1,007 research outputs found
Exclusive and photoproduction and the low gluon
We study exclusive vector meson photoproduction, with
or , at NLO in collinear factorisation, in order to
examine what may be learnt about the gluon distribution at very low . We
examine the factorisation scale dependence of the predictions. We argue that,
using knowledge of the NLO corrections, terms enhanced by a large
can be reabsorbed in the LO part by a choice of the factorisation scale. (In
these exclusive processes takes the role of Bjorken-.) Then, the scale
dependence coming from the remaining NLO contributions has no
enhancements. As a result, we find that predictions for the amplitude of
production are stable to within about . This will allow
data for the exclusive process at the LHC, particularly
from LHCb, to be included in global parton analyses to constrain the gluon PDF
down to . Moreover, the study of exclusive
photoproduction indicates that the gluon density found in the recent global PDF
analyses is too small at low and low scales.Comment: 21 pages, 6 figures. Text significantly improved, references added,
version to be published in J.Phys.
Massive Quark Production in Electron Positron Annihilation to Order
Recent analytical and numerical results for the three-loop polarization
function allow to present a phenomenological analysis of the cross section for
massive quark production in electron positron annihilation to order
. Numerical predictions based on fixed order perturbation theory
are presented for charm and bottom production above 5 and 11.5 GeV,
respectively. The contribution from these energy regions to ,
the running QED coupling constant at scale M_Z, are given. The dominant terms
close to threshold, i.e. in an expansion for small quark velocity , are
presented.Comment: 26 pages (Latex), 16 figures (Postscript
Amplified spontaneous emission in short-pulse excimer amplifiers.
We have found a simple analytical expression which describes the relation between amplified spontaneous emission (ASE) and small-signal gain in short-pulse amplifiers. It is also shown that the contrast of the short pulse to the ASE is weakly dependent on the saturation of the ASE, and influenced mainly by the saturation of the short pulse. The theoretical considerations were verified by measurements
Patron-Driven Acquisition: What Do We Know about Our Patrons?
This paper describes how Utrecht University Library is trying to reach a sustainable and efficient PDA model, offering more e-books to our users.
PDA e-books were made available in the catalog. We developed an efficient back office process for updates, deletions and financial administration. We did pilot programs to test PDA as an acquisition model. During the pilot anonymized user data was collected of patrons and their use of PDA e-books.
Due to heavy usage and too fast depletion of budget we had to adjust our PDA model. The collected data helped to understand the development of costs and to decide about the changes in the PDA model. After a year PDA pilot, we developed a predictable PDA model. However, for a sustainable model there are still challenges, not only due to dilemmas on restricting the PDA profile, but also due to publishers raising STL prices
The QCD description of diffractive rho meson electroproduction
We critically review the QCD predictions for the cross sections sigma_L and
sigma_T for diffractive rho meson electroproduction in longitudinally and
transversely polarised states in the HERA energy region. We show that both
perturbative and non-perturbative approaches which involve convolution with the
rho meson wave function predict values of sigma_T which fall-off too quickly
with increasing Q^2, in comparison with the data. We present a perturbative QCD
model based on the open production of light qqbar pairs and parton-hadron
duality, which describes all features of the data for rho electroproduction at
high Q^2 and, in particular, predicts a satisfactory Q^2 behaviour of
sigma_L/sigma_T. We find that precise measurements of the latter can give
valuable information on the Q^2 behaviour of the gluon distribution at small x.Comment: 19 pages, LaTeX, 6 Postscript figures. The complete paper, including
figures, is also available via anonymous ftp at
ftp://cpt1.dur.ac.uk/pub/preprints/dtp96/dtp9664
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