571 research outputs found
New techniques for one-loop scattering amplitudes
We review the main features of the Ossola, Papadopoulos and Pittau (OPP) method for the evaluation of one-loop amplitudes of arbitrary scattering processes. In this approach, the coefficients of the scalar integrals are extracted by means of simple algebraic equations constructed by numerically evaluating the numerator of the integrand for specific choices of the integration momentum. After a discussion of the first results obtained with this method, we will focus on the latest developments in the direction of a fully automatized approach to multi-loop NLO
calculation
Multiloop Integrand Reduction for Dimensionally Regulated Amplitudes
We present the integrand reduction via multivariate polynomial division as a
natural technique to encode the unitarity conditions of Feynman amplitudes. We
derive a recursive formula for the integrand reduction, valid for arbitrary
dimensionally regulated loop integrals with any number of loops and external
legs, which can be used to obtain the decomposition of any integrand
analytically with a finite number of algebraic operations. The general results
are illustrated by applications to two-loop Feynman diagrams in QED and QCD,
showing that the proposed reduction algorithm can also be seamlessly applied to
integrands with denominators appearing with arbitrary powers.Comment: Published version. 5 pages, 2 figure
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.
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
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
The Integrand Reduction of One- and Two-Loop Scattering Amplitudes
The integrand-level methods for the reduction of scattering amplitudes are
well-established techniques, which have already proven their effectiveness in
several applications at one-loop. In addition to the automation and refinement
of tools for one-loop calculations, during the past year we observed very
interesting progress in developing new techniques for amplitudes at two- and
higher-loops, based on similar principles. In this presentation, we review the
main features of integrand-level approaches with a particular focus on
algebraic techniques, such as Laurent series expansion which we used to improve
the one-loop reduction, and multivariate polynomial division which unveils the
structure of multi-loop amplitudes.Comment: 7 pages, v2: fixed typos, added references. Presented at "Loops and
Legs in Quantum Field Theory", Wernigerode, Germany, 15-20 April 201
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 Rational Terms of the one-loop amplitudes
The various sources of Rational Terms contributing to the one-loop amplitudes
are critically discussed. We show that the terms originating from the generic
(n-4)-dimensional structure of the numerator of the one-loop amplitude can be
derived by using appropriate Feynman rules within a tree-like computation. For
the terms that originate from the reduction of the 4-dimensional part of the
numerator, we present two different strategies and explicit algorithms to
compute them.Comment: 14 pages, 3 figures, uses axodraw.st
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