6,307 research outputs found
Study of Monte Carlo approach to experimental uncertainty propagation with MSTW 2008 PDFs
We investigate the Monte Carlo approach to propagation of experimental
uncertainties within the context of the established "MSTW 2008" global analysis
of parton distribution functions (PDFs) of the proton at next-to-leading order
in the strong coupling. We show that the Monte Carlo approach using replicas of
the original data gives PDF uncertainties in good agreement with the usual
Hessian approach using the standard Delta(chi^2) = 1 criterion, then we explore
potential parameterisation bias by increasing the number of free parameters,
concluding that any parameterisation bias is likely to be small, with the
exception of the valence-quark distributions at low momentum fractions x. We
motivate the need for a larger tolerance, Delta(chi^2) > 1, by making fits to
restricted data sets and idealised consistent or inconsistent pseudodata.
Instead of using data replicas, we alternatively produce PDF sets randomly
distributed according to the covariance matrix of fit parameters including
appropriate tolerance values, then we demonstrate a simpler method to produce
an arbitrary number of random predictions on-the-fly from the existing
eigenvector PDF sets. Finally, as a simple example application, we use Bayesian
reweighting to study the effect of recent LHC data on the lepton charge
asymmetry from W boson decays.Comment: 37 pages, 17 figures. v2: version published in JHEP. Supplementary
material at http://mstwpdf.hepforge.org/random
Recent Progress in Parton Distributions and Implications for LHC Physics
I outline some of the most recent developments on the global fit to parton
distributions performed by the MRST collaboration.Comment: 6 pages, 7 figures. To appear in proceedings of XIII International
Workshop on Deep Inelastic Scattering, April,27 - May,1, 2005, Madison,
Wisconsin, US
On the structure of line-driven winds near black holes
A general physical mechanism of the formation of line-driven winds at the
vicinity of strong gravitational field sources is investigated in the frame of
General Relativity. We argue that gravitational redshifting should be taken
into account to model such outflows. The generalization of the Sobolev
approximation in the frame of General Relativity is presented. We consider all
processes in the metric of a nonrotating (Schwarzschild) black hole. The
radiation force that is due to absorbtion of the radiation flux in lines is
derived. It is demonstrated that if gravitational redshifting is taken into
account, the radiation force becomes a function of the local velocity gradient
(as in the standard line-driven wind theory) and the gradient of . We
derive a general relativistic equation of motion describing such flow. A
solution of the equation of motion is obtained and confronted with that
obtained from the Castor, Abbott & Klein (CAK) theory. It is shown that the
proposed mechanism could have an important contribution to the formation of
line-driven outflows from compact objects.Comment: 20 pages, submitted to Ap
An Ordered Analysis of Heavy Flavour Production in Deep Inelastic Scattering
At low Q^2, charm production in deep-inelastic scattering is adequately
described by assuming generation in electroweak boson-light parton scattering
(dominantly boson-gluon fusion) which naturally incorporates the correct
threshold behaviour. At high Q^2 this description is inadequate, since it does
not sum logs in Q^2/m_c^2, and is replaced by the treatment of the charm quark
as a light parton. We show how the problem of going from one description to the
other can be solved in a satisfactory manner to all orders. The key ingredient
is the constraint of matching the evolution of the physical structure function
F_2 order by order in alpha_s(Q^2) in addition to the matching of the value of
F_2 itself. This leads to new expressions for the coefficient functions
associated with the charm parton which are unique in incorporating both the
correct threshold and asymptotic behaviours at each order in perturbation
theory. The use of these improved coefficients lead to an improvement in global
fits and an excellent description of the observed F_2,charm.Comment: Tex file, including a modification of Harvmac, 48 pages, 9 figures as
.ps file
Head-on infall of two compact objects: Third post-Newtonian Energy Flux
Head-on infall of two compact objects with arbitrary mass ratio is
investigated using the multipolar post-Minkowskian approximation method. At the
third post-Newtonian order the energy flux, in addition to the instantaneous
contributions, also includes hereditary contributions consisting of the
gravitational-wave tails, tails-of-tails and the tail-squared terms. The
results are given both for infall from infinity and also for infall from a
finite distance. These analytical expressions should be useful for the
comparison with the high accuracy numerical relativity results within the limit
in which post-Newtonian approximations are valid.Comment: 25 pages, 2 figures, This version includes the changes appearing in
the Erratum published in Phys. Rev.
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