470 research outputs found
Leptogenesis in the presence of exact flavor symmetries
In models with flavor symmetries in the leptonic sector leptogenesis can take
place in a very different way compared to the standard leptogenesis scenario.
We study the generation of a asymmetry in these kind of models in the
flavor symmetric phase pointing out that successful leptogenesis requires (i)
the right-handed neutrinos to lie in different representations of the flavor
group; (ii) the flavons to be lighter at least that one of the right-handed
neutrino representations. When these conditions are satisfied leptogenesis
proceeds due to new contributions to the CP violating asymmetry and -depending
on the specific model- in several stages. We demonstrate the validity of these
arguments by studying in detail the generation of the asymmetry in a
scenario of a concrete flavor model realization.Comment: 25 pages, 7 figures; version 2: A few clarifications added. Version
matches publication in JHE
Novel Higgs decay signals in R-parity violating models
In supersymmetric models the lightest Higgs boson may decay with a sizable
branching ratio into a pair of light neutralinos. We analyze such decays within
the context of the minimal supersymmetric standard model with R-parity
violation, where the neutralino itself is unstable and decays into Standard
Model fermions. We show that the R-parity violating couplings induce novel
Higgs decay signals that might facilitate the discovery of the Higgs boson at
colliders. At the LHC, the Higgs may be observed, for instance, through its
decay -via two neutralinos- into final states containing missing energy and
isolated charged leptons such as , and .
Another promising possibility is the search for the displaced vertices
associated with the neutralino decay. We also point out that Higgs searches at
the LHC might additionally provide the first evidence of R-parity violation.Comment: 15 pages, 7 figures. Corrected affiliation
Integration of stochastic models for long-term eruption forecasting into a Bayesian event tree scheme: a basis method to estimate the probability of volcanic unrest
Eruption forecasting refers, in general, to the
assessment of the occurrence probability of a given erup-
tive event, whereas volcanic hazards are normally associated
with the analysis of superficial and evident phenomena
that usually accompany eruptions (e.g., lava, pyroclastic
flows, tephra fall, lahars, etc.). Nevertheless, several hazards
of volcanic origin may occur in noneruptive phases dur-
ing unrest episodes. Among others, remarkable examples
are gas emissions, phreatic explosions, ground deforma-
tion, and seismic swarms. Many of such events may lead to
significant damages, and for this reason, the “risk” associ-
ated to unrest episodes could not be negligible with respect
to eruption-related phenomena. Our main objective in this
paper is to provide a quantitative framework to calculate
probabilities of volcanic unrest. The mathematical frame-
work proposed is based on the integration of stochastic mod-
els based on the analysis of eruption occurrence catalogs
into a Bayesian event tree scheme for eruption forecast-
ing and volcanic hazard assessment. Indeed, such models
are based on long-term eruption catalogs and in many
cases allow a more consistent analysis of long-term tem-
poral modulations of volcanic activity. The main result of
this approach is twofold: first, it allows to make inferences
about the probability of volcanic unrest; second, it allows
to project the results of stochastic modeling of the eruptive
history of a volcano toward the probabilistic assessment of
volcanic hazards. To illustrate the performance of the pro-
posed approach, we apply it to determine probabilities of
unrest at Miyakejima volcano, Japan
A Brownian Model for Recurrent Volcanic Eruptions: an Application to Miyakejima Volcano (Japan)
The definition of probabilistic models as mathematical structures to describe the
response of a volcanic system is a plausible approach to characterize the temporal behavior
of volcanic eruptions, and constitutes a tool for long-term eruption forecasting. This kind
of approach is motivated by the fact that volcanoes are complex systems in which a com-
pletely deterministic description of the processes preceding eruptions is practically impos-
sible. To describe recurrent eruptive activity we apply a physically-motivated probabilistic
model based on the characteristics of the Brownian passage-time (BPT) distribution; the
physical process defining this model can be described by the steady rise of a state variable
from a ground state to a failure threshold; adding Brownian perturbations to the steady load-
ing produces a stochastic load-state process (a Brownian relaxation oscillator) in which an
eruption relaxes the load state to begin a new eruptive cycle. The Brownian relaxation os-
cillator and Brownian passage-time distribution connect together physical notions of unob-
servable loading and failure processes of a point process with observable response statistics.
