273 research outputs found
Flavour Physics and CP Violation
These notes represent a summary of three lectures on flavour and CP
violation, given at the CERNs European School of High Energy Physics in 2014.
They cover flavour physics within the standard model, phenomenology of CP
violation in meson mixing and decays, as well as constraints of flavour
observableson physics beyond the standard model. In preparing the lectures (and
consequently this summary) I drew heavily from several existing excellent and
exhaustive sets of lecture notes and reviews on flavour physics and CP
violation [1]. The reader is encouraged to consult those as well as the
original literature for a more detailed study.Comment: 16 pages, contribution to the CERN in the Proceedings of the 2014
European School of High-Energy Physics, Garderen, the Netherlands, 18 June -
1 July 201
Flavor Changing Neutral Coupling Mediated Radiative Top Quark Decays at Next-to-Leading Order in QCD
We compute the branching ratios for the rare top quark decays t to c gamma
and t to c Z mediated by effective flavor changing neutral couplings at the
next-to-leading order in QCD including the effects due to operator mixing.
After re-suming contributions of order [alpha_s log (Lambda / m_t)]^n, where
Lambda is the scale at which the effective operators are generated, at leading
log level using renormalization group methods, we compute finite matrix element
corrections and study the effects of experimental kinematic cuts on the
extracted branching ratios. We find that the t to c gamma decay can be used to
probe also the effective operators mediating t to c g processes, since these
can naturaly contribute 10% or more to the radiative decay. Conversely, any
experimental signal of t to c g would indicate a natural lower bound on t to
cZ, gamma.Comment: 4 pages, 3 figure
QCD Corrections to Flavor Changing Neutral Coupling Mediated Rare Top Quark Decays
Recently we have presented an analysis of flavor changing neutral coupling
mediated radiative top quark decays at next-to-leading order in QCD. In the
present paper we provide the details of the calculation of QCD corrections to
t-> q gamma and t-> q Z decays within the effective theory approach including
operator mixing. In particular, we calculate virtual matrix element corrections
and the corresponding bremsstrahlung contributions. In the case of t-> q gamma
we study the effects of kinematic cuts on the extracted branching ratios.
Analytical formulae are given at all stages of the calculation. We find that
the t-> q gamma decay can be used to probe also the effective operators
mediating t-> q g processes, since these can naturally contribute 10% or more
to the radiative decay, given typical experimental cuts on the decay kinematics
at hadron colliders. Conversely, we argue that any positive experimental signal
of the t-> q g process would indicate a natural lower bound on t-> q gamma
decay rate.Comment: 12 page
Charm meson resonances in decays
Motivated by recent experimental results we reconsider semileptonic decays within a model which combines heavy quark symmetry and
properties of the chiral Lagrangian. We include excited charm meson states,
some of them recently observed, in our Lagrangian and determine their impact on
the charm meson semileptonic form factors. We find that the inclusion of
excited charm meson states in the model leads to a rather good agreement with
the experimental results on the shape of the form factor. We
also calculate branching ratios for all decays.Comment: 9 pages, 4 figures; minor corrections, added some discussion, version
as publishe
Light Colored Scalar as Messenger of Up-Quark Flavor Dynamics in Grand Unified Theories
The measured forward-backward asymmetry in the t tbar production at the
Tevatron might be explained by the additional exchange of a colored weak
singlet scalar. Such state appears in some of the grand unified theories and
its interactions with the up-quarks are purely antisymmetric in flavor space.
We systematically investigate the resulting impact on charm and top quark
physics. The constraints on the relevant Yukawa couplings come from the
experimentally measured observables related to D0--D0bar oscillations, as well
as di-jet and single top production measurements at the Tevatron. After fully
constraining the relevant Yukawa couplings, we predict possible signatures of
this model in rare top quark decays. In a class of grand unified models we
demonstrate how the obtained information enables to constrain the Yukawa
couplings of the up-quarks at very high energy scale.Comment: 13 pages, 11 figures, version as published in PR
Scalar diquark in top-antitop production and constraints on Yukawa sector of grand unified theories
International audienceA colored weak singlet scalar state with hypercharge 4/3 is one of the possible candidates for the explanation of the unexpectedly large forward-backward asymmetry in tt production as measured by the CDF and D0 experiments. We investigate the role of this state in a plethora of flavor chang- ing neutral current processes and precision observables of down-quarks and charged leptons. Our analysis includes tree- and loop-level mediated observables in the K and B systems, the charged lepton sector, as well as the Z → b ¯b width. We perform a fit of the relevant scalar couplings. This approach can explain the (g − 2)µ anomaly while tensions among the CP violating observables in the quark sector, most notably the nonstandard CP phase (and width difference) in the Bs system cannot be fully relaxed. The results are interpreted in a class of GUT models which allow for a light colored scalar with a mass below 1 TeV
Learning the latent structure of collider events
We describe a technique to learn the underlying structure of collider events directly from the data, without having a particular theoretical model in mind. It allows to infer aspects of the theoretical model that may have given rise to this structure, and can be used to cluster or classify the events for analysis purposes. The unsupervised machine-learning technique is based on the probabilistic (Bayesian) generative model of Latent Dirichlet Allocation. We pair the model with an approximate inference algorithm called Variational Inference, which we then use to extract the latent probability distributions describing the learned underlying structure of collider events. We provide a detailed systematic study of the technique using two example scenarios to learn the latent structure of di-jet event samples made up of QCD background events and either tt¯.Fil: Dillon, B. M.. Institute Jo?ef Stefan; EsloveniaFil: Faroughy, D. A.. Universitat Zurich; SuizaFil: Kamenik, J. F.. Institute Jo?ef Stefan; Eslovenia. University of Ljubljana; EsloveniaFil: Szewc, Manuel. Universidad Nacional de San Martín; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin
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