of the saturation model which provides good
description of inclusive and diffractive DIS at small x. The model uses parton
saturation ideas to take into account unitarity requirements. A new scaling
predicted by the model in the small x domain is successfully confronted with
the data.Comment: Presented at New Trends in HERA Physics 2001, Ringberg Castle,
Tegernsee, Germany, 17-22 June 2001, minor corrections, some references adde
Diffractive Meson Production and the Quark-Pomeron Coupling
Diffractive meson production at HERA offers interesting possibilities to
investigate diffractive processes and thus to learn something about the
properties of the pomeron. The most succesful phenomenological description of
the pomeron so far assumes it to couple like a isoscalar photon to
single quarks. This coupling leads, however, to problems for exclusive
diffractive reactions. We propose a new phenomenological pomeron vertex, which
leads to very good fits to the known data, but avoids the problems of the old
vertex.Comment: 20 pages, latex with uuencoded postscript, revised versio
Elastic and quasi-elastic and scattering in the Dipole Model
We have in earlier papers presented an extension of Mueller's dipole cascade
model, which includes sub-leading effects from energy conservation and running
coupling as well as colour suppressed saturation effects from pomeron loops via
a ``dipole swing''. The model was applied to describe the total and diffractive
cross sections in and collisions, and also the elastic cross
section in scattering.
In this paper we extend the model to describe the corresponding quasi-elastic
cross sections in , namely the exclusive production of vector mesons
and deeply virtual compton scattering. Also for these reactions we find a good
agrement with measured cross sections. In addition we obtain a reasonable
description of the -dependence of the elastic and quasi-elastic
cross sections
The dependence of the hard diffractive photoproduction of vector meson or photon and the range of pQCD validity
We consider two coupled problems.
We study the dependence on photon virtuality for the semihard
quasi--elastic photoproduction of neutral vector mesons on a quark, gluon or
real photon (at
GeV)). To this end we calculate the corresponding amplitudes (in an
analytical form) in the lowest nontrivial approximation of perturbative QCD. It
is shown that the amplitude for the production of light meson varies very
rapidly with the photon virtuality near .
We estimate the bound of the pQCD validity region for such processes. For the
real incident photon the obtained bound for the meson production is very
high. This bound decreases fast with the increase of , and we expect that
the virtual photoproduction at HERA gives opportunity to test the pQCD results.
The signature of this region is discussed. For the hard Compton effect the pQCD
should work good at not too high , and this effect seems measurable
at HERA.Comment: ReVTeX, 36 pages, 5 Postscript figures, uses epsf.st
Pomeron in diffractive processes and at large Q^2: the onset of pQCD
We study the reactions and
at large Q^2 and and small
momentum transfer, , to the nucleon where the pomeron exchange
dominates. At large Q^2 the virtual photon selects a hard pair, thus
selecting the hard pomeron component (the BFKL pomeron). The amplitudes for
both transverse and longitudinal polarizations of the initial photon and
outgoing -meson (photon) are calculated in the framework of the BFKL
pomeron exchange. Our calculations show that one cannot expect the early onset
of the pure perturbative regime in the discussed diffractive processes: the
small interquark distances, fm, start to dominate not
earlier than at in
and in
.Comment: 20 pages, LaTeX, epsfig.st
Unitarization of the BFKL Pomeron on a Nucleus
We analyze the evolution equation describing all multiple hard pomeron
exchanges in a hadronic or nuclear structure functions that was proposed
earlier. We construct a perturbation series providing us with an exact solution
to the equation outside of the saturation region. The series demonstrates how
at moderately high energies the corrections to the single BFKL pomeron exchange
contribution which are due to the multiple pomeron exchanges start unitarizing
total deep inelastic scattering cross section. We show that as energy increases
the scattering cross section of the quark-antiquark pair of a fixed transverse
separation on a hadron or nucleus given by the solution of our equation inside
of the saturation region unitarizes and becomes independent of energy. The
corresponding F_2 structure function also unitarizes and becomes linearly
proportional to ln s. We also discuss possible applications of the developed
technique to diffraction.Comment: REVTeX, 20 pages, 6 figure
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