537 research outputs found
Recursive numerical calculus of one-loop tensor integrals
A numerical approach to compute tensor integrals in one-loop calculations is
presented. The algorithm is based on a recursion relation which allows to
express high rank tensor integrals as a function of lower rank ones. At each
level of iteration only inverse square roots of Gram determinants appear. For
the phase-space regions where Gram determinants are so small that numerical
problems are expected, we give general prescriptions on how to construct
reliable approximations to the exact result without performing Taylor
expansions. Working in 4+epsilon dimensions does not require an analytic
separation of ultraviolet and infrared/collinear divergences, and, apart from
trivial integrals that we compute explicitly, no additional ones besides the
standard set of scalar one-loop integrals are needed.Comment: Typo corrected in formula 79. 22 pages, Latex, 1 figure, uses
axodraw.st
Extending CKKW-merging to One-Loop Matrix Elements
We extend earlier schemes for merging tree-level matrix elements with parton
showers to include also merging with one-loop matrix elements. In this paper we
make a first study on how to include one-loop corrections, not only for events
with a given jet multiplicity, but simultaneously for several different jet
multiplicities. Results are presented for the simplest non-trivial case of
hadronic events at LEP as a proof-of-concept
The Polar Bear Management Agreement for the Southern Beaufort Sea : An Evaluation of the First Ten Years of a Unique Conservation Agreement
Polar bears (Ursus maritimus) of the southern Beaufort Sea population, distributed from approximately Icy Cape, west of Point Barrow, Alaska, to Pearce Point, east of Paulatuk in Canada, are harvested by hunters from both countries. In Canada, quotas to control polar bear hunting have been in place, with periodic modifications, since 1968. In Alaska, passage of the United Sates Marine Mammal Protection Act (MMPA) of 1972 banned polar bear hunting unless done by Alaska Natives for subsistence hunt, leaving open the potential for an overharvest with no possible legal management response until the population was declared depleted. Recognizing that as a threat to the conservation of the shared polar bear population, the Inuvialuit Game Council from Canada and the North Slope Borough from Alaska negotiated and signed a user-to-user agreement, the Polar Bear Management Agreement for the Southern Beaufort Sea, in 1988. We reviewed the functioning of the agreement through its first 10 years and concluded that, overall, it has been successful because both the total harvest and the proportion of females in the harvest have been contained within sustainable limits. However, harvest monitoring needs to be improved in Alaska, and awareness of the need to prevent overharvest of females needs to be increased in both countries. This agreement is a useful model for other user-to-user conservation agreements.Les ours polaires (Ursus maritimus) constituant la population de la mer de Beaufort méridionale sont répartis d'environ Icy Cape, à l'ouest de Point Barrow (Alaska), à Pearce Point, à l'est de Paulatuk (Canada). Ils sont prélevés par des chasseurs des deux pays. Au Canada, les quotas visant le contrôle de la chasse à l'ours polaire sont en vigueur - avec des modifications périodiques - depuis 1968. En Alaska, l'adoption en 1972 de la loi américaine (MMPA) visant la protection des mammifères marins a interdit la chasse à l'ours polaire sauf la chasse de subsistance pratiquée par les Autochtones alaskiens. La MMPA n'a toutefois placé aucune restriction sur le nombre ou la composition de la chasse de subsistance, laissant la porte ouverte à une éventuelle surexploitation sans possibilité d'une réaction de gestion sur le plan légal jusqu'à ce que la population soit déclarée décimée. Reconnaissant en cela une menace à la conservation de la population commune d'ours polaires, le Conseil canadien de gestion du gibier et le North Slope Borough de l'Alaska ont négocié et signé en 1988 une entente entre usagers, le Polar Bear Management Agreement pour la mer de Beaufort méridionale. On a examiné le fonctionnement de l'entente durant sa première décennie pour conclure que, dans l'ensemble, elle a porté fruit car le total des prises et la proportion de femelles prélevées ont été maintenus dans des limites viables. Il faut toutefois améliorer le contrôle du prélèvement en Alaska et accroître dans les deux pays la sensibilisation à la nécessité de prévenir une surexploitation des femelles. Cette entente constitue un modèle pour d'autres accords entre usagers en matière de conservation
General subtraction method for numerical calculation of one-loop QCD matrix elements
We present a subtraction scheme for eliminating the ultraviolet, soft, and
collinear divergences in the numerical calculation of an arbitrary one-loop QCD
amplitude with an arbitrary number of external legs. The subtractions consist
of local counter terms in the space of the four-dimensional loop momentum. The
ultraviolet subtraction terms reproduce MSbar renormalization. The key point in
the method for the soft and collinear subtractions is that, although the
subtraction terms are defined graph-by-graph and the matrix element is also
calculated graph-by-graph, the sum over graphs of the integral of each the
subtraction term can be evaluated analytically and provides the well known
simple pole structure that arises from subtractions from real emission graphs,
