58,326 research outputs found
Studying the Perturbative Reggeon
We consider the flavour non-singlet Reggeon within the context of
perturbative QCD. This consists of ladders built out of ``reggeized'' quarks.
We propose a method for the numerical solution of the integro-differential
equation for the amplitude describing the exchange of such a Reggeon. The
solution is known to have a sharp rise at low values of Bjorken-x when applied
to non-singlet quantities in deep-inelastic scattering. We show that when the
running of the coupling is taken into account this sharp rise is further
enhanced, although the Q^2 dependence is suppressed by the introduction of the
running coupling. We also investigate the effects of simulating
non-perturbative physics by introducing a constituent mass for the soft quarks
and an effective mass for the soft gluons exchanged in the t-channel.Comment: LaTeX, 21 pages, 16 figure
Small and Large Time Stability of the Time taken for a L\'evy Process to Cross Curved Boundaries
This paper is concerned with the small time behaviour of a L\'{e}vy process
. In particular, we investigate the {\it stabilities} of the times,
\Tstarb(r) and \Tbarb(r), at which , started with , first leaves
the space-time regions (one-sided exit),
or (two-sided exit), , as
r\dto 0. Thus essentially we determine whether or not these passage times
behave like deterministic functions in the sense of different modes of
convergence; specifically convergence in probability, almost surely and in
. In many instances these are seen to be equivalent to relative stability
of the process itself. The analogous large time problem is also discussed
Stability of the Exit Time for L\'evy Processes
This paper is concerned with the behaviour of a L\'{e}vy process when it
crosses over a positive level, , starting from 0, both as becomes large
and as becomes small. Our main focus is on the time, , it takes the
process to transit above the level, and in particular, on the {\it stability}
of this passage time; thus, essentially, whether or not behaves
linearly as u\dto 0 or . We also consider conditional stability
of when the process drifts to , a.s. This provides
information relevant to quantities associated with the ruin of an insurance
risk process, which we analyse under a Cram\'er condition
CO adsorption on (111) and (100) surfaces of the Pt sub 3 Ti alloy. Evidence for parallel binding and strong activation of CO
The CO adsorption on a 40 atom cluster model of the (111) surface and a 36 atom cluster model of the (100) surface of the Pt3Ti alloy was studied. Parallel binding to high coordinate sites associated with Ti and low CO bond scission barriers are predicted for both surfaces. The binding of CO to Pt sites occurs in an upright orientation. These orientations are a consequence of the nature of the CO pi donation interactions with the surface. On the Ti sites the orbitals donate to the nearly empty Ti 3d band and the antibonding counterpart orbitals are empty. On the Pt sites, however, they are in the filled Pt 5d region of the alloy band, which causes CO to bond in a vertical orientation by 5 delta donation from the carbon end
Asymptotic Distributions of the Overshoot and Undershoots for the L\'evy Insurance Risk Process in the Cram\'er and Convolution Equivalent Cases
Recent models of the insurance risk process use a L\'evy process to
generalise the traditional Cram\'er-Lundberg compound Poisson model. This paper
is concerned with the behaviour of the distributions of the overshoot and
undershoots of a high level, for a L\'{e}vy process which drifts to
and satisfies a Cram\'er or a convolution equivalent condition. We derive these
asymptotics under minimal conditions in the Cram\'er case, and compare them
with known results for the convolution equivalent case, drawing attention to
the striking and unexpected fact that they become identical when certain
parameters tend to equality.
Thus, at least regarding these quantities, the "medium-heavy" tailed
convolution equivalent model segues into the "light-tailed" Cram\'er model in a
natural way. This suggests a usefully expanded flexibility for modelling the
insurance risk process. We illustrate this relationship by comparing the
asymptotic distributions obtained for the overshoot and undershoots, assuming
the L\'evy process belongs to the "GTSC" class
Bringing self assessment home: repository profiling and key lines of enquiry within DRAMBORA
Digital repositories are a manifestation of complex organizational, financial, legal, technological, procedural, and political interrelationships. Accompanying each of these are innate uncertainties, exacerbated by the relative immaturity of understanding prevalent within the digital preservation domain. Recent efforts have sought to identify core characteristics that must be demonstrable by successful digital repositories, expressed in the form of check-list documents, intended to support the processes of repository accreditation and certification. In isolation though, the available guidelines lack practical applicability; confusion over evidential requirements and difficulties associated with the diversity that exists among repositories (in terms of mandate, available resources, supported content and legal context) are particularly problematic. A gap exists between the available criteria and the ways and extent to which conformity can be demonstrated. The Digital Repository Audit Method Based on Risk Assessment (DRAMBORA) is a methodology for undertaking repository self assessment, developed jointly by the Digital Curation Centre (DCC) and DigitalPreservationEurope (DPE). DRAMBORA requires repositories to expose their organization, policies and infrastructures to rigorous scrutiny through a series of highly structured exercises, enabling them to build a comprehensive registry of their most pertinent risks, arranged into a structure that facilitates effective management. It draws on experiences accumulated throughout 18 evaluative pilot assessments undertaken in an internationally diverse selection of repositories, digital libraries and data centres (including institutions and services such as the UK National Digital Archive of Datasets, the National Archives of Scotland, Gallica at the National Library of France and the CERN Document Server). Other organizations, such as the British Library, have been using sections of DRAMBORA within their own risk assessment procedures.
Despite the attractive benefits of a bottom up approach, there are implicit challenges posed by neglecting a more objective perspective. Following a sustained period of pilot audits undertaken by DPE, DCC and the DELOS Digital Preservation Cluster aimed at evaluating DRAMBORA, it was stated that had respective project members not been present to facilitate each assessment, and contribute their objective, external perspectives, the results may have been less useful. Consequently, DRAMBORA has developed in a number of ways, to enable knowledge transfer from the responses of comparable repositories, and incorporate more opportunities for structured question sets, or key lines of enquiry, that provoke more comprehensive awareness of the applicability of particular threats and opportunities
Issues in digital preservation: towards a new research agenda
Digital Preservation has evolved into a specialized, interdisciplinary research discipline of its own, seeing significant increases in terms of research capacity, results, but also challenges. However, with this specialization and subsequent formation of a dedicated subgroup of researchers active in this field, limitations of the challenges addressed can be observed. Digital preservation research may seem to react to problems arising, fixing problems that exist now, rather than proactively researching new solutions that may be applicable only after a few years of maturing. Recognising the benefits of bringing together researchers and practitioners with various professional backgrounds related to digital preservation, a seminar was organized in Schloss Dagstuhl, at the Leibniz Center for Informatics (18-23 July 2010), with the aim of addressing the current digital preservation challenges, with a specific focus on the automation aspects in this field. The main goal of the seminar was to outline some research challenges in digital preservation, providing a number of "research questions" that could be immediately tackled, e.g. in Doctoral Thesis. The seminar intended also to highlight the need for the digital preservation community to reach out to IT research and other research communities outside the immediate digital preservation domain, in order to jointly develop solutions
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