190 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
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
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
Minimal lepton flavor violating realizations of minimal seesaw models
We study the implications of the global U(1)R symmetry present in minimal
lepton flavor violating implementations of the seesaw mechanism for neutrino
masses. In the context of minimal type I seesaw scenarios with a slightly
broken U(1)R, we show that, depending on the R-charge assignments, two classes
of generic models can be identified. Models where the right-handed neutrino
masses and the lepton number breaking scale are decoupled, and models where the
parameters that slightly break the U(1)R induce a suppression in the light
neutrino mass matrix. We show that within the first class of models,
contributions of right-handed neutrinos to charged lepton flavor violating
processes are severely suppressed. Within the second class of models we study
the charged lepton flavor violating phenomenology in detail, focusing on mu to
e gamma, mu to 3e and mu to e conversion in nuclei. We show that sizable
contributions to these processes are naturally obtained for right-handed
neutrino masses at the TeV scale. We then discuss the interplay with the
effects of the right-handed neutrino interactions on primordial B - L
asymmetries, finding that sizable right-handed neutrino contributions to
charged lepton flavor violating processes are incompatible with the requirement
of generating (or even preserving preexisting) B - L asymmetries consistent
with the observed baryon asymmetry of the Universe.Comment: 21 pages, 4 figures; version 2: Discussion on possible generic models
extended, typos corrected, references added. Version matches publication in
JHE
The Machine Learning Landscape of Top Taggers
Based on the established task of identifying boosted, hadronically decaying
top quarks, we compare a wide range of modern machine learning approaches.
Unlike most established methods they rely on low-level input, for instance
calorimeter output. While their network architectures are vastly different,
their performance is comparatively similar. In general, we find that these new
approaches are extremely powerful and great fun.Comment: Yet another tagger included
Implications of Flavor Dynamics for Fermion Triplet Leptogenesis
We analyze the importance of flavor effects in models in which leptogenesis
proceeds via the decay of Majorana electroweak triplets. We find that depending
on the relative strengths of gauge and Yukawa reactions the asymmetry can
be sizably enhanced, exceeding in some cases an order of magnitude level. We
also discuss the impact that such effects can have for TeV-scale triplets
showing that as long as the asymmetry is produced by the dynamics of the
lightest such triplet they are negligible, but open the possibility for
scenarios in which the asymmetry is generated above the TeV scale by heavier
states, possibly surviving the TeV triplet related washouts. We investigate
these cases and show how they can be disentangled at the LHC by using Majorana
triplet collider observables and, in the case of minimal type III see-saw
models even through lepton flavor violation observables.Comment: 22 pages, 9 figures, extended discussion on collider phenomenology,
references added. Version matches publication in JHE
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