but with the opposite sign.Comment: 38 pages, 10 figures, axodraw styl
Light-induced mass transport in amorphous chalcogenides/gold nanoparticles composites
We have established that mass-transport processes in two types of amorphous
materials, based on light-sensitive inorganic compounds like Se and As₂₀Se₈₀ chalcogenide
glasses (ChG), can be enhanced at the nanoscale in the presence of localized plasmonic
fields generated by visible light in gold nanoparticles (GNPs), if the condition of surface
plasmon resonance (SPR) is fulfilled. It was found that irradiation by light in the presence of
SPR produces profound surface nanostructurizations, and variation in topography follows
closely and permanently the underlying near field intensity pattern. We have proposed a
model of mass-transport in which the existence of moving anisotropic dipolar units and
internal electric field in ChG as a main driving force of this movement is suggested
Hadronic final states in deep-inelastic scattering with Sherpa
We extend the multi-purpose Monte-Carlo event generator Sherpa to include
processes in deeply inelastic lepton-nucleon scattering. Hadronic final states
in this kinematical setting are characterised by the presence of multiple
kinematical scales, which were up to now accounted for only by specific
resummations in individual kinematical regions. Using an extension of the
recently introduced method for merging truncated parton showers with
higher-order tree-level matrix elements, it is possible to obtain predictions
which are reliable in all kinematical limits. Different hadronic final states,
defined by jets or individual hadrons, in deep-inelastic scattering are
analysed and the corresponding results are compared to HERA data. The various
sources of theoretical uncertainties of the approach are discussed and
quantified. The extension to deeply inelastic processes provides the
opportunity to validate the merging of matrix elements and parton showers in
multi-scale kinematics inaccessible in other collider environments. It also
allows to use HERA data on hadronic final states in the tuning of hadronisation
models.Comment: 32 pages, 22 figure
PYTHIA 6.4 Physics and Manual
The PYTHIA program can be used to generate high-energy-physics `events', i.e.
sets of outgoing particles produced in the interactions between two incoming
particles. The objective is to provide as accurate as possible a representation
of event properties in a wide range of reactions, within and beyond the
Standard Model, with emphasis on those where strong interactions play a role,
directly or indirectly, and therefore multihadronic final states are produced.
The physics is then not understood well enough to give an exact description;
instead the program has to be based on a combination of analytical results and
various QCD-based models. This physics input is summarized here, for areas such
as hard subprocesses, initial- and final-state parton showers, underlying
events and beam remnants, fragmentation and decays, and much more. Furthermore,
extensive information is provided on all program elements: subroutines and
functions, switches and parameters, and particle and process data. This should
allow the user to tailor the generation task to the topics of interest.Comment: 576 pages, no figures, uses JHEP3.cls. The code and further
information may be found on the PYTHIA web page:
http://www.thep.lu.se/~torbjorn/Pythia.html Changes in version 2: Mistakenly
deleted section heading for "Physics Processes" reinserted, affecting section
numbering. Minor updates to take into account referee comments and new colour
reconnection option
Application of information entropy to nuclei
Shannon's information entropies in position- and momentum- space and their
sum are calculated for various - and - shell nuclei using a
correlated one-body density matrix depending on the harmonic oscillator size
and the short range correlation parameter which originates from a
Jastrow correlation function. It is found that the information entropy sum for
a nucleus depends only on the correlation parameter through the simple
relation , where , and
depend on the mass number . A similar approximate expression
is also valid for the root mean square radius of the nucleus as function of
leading to an approximate expression which connects with the root mean
square radius. Finally, we propose a method to determine the correlation
parameter from the above property of as well as the linear dependence of
on the logarithm of the number of nucleons.Comment: 10 pages, 10 EPS figures, RevTeX, Phys.Rev.C accepted for publicatio
Standard Model backgrounds to supersymmetry searches
This work presents a review of the Standard Model sources of backgrounds to
the search of supersymmetry signals. Depending on the specific model, typical
signals may include jets, leptons, and missing transverse energy due to the
escaping lightest supersymmetric particle. We focus on the simplest case of
multijets and missing energy, since this allows us to expose most of the issues
common to other more complex cases. The review is not exhaustive, and is aimed
at collecting a series of general comments and observations, to serve as
guideline for the process that will lead to a complete experimental
determination of size and features of such SM processes.Comment: To appear in the J. Wess memorial volume, "Supersymmetry on the Eve
of the LHC", to be published in European Physical Journal
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