3,373 research outputs found
A small collection of bryophytes from Montserrat, West Indies
Four species of liverworts and 13 taxa of mosses have been identified from samples collected in Montserrat in 1980, 11 of which appear to be new to the island
On dynamically generated parton distribution functions and their properties
The idea of ``dynamically'' generated parton distribution functions, based on
regular initial conditions at low momentum scale, is reanalyzed with particular
emphasize paid to its compatibility with the factorization mechanism. Basic
consequences of this approach are discussed and compared to those of the
conventional approach, employing singular initial distribution functions.Comment: 15 pages, Latex, 5 figures in PS format attache
Simultaneous QCD analysis of diffractive and inclusive DIS data
We perform a NLO QCD analysis of deep-inelastic scattering data, in which we
account for absorptive corrections. These corrections are determined from a
simultaneous analysis of diffractive deep-inelastic data. The absorptive
effects are found to enhance the size of the gluon distribution at small x,
such that a negative input gluon distribution at Q^2 = 1 GeV^2 is no longer
required. We discuss the problem that the gluon distribution is valence-like at
low scales, whereas the sea quark distribution grows with decreasing x. Our
study hints at the possible importance of power corrections for Q^2 \simeq 1--2
GeV^2.Comment: 11 pages, 3 figures. Version published as a Rapid Communication in
Phys. Rev.
Predictions for dijet production in DIS using small x dynamics
We study the properties of dijet production in deep inelastic scattering
using a unified BFKL/DGLAP framework, which includes important subleading ln
(1/x) contributions. We calculate the azimuthal decorrelation between the jets.
We compute the cross section for dijet production as a function of Q^2 and the
jet transverse momentum, as well as calculate the total dijet rate. We compare
the predictions with HERA data.Comment: 7 pages, LaTeX with 4 eps figure
Upsilon photoproduction at HERA compared to estimates of perturbative QCD
We estimate the cross section for gamma p -> Upsilon p by two independent
methods. First, by studying the corrections to the naive leading-order QCD
formula and, second, by using parton-hadron duality. The estimates are in good
agreement with each other and with the recent measurements of the cross section
at HERA.Comment: 9 pages, LaTeX, one figur
Higher twists in deep inelastic scattering
We perform an exploratory study of higher twist contributions to deep
inelastic scattering. We estimate the size of two major sources of higher
twist, namely absorptive corrections and the vector meson dominance (VMD)
contribution. We find that they give a sizeable higher twist component of F_2.
For example at x = 0.01 it is about 8% (17%) at Q^2 = 10 GeV^2 (4 GeV^2),
reaching up to 27% at x = 10^{-4} and Q^2 = 4 GeV^2. At the smaller x value the
largest contribution comes from absorptive corrections, while at the larger
values of x the VMD term dominates.Comment: The sign of the gluon rescattering twist-4 component has been
corrected and the manuscript modified accordingl
Skewed distributions fixed by diagonal partons at small x,\xi and at HERA
We show that the skewed parton distributions are completely determined at
small x and \xi by the conventional diagonal partons. We study the application
to diffractive vector meson production at HERA.Comment: 3 pages, LATeX, no figures, uses npb.sty, presented at the DIS99
Workshop, Zeuthen, Germany, April 19-23, 1999, to appear in the proceeding
Optimal choice of factorization scales for the description of jet production at the LHC
To obtain more precise parton distribution functions (PDFs) it is important to include data on inclusive high transverse energy jet production in the global parton analyses. These data have high statistics and the NNLO terms in the perturbative QCD (pQCD) description are now available. Our aim is to reduce the uncertainty in the comparison of the jet data with pQCD. To ensure the best convergence of the pQCD series it is important to choose the appropriate factorization scales, μFμF . We show that it is possible to absorb and resum in the incoming PDFs and fragmentation function (D) an essential part of the higher αsαs -order corrections by determining the ‘optimal’ values of μFμF . We emphasize that it is necessary to optimize different factorization scales for the various factors in the cross section: indeed, both of the PDFs, and also the fragmentation function, have their own optimal scale. We show how the values of these scales can be calculated for the LO (NLO) part of the pQCD prediction of the cross section based on the theoretically known NLO (NNLO) corrections. After these scales are fixed at their optimal values, the residual factorization scale dependence is much reduced
An independent estimate of the triple-Pomeron coupling
The value of the important triple-Pomeron coupling is estimated by an
unorthodox procedure using the known diffractive parton distribution functions.
The result is g_{3P}\simeq 0.2g_N, where g_N is the Pomeron-nucleon coupling.
This is in excellent agreement with an independent determination, g_{3P}\simeq
0.2g_N, previously obtained by analysing the available data in the triple-Regge
region with absorptive effects taken into account.Comment: 7 pages, 5 figure
Diffractive parton distributions from H1 data
We analyse the latest H1 large rapidity gap data to obtain diffractive parton
distributions, using a procedure based on perturbative QCD, and compare them
with distributions obtained from the simplified Regge factorisation type of
analysis. The diffractive parton densities and structure functions are made
publically available.Comment: 9 pages, 2 figures. Fortran code for diffractive parton densities and
structure functions can be found at http://durpdg.dur.ac.uk/hepdata/mrw.html
. Version to appear in Phys. Lett. B; final paragraph added, with curves from
H1 incl.+dijet fit added to Fig.
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