1,974 research outputs found
Gauge boson couplings at LEP
A review is given of the measurements of triple and quartic couplings among
the electroweak gauge bosons performed at LEP by the four experiments ALEPH,
DELPHI, L3 and OPAL. Emphasis is placed on recently published results and on
combinations of results performed by the LEP electroweak gauge-couplings group.
All measurements presented are consistent with the Standard Model expectations.Comment: 6 pages, 7 figures. To be published in the proceedings of the BEACH04
conference, Chicago, June 27-July 3 200
Light-by-light scattering sum rules constraining meson transition form factors
Relating the forward light-by-light scattering to energy weighted integrals
of the \gamma* \gamma -fusion cross sections, with one real photon (\gamma) and
one virtual photon (\gamma*), we find two new exact super-convergence
relations. They complement the known super-convergence relation based on the
extension of the GDH sum rule to the light-light system. We also find a set of
sum rules for the low-energy photon-photon interaction. All of the new
relations are verified here exactly at leading order in scalar and spinor QED.
The super-convergence relations, applied to the \gamma* \gamma -production of
mesons, lead to intricate relations between the \gamma \gamma -decay widths or
the \gamma* \gamma -transition form factors for (pseudo-) scalar, axial-vector
and tensor mesons. We discuss the phenomenological implications of these
results for mesons in both the light-quark sector and the charm-quark sector.Comment: 32 pages, 7 figure
Evolution rĂ©cente de la population, de lâoccupation des sols et de la diversitĂ© floristique sur un terroir agricole du Sud-Ouest du Niger
Au cours des 50 derniĂšres annĂ©es, un fort accroissement de la population a Ă©tĂ© enregistrĂ© dans la partie Sud-Ouest du Niger. Ce phĂ©nomĂšne a entraĂźnĂ© de profonds changements dans lâoccupation des sols et une pression accrue sur les ressources naturelles qui ont eu une incidence importante sur lâallure de la vĂ©gĂ©tation. Lâobjectif de cette Ă©tude est de faire le point sur la richesse spĂ©cifique du terroir au regard de cette Ă©volution. Les rĂ©sultats montrent que la richesse floristique est assez Ă©levĂ©e Ă Ticko. Ainsi, 400 espĂšces, rĂ©parties entre 66 familles ont Ă©tĂ© recensĂ©es. Parmi celles-ci, on compte 85 espĂšces ligneuses regroupĂ©es en 32 familles et 315 espĂšces herbacĂ©es, ou ligneuses basses, rĂ©parties entre 44 familles, dont 64% de ThĂ©rophytes. Cette flore reprĂ©sente 65% des espĂšces et 71% des familles prĂ©sentes dans la rĂ©gion du Liptako. AprĂšs 25 ans de forte croissance dĂ©mographique Ă Ticko, la diversitĂ© floristique reste encore Ă peu prĂšs intacte dans ce terroir. On note cependant une rĂ©gression des populations dâespĂšces herbacĂ©es et ligneuses, autrefois bien reprĂ©sentĂ©es sur le terroir, qui pourrait conduire dans les annĂ©es Ă venir Ă la disparition de ces taxons, et plus particuliĂšrement dâune vingtaine de taxons ligneux.Mots clĂ©s : population humaine, diversitĂ© floristique, Ticko, Niger
Magnetic imaging with an ensemble of Nitrogen Vacancy centers in diamond
The nitrogen-vacancy (NV) color center in diamond is an atom-like system in
the solid-state which specific spin properties can be efficiently used as a
sensitive magnetic sensor. An external magnetic field induces Zeeman shifts of
the NV center levels which can be measured using Optically Detected Magnetic
Resonance (ODMR). In this work, we exploit the ODMR signal of an ensemble of NV
centers in order to quantitatively map the vectorial structure of a magnetic
field produced by a sample close to the surface of a CVD diamond hosting a thin
layer of NV centers. The reconstruction of the magnetic field is based on a
maximum-likelihood technique which exploits the response of the four intrinsic
orientations of the NV center inside the diamond lattice. The sensitivity
associated to a 1 {\mu}m^2 area of the doped layer, equivalent to a sensor
consisting of approximately 10^4 NV centers, is of the order of 2
{\mu}T/sqrt{Hz}. The spatial resolution of the imaging device is 400 nm,
limited by the numerical aperture of the optical microscope which is used to
collect the photoluminescence of the NV layer. The versatility of the sensor is
illustrated by the accurate reconstruction of the magnetic field created by a
DC current inside a copper wire deposited on the diamond sample.Comment: 11 pages, 5 figures, figure 4 added, results unchange
