10,497 research outputs found
Assessing consistency of fish survey data : uncertainties in the estimation of mackerel icefish (Champsocephalus gunnari) abundance at South Georgia
Acknowledgments The authors wish to thank the crews, fishermen and scientists who conducted the various surveys from which data were obtained, and Mark Belchier and Simeon Hill for their contributions. This work was supported by the Government of South Georgia and South Sandwich Islands. Additional logistical support provided by The South Atlantic Environmental Research Institute with thanks to Paul Brickle. Thanks to Stephen Smith of Fisheries and Oceans Canada (DFO) for help in constructing bootstrap confidence limits. Paul Fernandes receives funding from the MASTS pooling initiative (The Marine Alliance for Science and Technology for Scotland), and their support is gratefully acknowledged. MASTS is funded by the Scottish Funding Council (grant reference HR09011) and contributing institutions. We also wish to thank two anonymous referees for their helpful suggestions on earlier versions of this manuscript.Peer reviewedPostprin
Generalized parton correlation functions for a spin-1/2 hadron
The fully unintegrated, off-diagonal quark-quark correlator for a spin-1/2
hadron is parameterized in terms of so-called generalized parton correlation
functions. Such objects, in particular, can be considered as mother
distributions of generalized parton distributions on the one hand and
transverse momentum dependent parton distributions on the other. Therefore, our
study provides new, model-independent insights into the recently proposed
nontrivial relations between generalized and transverse momentum dependent
parton distributions. We find that none of these relations can be promoted to a
model-independent status. As a by-product we obtain the first complete
classification of generalized parton distributions beyond leading twist. The
present paper is a natural extension of our previous corresponding analysis for
spin-0 hadrons.Comment: 41 pages, 3 figures; v2: added referenc
NLO prescription for unintegrated parton distributions
We show how parton distributions unintegrated over the parton transverse
momentum, k_t, may be generated, at NLO accuracy, from the known integrated
(DGLAP-evolved) parton densities determined from global data analyses. A few
numerical examples are given, which demonstrate that sufficient accuracy is
obtained by keeping only the LO splitting functions together with the NLO
integrated parton densities. However, it is important to keep the precise
kinematics of the process, by taking the scale to be the virtuality rather than
the transverse momentum, in order to be consistent with the calculation of the
NLO splitting functions.Comment: 20 pages, 8 figures. v2: version to appear in Eur. Phys. J.
Deeply virtual Compton scattering in next-to-leading order
We study the amplitude of deeply virtual Compton scattering in
next-to-leading order of perturbation theory including the two-loop evolution
effects for different sets of skewed parton distributions (SPDs). It turns out
that in the minimal subtraction scheme the relative radiative corrections are
of order 20-50%. We analyze the dependence of our predictions on the choice of
SPD, that will allow to discriminate between possible models of SPDs from
future high precision experimental data, and discuss shortly theoretical
uncertainties induced by the radiative corrections.Comment: 10 pages, LaTeX, 3 figure
Comparison of the color-evaporation model and the NRQCD factorization approach in charmonium production
We compare the color-evaporation model (CEM) and nonrelativistic QCD (NRQCD)
factorization predictions for inclusive quarkonium production. Using the NRQCD
factorization formulas for quarkonium production and for perturbative QQ-bar
production, we deduce relationships that are implied by the CEM between the
nonperturbative NRQCD matrix elements that appear in the factorization formula
for quarkonium production. These relationships are at odds with the
phenomenological values of the matrix elements that have been extracted from
the Tevatron data for charmonium production at large transverse momentum. A
direct comparison of the CEM and NRQCD factorization predictions with the CDF
charmonium production data reveals that the CEM fits to the data are generally
unsatisfactory, while the NRQCD factorization fits are generally compatible
with the data. The inclusion of k_T smearing improves the CEM fits
substantially, but significant incompatibilities remain. The NRQCD
factorization fits to the chi_c data indicate that multiple gluon radiation is
an essential ingredient in obtaining the correct shape of the cross section as
a function of p_T.Comment: 26 pages, 6 figures, 7 tables, Some changes of emphasis in the
conclusions, additional discussion of theoretical uncertainties, minor
revisions and corrections, version to be published in Physical Review
Radion Induced Brane Preheating
When the interbrane separation in compact Randall-Sundrum models is
stabilized via the Goldberger-Wise mechanism, a potential is generated for the
four dimensional field that encodes this geometric information, the so-called
radion. Due to its origin as a part of the full five dimensional metric, the
radion couples directly to particles on both branes. We exhibit the exponential
growth in the number of brane particles due to parametric amplification from
radion oscillations and discuss some of the consequences of this process for
brane cosmology.Comment: 4 pages, uses ReVTeX, 3 postscript figure
Diffractive parton distributions from perturbative QCD
The asymptotic collinear factorisation theorem, which holds for diffractive
deep-inelastic scattering, has important modifications in the sub-asymptotic
HERA regime. We use perturbative QCD to quantify these modifications. The
diffractive parton distributions are shown to satisfy an inhomogeneous
evolution equation. We emphasise that it is necessary to include both the
gluonic and sea-quark t-channel components of the perturbative Pomeron. The
corresponding Pomeron-to-parton splitting functions are derived in the
Appendix.Comment: 34 pages, 13 figures. Version published in Eur. Phys. J.
Double transverse spin asymmetries in vector boson production
We investigate a helicity non-flip double transverse spin asymmetry in vector
boson production in hadron-hadron scattering, which was first considered by
Ralston and Soper at the tree level. It does not involve transversity functions
and in principle also arises in W-boson production for which we present the
expressions. The asymmetry requires observing the transverse momentum of the
vector boson, but it is not suppressed by explicit inverse powers of a large
energy scale. However, as we will show, inclusion of Sudakov factors causes
suppression of the asymmetry, which increases with energy. Moreover, the
asymmetry is shown to be approximately proportional to x_1 g_1(x_1) x_2 \bar
g_1(x_2), which gives rise to additional suppression at small values of the
light cone momentum fractions. This implies that it is negligible for Z or W
production and is mainly of interest for \gamma^* at low energies. We also
compare the asymmetry with other types of double transverse spin asymmetries
and discuss how to disentangle them.Comment: 12 pages, Revtex, 2 Postscript figures, uses aps.sty, epsf.sty;
figures replaced, a few minor other correction
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