43 research outputs found
On the mistake in the implementation of the minimal model of the dual parameterization and resulting inability to describe the high-energy DVCS data
We correct the mistaken claim made in \cite{Guzey:2005ec,Guzey:2006xi} that
the minimal model of the dual parameterization of nucleon generalized parton
distributions (GPDs) gives a good, essentially model-independent description of
high-energy data on deeply virtual Compton scattering (DVCS). In the
implementation of the dual parameterization in
\cite{Guzey:2005ec,Guzey:2006xi}, the numerical prefactor of two in front of
the DVCS amplitude was missing. We show that the corrected minimal model of the
dual parameterization significantly overestimates the HERA data (H1 and ZEUS)
on the DVCS cross section.Comment: 8 pages, 1 figur
Intrinsic transverse parton momenta in deeply inelastic reactions
Intrinsic transverse parton momenta pT play an important role in the
understanding of azimuthal/spin asymmetries in semi-inclusive deep-inelastic
scattering (SIDIS) and the Drell-Yan process (DY). We review and update what is
presently known about pT from these processes. In particular, we address the
question to which extent data support the popular Gauss model for the
pT-distributions. We find that the Gauss model works very well, and observe
that the intrinsic transverse momenta in SIDIS and DY are compatible, which is
a support for the factorization approach. As a byproduct we recover a simple
but practical way of taking into account the energy dependence of
pT-distributions.Comment: 19 pages, 11 figure
Lorentz invariance relations and Wandzura-Wilczek approximation
A complete list of the so-called Lorentz invariance relations between parton
distribution functions is given and some of their consequences are discussed,
such as the Burkhardt-Cottingham sum rule. The violation of these relations is
considered in a model independent way. It is shown that several Lorentz
invariance relations are not violated in a generalized Wandzura-Wilczek
approximation, indicating that numerically their violation may be small.Comment: 10 pages; Proceedings of the workshop "Recent Advances in
Perturbative QCD and Hadronic Physics", July 20-24, 2009, at ECT*, Trento
(Italy), in honor of Anatoli V. Efremov on the occasion of his 75th birthday;
to appear in Mod. Phys. Lett.
The dual parameterization of the proton generalized parton distribution functions H and E and description of the DVCS cross sections and asymmetries
We develop the minimal model of a new leading order parameterization of GPDs
introduced by Shuvaev and Polyakov. The model for GPDs H and E is formulated in
terms of the forward quark distributions, the Gegenbauer moments of the D-term
and the forward limit of the GPD E. The model is designed primarely for small
and medium-size values of x_B, x_B \leq 0.2.
We examined two different models of the t-dependence of the GPDs: The
factorized exponential model and the non-factorized Regge-motivated model.
Using our model, we successfully described the DVCS cross section measured by
H1 and ZEUS, the moments of the beam-spin A_{LU}^{\sin \phi}, beam-charge
A_{C}^{\cos \phi} and transversely-polarized target A_{UT}^{\sin \phi \cos
\phi} DVCS asymmetries measured by HERMES and A_{LU}^{\sin \phi} measured by
CLAS. The data on A_{C}^{\cos \phi} prefers the Regge-motivated model of the
t-dependence of the GPDs. The data on A_{UT}^{\sin \phi \cos \phi} indicates
that the u and d quarks carry only a small fraction of the proton total angular
momentum.Comment: 33 pages, 11 figure
Are there approximate relations among transverse momentum dependent distribution functions?
Certain exact relations among transverse momentum dependent parton
distribution functions due to QCD equations of motion turn into approximate
ones upon the neglect of pure twist-3 terms. On the basis of available data
from HERMES we test the practical usefulness of one such
``Wandzura-Wilczek-type approximation'', namely of that connecting
h_{1L}^{\perp(1)a}(x) to h_L^a(x), and discuss how it can be further tested by
future CLAS and COMPASS data.Comment: 9 pages, 3 figure
A Hierarchical Multilevel Markov Chain Monte Carlo Algorithm with Applications to Uncertainty Quantification in Subsurface Flow
In this paper we address the problem of the prohibitively large computational
cost of existing Markov chain Monte Carlo methods for large--scale applications
with high dimensional parameter spaces, e.g. in uncertainty quantification in
porous media flow. We propose a new multilevel Metropolis-Hastings algorithm,
and give an abstract, problem dependent theorem on the cost of the new
multilevel estimator based on a set of simple, verifiable assumptions. For a
typical model problem in subsurface flow, we then provide a detailed analysis
of these assumptions and show significant gains over the standard
Metropolis-Hastings estimator. Numerical experiments confirm the analysis and
demonstrate the effectiveness of the method with consistent reductions of more
than an order of magnitude in the cost of the multilevel estimator over the
standard Metropolis-Hastings algorithm for tolerances
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An overview of ADSL Homed Nepenthes Honeypots In Western Australia
This paper outlines initial analysis from research in progress into ADSL homed Nepenthes honeypots. One of the Nepenthes honeypots prime objective in this research was the collection of malware for analysis and dissection. A further objective is the analysis of risks that are circulating within ISP networks in Western Australian. What differentiates Nepenthes from many traditional honeypot designs it that is has been engineered from a distributed network philosophy. The program allows distribution of results across a network of sensors and subsequent aggregation of malware statistics readily within a large network environment
Lorentz invariance relations between parton distributions and the Wandzura-Wilczek approximation
The violation of the so-called Lorentz invariance relations between parton
distribution functions is considered in a model independent way. It is shown
that these relations are not violated in a generalized Wandzura-Wilczek
approximation, indicating that numerically their violation may be small.Comment: 13 pages, added references, minor changes, to appear in Phys. Lett.
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Quantitative assessment of fire and vegetation properties in simulations with fire-enabled vegetation models from the Fire Model Intercomparison Project
Global fire-vegetation models are widely used to assess impacts of environmental change on fire regimes and the carbon cycle and to infer relationships between climate, land use and fire. However, differences in model structure and parameterizations, in both the vegetation and fire components of these models, could influence overall model performance, and to date there has been limited evaluation of how well different models represent various aspects of fire regimes. The Fire Model Intercomparison Project (FireMIP) is coordinating the evaluation of state-of-the-art global fire models, in order to improve projections of fire characteristics and fire impacts on ecosystems and human societies in the context of global environmental change. Here we perform a systematic evaluation of historical simulations made by nine FireMIP models to quantify their ability to reproduce a range of fire and vegetation benchmarks. The FireMIP models simulate a wide range in global annual total burnt area (39–536 Mha) and global annual fire carbon emission (0.91–4.75 Pg C yr−1) for modern conditions (2002–2012), but most of the range in burnt area is within observational uncertainty (345–468 Mha). Benchmarking scores indicate that seven out of nine FireMIP models are able to represent the spatial pattern in burnt area. The models also reproduce the seasonality in burnt area reasonably well but struggle to simulate fire season length and are largely unable to represent interannual variations in burnt area. However, models that represent cropland fires see improved simulation of fire seasonality in the Northern Hemisphere. The three FireMIP models which explicitly simulate individual fires are able to reproduce the spatial pattern in number of fires, but fire sizes are too small in key regions, and this results in an underestimation of burnt area. The correct representation of spatial and seasonal patterns in vegetation appears to correlate with a better representation of burnt area. The two older fire models included in the FireMIP ensemble (LPJ–GUESS–GlobFIRM, MC2) clearly perform less well globally than other models, but it is difficult to distinguish between the remaining ensemble members; some of these models are better at representing certain aspects of the fire regime; none clearly outperforms all other models across the full range of variables assessed