861 research outputs found
Constraints on the Gluon Density from Lepton Pair Production
The hadroproduction of lepton pairs with mass Q and finite transverse
momentum Q_T is described in perturbative QCD by the same partonic subprocesses
as prompt photon production. We demonstrate that, like prompt photon
production, lepton pair production is dominated by quark-gluon scattering in
the region Q_T>Q/2. This feature leads to sensitivity to the gluon density in
kinematical regimes accessible in collider and fixed target experiments, and it
provides a new independent method for constraining the gluon density.Comment: 5 pages, 7 figures, to appear in the Proceedings of the Fermilab
Physics at Run II Workshop on ``QCD and Weak Boson Physics
A minimal model for two-component dark matter
We propose and study a new minimal model for two-component dark matter. The
model contains only three additional fields, one fermion and two scalars, all
singlets under the Standard Model gauge group. Two of these fields, one fermion
and one scalar, are odd under a symmetry that renders them simultaneously
stable. Thus, both particles contribute to the observed dark matter density.
This model resembles the union of the singlet scalar and the singlet fermionic
models but it contains some new features of its own. We analyze in some detail
its dark matter phenomenology. Regarding the relic density, the main novelty is
the possible annihilation of one dark matter particle into the other, which can
affect the predicted relic density in a significant way. Regarding dark matter
detection, we identify a new contribution that can lead either to an
enhancement or to a suppression of the spin-independent cross section for the
scalar dark matter particle. Finally, we define a set of five benchmarks models
compatible with all present bounds and examine their direct detection prospects
at planned experiments. A generic feature of this model is that both particles
give rise to observable signals in 1-ton direct detection experiments. In fact,
such experiments will be able to probe even a subdominant dark matter component
at the percent level.Comment: 18 pages, 10 figure
Calculating two- and three-body decays with FeynArts and FormCalc
The Feynman diagram generator FeynArts and the computer algebra program
FormCalc allow for an automatic computation of 2->2 and 2->3 scattering
processes in High Energy Physics. We have extended this package by four new
kinematical routines and adapted one existing routine in order to accomodate
also two- and three-body decays of massive particles. This makes it possible to
compute automatically two- and three-body particle decay widths and decay
energy distributions as well as resonant particle production within the
Standard Model and the Minimal Supersymmetric Standard Model at the tree- and
loop-level. The use of the program is illustrated with three standard examples:
h->b\bar{b}, \mu->e\bar{\nu}_e\nu_\mu, and Z->\nu_e\bar{\nu}_e.Comment: 8 pages, 1 figur
Next-to-leading Order SUSY-QCD Calculation of Associated Production of Gauginos and Gluinos
Results are presented of a next-to-leading order calculation in perturbative
QCD of the production of charginos and neutralinos in association with gluinos
at hadron colliders. Predictions for cross sections are shown at the energies
of the Fermilab Tevatron and CERN Large Hadron Collider for a typical
supergravity (SUGRA) model of the sparticle mass spectrum and for a light
gluino model.Comment: 3 pages, latex, 4 figures, paper presented by E. L. Berger at ICHEP
2000, the XXXth International Conference on High Energy Physics July 27 -
August 2, 2000, Osaka, Japa
Exotic Rickettsiae in Ixodes ricinus: fact or artifact?
Several pathogenic Rickettsia species can be transmitted via Ixodes ricinus ticks to humans and animals. Surveys of I. ricinus for the presence of Rickettsiae using part of its 16S rRNA gene yield a plethora of new and different Rickettsia sequences. Interpreting these data is sometimes difficult and presenting these findings as new or potentially pathogenic Rickettsiae should be done with caution: a recent report suggested presence of a known human pathogen, R. australis, in questing I. ricinus ticks in Europe. A refined analysis of these results revealed that R. helvetica was most likely to be misinterpreted as R. australis. Evidence in the literature is accumulating that rickettsial DNA sequences found in tick lysates can also be derived from other sources than viable, pathogenic Rickettsiae. For example, from endosymbionts, environmental contamination or even horizontal gene transfer
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