281 research outputs found
One-loop leading logarithms in electroweak radiative corrections, I. Results
We present results for the complete one-loop electroweak logarithmic
corrections for general processes at high energies and fixed angles. Our
results are applicable to arbitrary matrix elements that are not
mass-suppressed. We give explicit results for 4-fermion processes and
gauge-boson-pair production in electron-positron annihilation.Comment: 35 pages, latex, 4 postscript figures, some misprints correcte
One-loop leading logarithms in electroweak radiative corrections: II. Factorization of collinear singularities
We discuss the evaluation of the collinear single-logarithmic contributions
to virtual electroweak corrections at high energies. More precisely, we proof
the factorization of the mass singularities originating from loop diagrams
involving collinear virtual gauge bosons coupled to external legs. We discuss,
in particular, processes involving external longitudinal gauge bosons, which
are treated using the Goldstone-boson equivalence theorem. The proof of
factorization is performed within the 't Hooft--Feynman gauge at one-loop order
and applies to arbitrary electroweak processes that are not mass-suppressed at
high energies. As basic ingredient we use Ward identities for Green functions
with arbitrary external particles involving a gauge boson collinear to one of
these. The Ward identities are derived from the BRS invariance of the
spontaneously broken electroweak gauge theory.Comment: 28 pages, late
On-the-fly reduction of open loops
Building on the open-loop algorithm we introduce a new method for the
automated construction of one-loop amplitudes and their reduction to scalar
integrals. The key idea is that the factorisation of one-loop integrands in a
product of loop segments makes it possible to perform various operations
on-the-fly while constructing the integrand. Reducing the integrand on-the-fly,
after each segment multiplication, the construction of loop diagrams and their
reduction are unified in a single numerical recursion. In this way we entirely
avoid objects with high tensor rank, thereby reducing the complexity of the
calculations in a drastic way. Thanks to the on-the-fly approach, which is
applied also to helicity summation and for the merging of different diagrams,
the speed of the original open-loop algorithm can be further augmented in a
very significant way. Moreover, addressing spurious singularities of the
employed reduction identities by means of simple expansions in rank-two Gram
determinants, we achieve a remarkably high level of numerical stability. These
features of the new algorithm, which will be made publicly available in a
forthcoming release of the OpenLoops program, are particularly attractive for
NLO multi-leg and NNLO real-virtual calculations.Comment: v2 as accepted by EPJ C: extended discussion of the triangle
reduction and its numerical stability in section 5.4.2; speed benchmarks for
2->5 processes included in section 6.2.1; ref. adde
NLO QCD corrections to top anti-top bottom anti-bottom production at the LHC: 1. quark-antiquark annihilation
The process pp -> top anti-top bottom anti-bottom + X represents a very
important background reaction to searches at the LHC, in particular to top
anti-top H production where the Higgs boson decays into a bottom anti-bottom
pair. A successful analysis of top anti-top H at the LHC requires the knowledge
of direct top anti-top bottom anti-bottom production at next-to-leading order
in QCD. We take the first step in this direction upon calculating the
next-to-leading-order QCD corrections to the subprocess initiated by quark
anti-quark annihilation. We devote an appendix to the general issue of rational
terms resulting from ultraviolet or infrared (soft or collinear) singularities
within dimensional regularization. There we show that, for arbitrary processes,
in the Feynman gauge, rational terms of infrared origin cancel in truncated
one-loop diagrams and result only from trivial self-energy corrections.Comment: 30 pages, LaTeX, 12 postscript figure
A unified NLO description of top-pair and associated Wt production
We present an NLO simulation of WWbb production with massive b-quarks at the
LHC. Off-shell and non-resonant contributions associated with top-pair and
single-top channels and with leptonic W-boson decays are consistently taken
into account using the complex-mass scheme. Thanks to the finite b-quark mass,
WWbb predictions can be extended to the whole b-quark phase space, thereby
including Wt-channel single-top contributions that originate from collinear
g->bb splittings in the four-flavour scheme. This provides a consistent NLO
description of tt and Wt production and decay, including quantum interference
effects. The simulation is also applicable to exclusive 0- and 1-jet bins,
which is of great importance for Higgs-boson studies in the H->WW channel and
for any other analysis with large top backgrounds and jet vetoes or jet bins.Comment: 8pp. Minor revision, results unchange
Electroweak radiative corrections at high energies
This PhD thesis is concerned with one-loop virtual electroweak corrections to
arbitrary processes in the high-energy limit. Complete results are presented
for the leading and subleading logarithms of large ratios of energy scales to
mass scales. These results include the logarithmic dependence on the photon and
weak-boson masses, on the light-fermion masses, as well as on the Higgs- and
top-masses in the heavy-Higgs and heavy-top limit.
All sources of electroweak logarithmic corrections are taken into account,
including the exchange of soft and/or collinear electroweak gauge bosons as
well as the renormalization-group running of the gauge, scalar and Yukawa
couplings.
The logarithmic corrections are derived in a process-independent way,
resulting in simple analytic formulas that apply to arbitrary electroweak
processes that are not mass-suppressed in the high-energy limit. We also
present analytical and numerical applications for the processes: e^+e^- \to
f\bar{f}, e^+ e^- \to W^+W^-, ZZ, Z\gamma, \gamma\gamma and \bar{d}u \to W^+Z,
W^+\gamma.Comment: PhD thesis, 131 page
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