10,963 research outputs found
Non-Abelian Discrete Groups from the Breaking of Continuous Flavor Symmetries
We discuss the possibility of obtaining a non-abelian discrete flavor
symmetry from an underlying continuous, possibly gauged, flavor symmetry SU(2)
or SU(3) through spontaneous symmetry breaking. We consider all possible cases,
where the continuous symmetry is broken by small representations. "Small"
representations are these which couple at leading order to the Standard Model
fermions transforming as two- or three-dimensional representations of the
flavor group. We find that, given this limited representation content, the only
non-abelian discrete group which can arise as a residual symmetry is the
quaternion group D_2'.Comment: 15 page
Systematic approach to leptogenesis in nonequilibrium QFT: self-energy contribution to the CP-violating parameter
In the baryogenesis via leptogenesis scenario the self-energy contribution to
the CP-violating parameter plays a very important role. Here, we calculate it
in a simple toy model of leptogenesis using the Schwinger-Keldysh/Kadanoff-Baym
formalism as starting point. We show that the formalism is free of the
double-counting problem typical for the canonical Boltzmann approach. Within
the toy model, medium effects increase the CP-violating parameter. In contrast
to results obtained earlier in the framework of thermal field theory, the
medium corrections are linear in the particle number densities. In the resonant
regime quantum corrections lead to modified expressions for the CP-violating
parameter and for the decay width. Most notably, in the maximal resonant regime
the Boltzmann picture breaks down and an analysis in the full Kadanoff-Baym
formalism is required.Comment: 28 pages, 14 figure
Leptogenesis in models with keV sterile neutrino dark matter
We analyze leptogenesis in gauge extensions of the Standard Model with keV
sterile neutrino dark matter. We find that both the observed dark matter
abundance and the correct baryon asymmetry of the Universe can simultaneously
emerge in these models. Both the dark matter abundance and the leptogenesis are
controlled by the out of equilibrium decays of the same heavy right handed
neutrino.Comment: 6 pages, 1 figur
Estimating the COGARCH(1,1) model - a first go
We suggest moment estimators for the parameters of a continuous time GARCH(1,1) process based on equally spaced observations. Using the fact that the increments of the COGARCH(1,1) process are ergodic, the resulting estimators are consistent. We investigate the quality of our estimators in a simulation study based on the compound Poisson driven COGARCH model. The estimated volatility with corresponding residual analysis is also presented
Continuous time volatility modelling: COGARCH versus Ornstein-Uhlenbeck models
We compare the probabilistic properties of the non-Gaussian Ornstein-Uhlenbeck based stochastic volatility model of Barndorff-Nielsen and Shephard (2001) with those of the COGARCH process. The latter is a continuous time GARCH process introduced by the authors (2004). Many features are shown to be shared by both processes, but differences are pointed out as well. Furthermore, it is shown that the COGARCH process has Pareto like tails under weak regularity conditions
A Continuous Time GARCH Process Driven by a Lévy Process: Stationarity and Second Order Behaviour
We use a discrete time analysis, giving necessary and sufficient conditions for the almost sure convergence of ARCH(1) and GARCH(1,1) discrete time models, tosuggest an extension of the (G)ARCH concept to continuous time processes. Our "COGARCH" (continuous time GARCH) model, based on a single background driving Levy process, is different from, though related to, other continuous time stochastic volatility models that have been proposed. The model generalises the essential features of discrete time GARCH processes, and is amenable to further analysis, possessing useful Markovian and stationarity properties
Universal transport signatures of Majorana fermions in superconductor-Luttinger liquid junctions
One of the most promising proposals for engineering topological
superconductivity and Majorana fermions employs a spin-orbit coupled nanowire
subjected to a magnetic field and proximate to an s-wave superconductor. When
only part of the wire's length contacts to the superconductor, the remaining
conducting portion serves as a natural lead that can be used to probe these
Majorana modes via tunneling. The enhanced role of interactions in one
dimension dictates that this configuration should be viewed as a
superconductor-Luttinger liquid junction. We investigate such junctions between
both helical and spinful Luttinger liquids, and topological as well as
non-topological superconductors. We determine the phase diagram for each case
and show that universal low-energy transport in these systems is governed by
fixed points describing either perfect normal reflection or perfect Andreev
reflection. In addition to capturing (in some instances) the familiar
Majorana-mediated `zero-bias anomaly' in a new framework, we show that
interactions yield dramatic consequences in certain regimes. Indeed, we
establish that strong repulsion removes this conductance anomaly altogether
while strong attraction produces dynamically generated effective Majorana modes
even in a junction with a trivial superconductor. Interactions further lead to
striking signatures in the local density of states and the line-shape of the
conductance peak at finite voltage, and also are essential for establishing
smoking-gun transport signatures of Majorana fermions in spinful Luttinger
liquid junctions.Comment: 25 pages, 6 figures, v
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