The Brownian passage-time model is parameterized by the mean rate of event occurrence,
μ , and the aperiodicity about the mean, α . We apply this model to analyze the eruptive his-
tory of Miyakejima volcano, Japan, finding a value of 44.2(±6.5 years) for the μ parameter
and 0.51(±0.01) for the (dimensionless) α parameter. The comparison with other models
often used in volcanological literature shows that this pysically-motivated model may be a
good descriptor of volcanic systems that produce eruptions with a characteristic size. BPT
is clearly superior to the exponential distribution and the fit to the data is comparable to
other two-parameters models. Nonetheless, being a physically-motivated model, it provides
an insight into the macro-mechanical processes driving the system
Reactor mixing angle from hybrid neutrino masses
In terms of its eigenvector decomposition, the neutrino mass matrix (in the
basis where the charged lepton mass matrix is diagonal) can be understood as
originating from a tribimaximal dominant structure with small deviations, as
demanded by data. If neutrino masses originate from at least two different
mechanisms, referred to as "hybrid neutrino masses", the experimentally
observed structure naturally emerges provided one mechanism accounts for the
dominant tribimaximal structure while the other is responsible for the
deviations. We demonstrate the feasibility of this picture in a fairly
model-independent way by using lepton-number-violating effective operators,
whose structure we assume becomes dictated by an underlying flavor
symmetry. We show that if a second mechanism is at work, the requirement of
generating a reactor angle within its experimental range always fixes the solar
and atmospheric angles in agreement with data, in contrast to the case where
the deviations are induced by next-to-leading order effective operators. We
prove this idea is viable by constructing an -based ultraviolet
completion, where the dominant tribimaximal structure arises from the type-I
seesaw while the subleading contribution is determined by either type-II or
type-III seesaw driven by a non-trivial singlet (minimal hybrid model).
After finding general criteria, we identify all the symmetries
capable of producing such -based minimal hybrid models.Comment: 18 pages, 5 figures. v3: section including sum rules added, accepted
by JHE
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
On fast CP violating interactions in leptogenesis
We show that when the relevant CP violating interactions in leptogenesis are
fast, the different matter density asymmetries are determined at each instant
by a balance condition between the amount of asymmetry being created and
destroyed. This fact allows to understand in a simple way many features of
leptogenesis in the strong washout regime. In particular, we find some
non-trivial effects of flavour changing interactions that conserve lepton
number, which are specially relevant in models for leptogenesis that rely
heavily on flavour effects.Comment: V2: To match published version in JCAP. Minor changes, including one
figure, with respect to V1. 17 pages, 4 figure
Leptogenesis with a dynamical seesaw scale
In the simplest type-I seesaw leptogenesis scenario right-handed neutrino annihilation processes are absent. However, in the presence of new interactions these processes are possible and can affect the resulting B ¿ L asymmetry in an important way. A prominent example is provided by models with spontaneous lepton number violation, where the existence of new dynamical degrees of freedom can play a crucial role. In this context, we provide a model-independent discussion of the effects of right-handed neutrino annihilations. We show that in the weak washout regime, as long as the scattering processes remain slow compared with the Hubble expansion rate throughout the relevant temperature range, the efficiency can be largely enhanced, reaching in some cases maximal values. Moreover, the B ¿ L asymmetry yield turns out to be independent upon initial conditions, in contrast to the ¿standard¿ case. On the other hand, when the annihilation processes are fast, the right-handed neutrino distribution tends to a thermal one down to low temperatures, implying a drastic suppression of the efficiency which in some cases can render the B ¿ L generation mechanism inoperative
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