157 research outputs found
Helicity Amplitudes for Single-Top Production
Single top quark production at hadron colliders allows a direct measurement
of the top quark charged current coupling. We present the complete tree-level
helicity amplitudes for four processes involving the production and
semileptonic decay of a single top quark: W-gluon fusion, flavor excitation,
s-channel production and W-associated production. For the first three processes
we study the quality of the narrow top width approximation. We also examine
momentum and angular distributions of some of the final state particles.Comment: 27 pages, 7 figures, final versio
Fully differential QCD corrections to single top quark final states
A new next-to-leading order Monte Carlo program for calculation of fully
differential single top quark final states is described and first results
presented. Both the s- and t-channel contributions are included.Comment: 3 pages, 3 figures, talk presented at DPF2000, August 9-12, 2000. To
appear in International Journal of Modern Physics
The singular behavior of massive QCD amplitudes
We discuss the structure of infrared singularities in on-shell QCD amplitudes
with massive partons and present a general factorization formula in the limit
of small parton masses. The factorization formula gives rise to an all-order
exponentiation of both, the soft poles in dimensional regularization and the
large collinear logarithms of the parton masses. Moreover, it provides a
universal relation between any on-shell amplitude with massive external partons
and its corresponding massless amplitude. For the form factor of a heavy quark
we present explicit results including the fixed-order expansion up to three
loops in the small mass limit. For general scattering processes we show how our
constructive method applies to the computation of all singularities as well as
the constant (mass-independent) terms of a generic massive n-parton QCD
amplitude up to the next-to-next-to-leading order corrections.Comment: version to appear in JHEP (sec. 3 with expanded discussion and
appendix with added results
The treatment of the infrared region in perturbative QCD
We discuss the contribution coming from the infrared region to NLO matrix
elements and/or coefficient functions of hard QCD processes. Strictly speaking,
this contribution is not known theoretically, since it is beyond perturbative
QCD. For DGLAP evolution all the infrared contributions are collected in the
phenomenological input parton distribution functions (PDFs), at some relatively
low scale Q_0; functions which are obtained from a fit to the `global' data.
However dimensional regularization sometimes produces a non-zero result coming
from the infrared region. Instead of this conventional regularization
treatment, we argue that the proper procedure is to first subtract from the NLO
matrix element the contribution already generated at the same order in \alpha_s
by the LO DGLAP splitting function convoluted with the LO matrix element. This
prescription eliminates the logarithmic infrared divergence, giving a
well-defined result which is consistent with the original idea that everything
below Q_0 is collected in the PDF input. We quantify the difference between the
proposed treatment and the conventional approach using low-mass Drell-Yan
production and deep inelastic electron-proton scattering as examples; and
discuss the potential impact on the `global' PDF analyses. We present arguments
to show that the difference cannot be regarded as simply the use of an
alternative factorization scheme.Comment: 15 pages, 5 figures, title changed, text considerably modified to
improve presentation, and discussion section enlarge
Expansion around half-integer values, binomial sums and inverse binomial sums
I consider the expansion of transcendental functions in a small parameter
around rational numbers. This includes in particular the expansion around
half-integer values. I present algorithms which are suitable for an
implementation within a symbolic computer algebra system. The method is an
extension of the technique of nested sums. The algorithms allow in addition the
evaluation of binomial sums, inverse binomial sums and generalizations thereof.Comment: 21 page
The infrared structure of e+ e- --> 3 jets at NNLO reloaded
This paper gives detailed information on the structure of the infrared
singularities for the process e+ e- --> 3 jets at next-to-next-to-leading order
in perturbation theory. Particular emphasis is put on singularities associated
to soft gluons. The knowledge of the singularity structure allows the
construction of appropriate subtraction terms, which in turn can be implemented
into a numerical Monte Carlo program.Comment: 59 pages, additional comments added, version to be publishe
Higgs Radiation off Top Quarks at the Tevatron and the LHC
Higgs bosons can be searched for in the channels
at the Tevatron and the LHC. We have calculated the QCD corrections to these
processes in the Standard Model at next-to-leading order. The higher-order
corrections reduce the renormalization and factorization scale dependence
considerably and stabilize the theoretical predictions for the cross sections.
At the central scale the properly defined factors are
slightly below unity for the Tevatron () and slightly above unity
for the LHC ().Comment: 5 pages, latex, 2 figure
Polarizing the Dipoles
We extend the massless dipole formalism of Catani and Seymour, as well as its
massive version as developed by Catani, Dittmaier, Seymour and Trocsanyi, to
arbitrary helicity eigenstates of the external partons. We modify the real
radiation subtraction terms only, the primary aim being an improved efficiency
of the numerical Monte Carlo integration of this contribution as part of a
complete next-to-leading order calculation. In consequence, our extension is
only applicable to unpolarized scattering. Upon summation over the helicities
of the emitter pairs, our formulae trivially reduce to their original form. We
implement our extension within the framework of Helac-Phegas, and give some
examples of results pertinent to recent studies of backgrounds for the LHC. The
code is publicly available. Since the integrated dipole contributions do not
require any modifications, we do not discuss them, but they are implemented in
the software.Comment: 20 pages, 4 figures, Integrated dipoles implemented for massless and
massive case
Subtraction terms for one-loop amplitudes with one unresolved parton
Fully differential next-to-next-to-leading order calculations require a
method to cancel infrared singularities. In a previous publication, I discussed
the general setup for the subtraction method at NNLO. In this paper I give all
subtraction terms for electron-positron annihilation associated with one-loop
amplitudes with one unresolved parton. These subtraction terms are integrated
within dimensional regularization over the unresolved one-particle phase space.
The results can be used with all variants of dimensional regularization
(conventional dimensional regularization, the 't Hooft-Veltman scheme and the
four-dimensional scheme).Comment: 27 page
Subtraction terms at NNLO
Perturbative calculations at next-to-next-to-leading order for multi-particle
final states require a method to cancel infrared singularities. I discuss the
subtraction method at NNLO. As a concrete example I consider the leading-colour
contributions to e+ e- --> 2 jets. This is the simplest example which exhibits
all essential features. For this example, explicit subtraction terms are given,
which approximate the four-parton and three-parton final states in all double
and single unresolved limits, such that the subtracted matrix elements can be
integrated numerically.Comment: 41 page
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