876 research outputs found
Single top-quark production with the Matrix Element Method in next-to-leading order accuracy
Single top-quark production offers a unique laboratory for precision tests of
the Standard Model and searches of possible extensions. Furthermore, assuming
the Standard Model, single top-quark production can be used to determine
top-quark related couplings. For precise determinations of parameters like the
electroweak gauge couplings or the mass of the top quark, efficient, unbiased,
and theoretically unambiguous analysis methods are needed. Within this context,
the Matrix Element Method (MEM) has been established in hadron collider
analyses due to its possibility to top out at utilising the information
available in experimental data. However, so far it has mostly been applied in
Born approximation only. We discuss the extension to next-to-leading order
(NLO) accuracy. As a necessary prerequisite we introduce an efficient method to
calculate NLO QCD weights for jet events. As proof of concept and
representative example we use the MEM at NLO to reproduce the top-quark mass in
a toy experiment where we treat single top-quark events generated at NLO
accuracy as pseudo-data. This is the first application of the MEM at NLO
accuracy to the hadronic production of jets originating from coloured final
state partons. We observe that analysing NLO events with Born likelihoods can
introduce a pronounced bias in the extracted mass which would require
significant calibration with associated uncertainties. Although we focus on
parameter determinations, the methods presented here can also be used to search
for new physics using likelihood ratios.Comment: 5 pages with 4 figures; proccedings to XXV International Workshop on
Deep-Inelastic Scattering and Related Subjects, 3-7 April 2017, University of
Birmingham, U
Extending the Matrix Element Method beyond the Born approximation: Calculating event weights at next-to-leading order accuracy
In this article we illustrate how event weights for jet events can be
calculated efficiently at next-to-leading order (NLO) accuracy in QCD. This is
a crucial prerequisite for the application of the Matrix Element Method in NLO.
We modify the recombination procedure used in jet algorithms, to allow a
factorisation of the phase space for the real corrections into resolved and
unresolved regions. Using an appropriate infrared regulator the latter can be
integrated numerically. As illustration, we reproduce differential
distributions at NLO for two sample processes. As further application and proof
of concept, we apply the Matrix Element Method in NLO accuracy to the mass
determination of top quarks produced in e+e- annihilation. This analysis is
relevant for a future Linear Collider. We observe a significant shift in the
extracted mass depending on whether the Matrix Element Method is used in
leading or next-to-leading order.Comment: 35 pages, 12 figures, references & acknowledgments added, typos
corrected, matches published versio
Kira - A Feynman Integral Reduction Program
In this article, we present a new implementation of the Laporta algorithm to
reduce scalar multi-loop integrals---appearing in quantum field theoretic
calculations---to a set of master integrals. We extend existing approaches by
using an additional algorithm based on modular arithmetic to remove linearly
dependent equations from the system of equations arising from
integration-by-parts and Lorentz identities. Furthermore, the algebraic
manipulations required in the back substitution are optimized. We describe in
detail the implementation as well as the usage of the program. In addition, we
show benchmarks for concrete examples and compare the performance to Reduze 2
and FIRE 5.
In our benchmarks we find that Kira is highly competitive with these existing
tools.Comment: 37 pages, 3 figure
Top-Quark Physics at the LHC
The top quark is the heaviest of all known elementary particles. It was
discovered in 1995 by the CDF and D0 experiments at the Tevatron. With the
start of the LHC in 2009, an unprecedented wealth of measurements of the top
quark's production mechanisms and properties have been performed by the ATLAS
and CMS collaborations, most of these resulting in smaller uncertainties than
those achieved previously. At the same time, huge progress was made on the
theoretical side yielding significantly improved predictions up to
next-to-next-to-leading order in perturbative QCD. Due to the vast amount of
events containing top quarks, a variety of new measurements became feasible and
opened a new window to precisions tests of the Standard Model and to
contributions of new physics. In this review, originally written for a recent
book on the results of LHC Run 1, top-quark measurements obtained so far from
the LHC Run 1 are summarised and put in context with the current understanding
of the Standard Model.Comment: 35 pages, 25 figures. To appear in "The Large Hadron Collider --
Harvest of Run 1", Thomas Sch\"orner-Sadenius (ed.), Springer, 2015 (532
pages, 253 figures; ISBN 978-3-319-15000-0; eBook ISBN 978-3-319-15001-7, for
more details, see http://www.springer.com/de/book/9783319150000
Heavy Higgs boson resonances and their decay into top quarks at the LHC
We investigate, within the type-II two-Higgs-doublet extension of the standard model (SM), the impact of heavy neutral Higgs boson resonances with unsuppressed Yukawa couplings to top quarks on top-quark pair production at the LHC at next-to-leading order (NLO) in the strong coupling constant. We take into account the resonant Higgs boson contributions, the non-resonant SM t<span style="text-decoration: overline">t</span> continuum and the interference of these two contributions. The NLO QCD corrections to heavy Higgs production and the interference contributions are calculated in the large top-quark mass (m<sub>t</sub>) limit, including an effective K-factor rescaling. Our evaluation of the QCD-Higgs interference is focused on the Higgs resonance region. Using representative CP-conserving as well as CP-violating parameter scenarios phenomenological results are presented for different observables
Computation of multi-leg amplitudes with NJet
In these proceedings we report our progress in the development of the
publicly available C++ library NJet for accurate calculations of
high-multiplicity one-loop amplitudes. As a phenomenological application we
present the first complete next-to-leading order (NLO) calculation of five jet
cross section at hadron colliders.Comment: 8 pages, 5 figures, Contribution to the proceedings of "ACAT 2013"
conference, Beijing, China, May 201
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