Test of the Ï-model of BoseâEinstein correlations and reconstruction of the source function in hadronic Z-boson decay at LEP
BoseâEinstein correlations of pairs of identical charged pions produced in hadronic Z decays are analyzed in terms of various parametrizations. A good description is achieved using a LĂ©vy stable distribution in conjunction with a model where a particleâs momentum is correlated with its spaceâtime point of production, the Ï-model. Using this description and the measured rapidity and transverse momentum distributions, the spaceâtime evolution of particle emission in two-jet events is reconstructed. However, the elongation of the particle emission region previously observed is not accommodated in the Ï-model, and this is investigated using an ad hoc modification
Good Learning and Implicit Model Enumeration
MathSBML is an open-source, freely-downloadable Mathematica package that facilitates working with Systems Biology Markup Language (SBML) models. SBML is a toolneutral,computer-readable format for representing models of biochemical reaction networks, applicable to metabolic networks, cell-signaling pathways, genomic regulatory networks, and other modeling problems in systems biology that is widely supported by the systems biology community. SBML is based on XML, a standard medium for representing and transporting data that is widely supported on the internet as well as in computational biology and bioinformatics. Because SBML is tool-independent, it enables model transportability, reuse, publication and survival. In addition to MathSBML, a number of other tools that support SBML model examination and manipulation are provided on the sbml.org website, including libSBML, a C/C++ library for reading SBML models; an SBML Toolbox for MatLab; file conversion programs; an SBML model validator and visualizer; and SBML specifications and schemas. MathSBML enables SBML file import to and export from Mathematica as well as providing an API for model manipulation and simulation
Flavour structure of low-energy hadron pair photoproduction
We consider the process where and
are either mesons or baryons. The experimental findings for such quantities as
the and differential cross sections, in the energy range
currently probed, are found often to be in disparity with the scaling behaviour
expected from hard constituent scattering. We discuss the long-distance
pole--resonance contribution in understanding the origin of these phenomena, as
well as the amplitude relations governing the short-distance contribution which
we model as a scaling contribution. When considering the latter, we argue that
the difference found for the and the integrated cross
sections can be attributed to the s-channel isovector component. This
corresponds to the subprocess in the VMD
(vector-meson-dominance) language. The ratio of the two cross sections is
enhanced by the suppression of the component, and is hence constrained.
We give similar constraints to a number of other hadron pair production
channels. After writing down the scaling and pole--resonance contributions
accordingly, the direct summation of the two contributions is found to
reproduce some salient features of the and data.Comment: 12 pages, 9 figures, revised version to be published in EPJ
A qualitative optimization technique for biophysical neuron models with many parameters
We present a novel computational technique that enables more efficient optimization of qualitative features in biophysical neural models
Neutrino Masses from Fine Tuning
We present a new approach for generating tiny neutrino masses. The Dirac
neutrino mass matrix gets contributions from two new Higgs doublets with their
vevs at the electroweak (EW) scale. Neutrino masses are tiny not because of
tiny Yukawa couplings, or very heavy () right handed
neutrinos. They are tiny because of a cancelation in the Dirac neutrino mass
matrix (fine tuning). After fine tuning to make the Dirac neutrino mass matrix
at the GeV scale, light neutrino masses are obtained in the correct
scale via the see-saw mechanism with the right handed neutrino at the EW scale.
The proposal links neutrino physics to collider physics. The Higgs search
strategy is completely altered. For a wide range of Higgs masses, the Standard
Model Higgs decays dominantly to mode giving rise to the final
state , or . This can be
tested at the LHC, and possibly at the Tevatron.Comment: 12 pages, 4 figures and 3 table
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