38 research outputs found
Bread and Butter Standard Model
Lectures given at the International School of Subnuclear Physics: 37th
Course: Basics and Highlights of Fundamental Physics, Erice, Italy, 29 Aug - 7
Sep 1999
One-loop effective lagrangians after matching
We discuss the limitations of the covariant derivative expansion prescription
advocated to compute the one-loop Standard Model (SM) effective lagrangian when
the heavy fields couple linearly to the SM. In particular, one-loop
contributions resulting from the exchange of both heavy and light fields must
be explicitly taken into account through matching because the proposed
functional approach alone does not account for them. We review a simple case
with a heavy scalar singlet of charge to illustrate the argument. As two
other examples where this matching is needed and this functional method gives a
vanishing result, up to renormalization of the heavy sector parameters, we
re-evaluate the one-loop corrections to the T--parameter due to a heavy scalar
triplet with vanishing hypercharge coupling to the Brout-Englert-Higgs boson
and to a heavy vector-like quark singlet of charged mixing with the top
quark, respectively. In all cases we make use of a new code for matching
fundamental and effective theories in models with arbitrary heavy field
additions.Comment: 22 pages; v2: comments and references added; v3: published version,
typos correcte
One-loop radiative corrections to the helicity amplitudes of QCD processes involving four quarks and one gluon
We present analytic results for the one-loop corrections of the helicity
amplitudes of the QCD five-parton subprocesses involving four quarks and one
gluon obtained with a standard Feynman diagram calculation using dimensional
reduction.Comment: 9 pages LaTeX, ETH-TH/94-14, KLTE-DTP/94-
Two-loop amplitudes and master integrals for the production of a Higgs boson via a massive quark and a scalar-quark loop
We compute all two-loop master integrals which are required for the
evaluation of next-to-leading order QCD corrections in Higgs boson production
via gluon fusion. Many two-loop amplitudes for 2 -> 1 processes in the Standard
Model and beyond can be expressed in terms of these integrals using automated
reduction techniques. These integrals also form a subset of the master
integrals for more complicated 2 -> 2 amplitudes with massive propagators in
the loops. As a first application, we evaluate the two-loop amplitude for Higgs
boson production in gluon fusion via a massive quark. Our result is the first
independent check of the calculation of Spira, Djouadi, Graudenz and Zerwas. We
also present for the first time the two-loop amplitude for gg -> h via a
massive squark
Unitarity cuts and reduction to master integrals in d dimensions for one-loop amplitudes
We present an alternative reduction to master integrals for one-loop
amplitudes using a unitarity cut method in arbitrary dimensions. We carry out
the reduction in two steps. The first step is a pure four-dimensional
cut-integration of tree amplitudes with a mass parameter, and the second step
is applying dimensional shift identities to master integrals. This reduction is
performed at the integrand level, so that coefficients can be read out
algebraically.Comment: 38 Pages. v2: reference added, typos fixe
Singular terms of helicity amplitudes at one-loop in QCD and the soft limit of the cross sections of multi-parton processes
We describe a general method that enables us to obtain all the singular terms
of helicity amplitudes of n-parton processes at one loop. The algorithm uses
helicity amplitudes at tree level and simple color algebra.
We illustrate the method by calculating the singular part of the one loop
helicity amplitudes of all parton subprocesses. The results are used
to derive the soft gluon limit of the cross sections of all parton
scattering subprocesses which provide a useful initial condition for the
angular ordering approximation to coherent multiple soft gluon emission,
incorporated in existing Monte Carlo simulation programs.Comment: Latex,13 pages, ETH-TH/94-
One-loop helicity amplitudes for all 2 -> 2 processes in QCD and N=1 supersymmetric Yang-Mills theory
One loop corrections to the helicity amplitudes of all 2 -> 2 subprocesses
are calculated in QCD and in N=1 supersymmetric Yang-Mills theory using two
versions of dimensional regularization: the `t~Hooft-Veltman scheme and
dimensional reduction. Studying the structure of the soft and collinear
singularities, we found universal transition rules for the squared matrix
element which can be used to translate the results obtained in these schemes to
the results valid in the conventional dimensional regularization scheme. With
explicit calculation it is demonstrated that the one loop helicity amplitudes
of the 2 -> 2 subprocesses calculated using dimensional reduction in the N=1
supersymmetric gauge theory respect the supersymmetry Ward identities.
Our transition rules can also be used to calculate the next-to-leading order
Altarelli-Parisi kernels in the dimensional reduction scheme when they satisfy
supersymmetry Ward identities as well.Comment: 40 pages + 1 table, uses LaTeX and table.tex, ETH-TH/93-1
Jet physics at hadron colliders
I give a short summary of jet definition algorithms and recent progress in
the quantitative description of jet production.Comment: 9 pages, LaTex, 6 postscript figures, Proceedings of Quark Matter
2002, Nantes, France, July 18-24, 200