599 research outputs found
Search for fermiophobic Higgs bosons in final states with photons at LEP 2
Higgs boson production with subsequent decay to photons was searched for in
the data collected by the DELPHI detector at centre-of-mass energies between
183 GeV and 209 GeV, corresponding to a total integrated luminosity of nearly
650 pb^{-1}. No evidence for a signal was found, and limits were set on h0Z0
and h0A0 production with h0 decay to photons. These results were used to
exclude regions in the parameter space of fermiophobic scenarios of Two Higgs
Doublet Models.Comment: 24 pages, 6 figures, Accepted by Eur. Phys. J.
Complete (O_7,O_8) contribution to B -> X_s gamma at order alpha_s^2
We calculate the set of O(alpha_s^2) corrections to the branching ratio and
to the photon energy spectrum of the decay process B -> X_s gamma originating
from the interference of diagrams involving the electromagnetic dipole operator
O_7 with diagrams involving the chromomagnetic dipole operator O_8. The
corrections evaluated here are one of the elements needed to complete the
calculations of the B -> X_s gamma branching ratio at next-to-next-to-leading
order in QCD. We conclude that this set of corrections does not change the
central value of the Standard Model prediction for Br(B -> X_s gamma) by more
than 1 %.Comment: 19 pages, 8 figure
Biomasse da praterie seminaturali come fonte di energia: un fuoco di paglia?
Biomass from seminatural grasslands as energy source: flash in the pan? - The aim of the study is to evaluate the potentiality of four seminatural herbaceous communities (grasslands of Arrhenatherion, Nardo-Agrostion tenuis, Bromion erecti and Polygono-Trisetion alliances) in the Prealpine Lombardy. We quantified the biomass production (stubble height 5 cm), the main bioenergetic features, such as ash, carbon, hydrogen and nitrogen content, and the Higher Heating Value. Biomass values vary greatly, with means between 513 (Arrhenatherion) to 227 g m-2 (Bromion erecti). The mean ash content ranges between 4.3-7.7% and mean HHV from 18.3 to 19.6 MJ kg-1. The mean energy per unit area varies between 4.5-9.5 MJ m-2.We suggest that the management of seminatural grasslands could provide biomass suitable for bioenergy production instead of common energy crops and at the same time promote conservation of habitats and their biodiversity, mostly in protected areas
On the Numerical Evaluation of One-Loop Amplitudes: the Gluonic Case
We develop an algorithm of polynomial complexity for evaluating one-loop
amplitudes with an arbitrary number of external particles. The algorithm is
implemented in the Rocket program. Starting from particle vertices given by
Feynman rules, tree amplitudes are constructed using recursive relations. The
tree amplitudes are then used to build one-loop amplitudes using an integer
dimension on-shell cut method. As a first application we considered only three
and four gluon vertices calculating the pure gluonic one-loop amplitudes for
arbitrary external helicity or polarization states. We compare our numerical
results to analytical results in the literature, analyze the time behavior of
the algorithm and the accuracy of the results, and give explicit results for
fixed phase space points for up to twenty external gluons.Comment: 22 pages, 9 figures; v2: references added, version accepted for
publicatio
Feynman rules for the rational part of the Electroweak 1-loop amplitudes
We present the complete set of Feynman rules producing the rational terms of
kind R_2 needed to perform any 1-loop calculation in the Electroweak Standard
Model. Our results are given both in the 't Hooft-Veltman and in the Four
Dimensional Helicity regularization schemes. We also verified, by using both
the 't Hooft-Feynman gauge and the Background Field Method, a huge set of Ward
identities -up to 4-points- for the complete rational part of the Electroweak
amplitudes. This provides a stringent check of our results and, as a
by-product, an explicit test of the gauge invariance of the Four Dimensional
Helicity regularization scheme in the complete Standard Model at 1-loop. The
formulae presented in this paper provide the last missing piece for completely
automatizing, in the framework of the OPP method, the 1-loop calculations in
the SU(3) X SU(2) X U(1) Standard Model.Comment: Many thanks to Huasheng Shao for having recomputed, independently of
us, all of the effective vertices. Thanks to his help and by
comparing with an independent computation we performed in a general
gauge, we could fix, in the present version, the following formulae: the
vertex in Eq. (3.6), the vertex in Eq. (3.8),
Eqs (3.16), (3.17) and (3.18
Feynman Rules for the Rational Part of the QCD 1-loop amplitudes
We compute the complete set of Feynman Rules producing the Rational Terms of
kind R_2 needed to perform any QCD 1-loop calculation. We also explicitly check
that in order to account for the entire R_2 contribution, even in case of
processes with more than four external legs, only up to four-point vertices are
needed. Our results are expressed both in the 't Hooft Veltman regularization
scheme and in the Four Dimensional Helicity scheme, using explicit color
configurations as well as the color connection language.Comment: 18 pages, 11 figures. Misprints corrected in Appendix A. Version to
be published in JHE
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