369 research outputs found

### Extracting Majorana Properties in the Throat of Neutrinoless Double Beta Decay

Assuming that neutrinos are Majorana particles, we explore what information
can be inferred from future strong limits (i.e. non-observation) for
neutrinoless double beta decay. Specifically we consider the case where the
mass hierarchy is normal and the different contributions to the effective mass
$\langle m \rangle_{ee}$ partly cancel. We discuss how this fixes the two
Majorana CP phases simultaneously from the Majorana Triangle and how it limits
the lightest neutrino mass $m_1$ within a narrow window. The two Majorana CP
phases are in this case even better determined than in the usual case for
larger $\langle m \rangle_{ee}$. We show that the uncertainty in these
predictions can be significantly reduced by the complementary measurement of
reactor neutrino experiments, especially the medium baseline version
JUNO/RENO-50. We also estimate the necessary precision on $\langle m
\rangle_{ee}$ to infer non-trivial Majorana CP phases and the upper limit
\langle m \rangle_{ee} \lesssim 1\,\mbox{meV} sets a target for the design of
future neutrinoless double beta decay experiments.Comment: 21 pages, 10 figure

### Dark matter and U(1)' symmetry for the right-handed neutrinos

We consider a U(1)' gauge symmetry acting on three generations of
right-handed neutrinos. The U(1)' symmetry is broken at the TeV scale and its
remnant discrete symmetry makes one of the right-handed neutrinos stable. As a
natural consequence of the anomaly cancellation, the neutrino mass matrix
consists of a combination of Type I (TeV scale) seesaw and radiative
correction. The stable right-handed neutrino communicates with the Standard
Model via s-channel exchange of the Higgs field and the U(1)' gauge boson, so
that the observed relic density for dark matter is obtained in a wide range of
the parameter space. The experimental signatures in collider and other
experiments are briefly discussed.Comment: 16 pages, 4 figure

### Revisiting Large Neutrino Magnetic Moments

Current experimental sensitivity on neutrino magnetic moments is many orders
of magnitude above the Standard Model prediction. A potential measurement of
next-generation experiments would therefore strongly request new physics beyond
the Standard Model. However, large neutrino magnetic moments generically tend
to induce large corrections to the neutrino masses and lead to fine-tuning. We
show that in a model where neutrino masses are proportional to neutrino
magnetic moments. We revisit, discuss and propose mechanisms that still provide
theoretical consistent explanations for a potential measurement of large
neutrino magnetic moments. We find only two viable mechanisms to realize large
transition magnetic moments for Majorana neutrinos only.Comment: 12 pages, 7 figure

### Discovery reach for non-standard interactions in a neutrino factory

We study the discovery reach for Non-Standard Interactions (NSIs) in a
neutrino factory experiment. After giving a theoretical, but model-independent,
overview of the most relevant classes of NSIs, we present detailed numerical
results for some of them. Our simulations take into account matter effects,
uncertainties in the neutrino oscillation parameters, systematical errors,
parameter correlations, and degeneracies. We perform scans of the parameter
space, and show that a neutrino factory has excellent prospects of detecting
NSIs originating from new physics at around 1 TeV, which is a scale favored by
many extensions of the standard model. It will also turn out that the discovery
reach depends strongly on the standard and non-standard CP violating phases in
the Lagrangian.Comment: RevTeX 4, 10 pages, 5 figures, extended discussion of systematical
errors and of existing bounds, matches published versio

### Consistency test of neutrinoless double beta decay with one isotope

We discuss a consistency test which makes it possible to discriminate unknown
nuclear background lines from neutrinoless double beta decay with only one
isotope. By considering both the transition to the ground state and to the
first excited $0^+$ state, a sufficiently large detector can reveal if
neutrinoless double beta decay or some other nuclear physics process is at
work. Such a detector could therefore simultaneously provide a consistency test
for a certain range of Majorana masses and be sensitive to lower values of the
effective Majorana mass.Comment: 1+12 pages, 4 figures; v2: discussion enhanced, figures improved,
matches journal versio

### See-saw Mechanisms for Dirac and Majorana Neutrino Masses

We investigate the see-saw mechanism for generally non-fine-tuned $n \times
n$ mass matrices involving both Dirac and Majorana neutrinos. We specifically
show that the number of naturally light neutrinos cannot exceed half of the
dimension of the considered mass matrix. Furthermore, we determine a criterion
for mass matrix textures leading to light Dirac neutrinos with the see-saw
mechanism. Especially, we study $4 \times 4$ and $6 \times 6$ mass matrix
textures and give some examples in order to highlight these types of textures.
Next, we present a model scheme based on non-Abelian and discrete symmetries
fulfilling the above mentioned criterion for light Dirac neutrinos. Finally, we
investigate the connection between symmetries and the invariants of a mass
matrix on a formal level.Comment: 12 pages, RevTeX. Final version to be published in Phys. Rev.

### The Inverse Seesaw in Conformal Electro-Weak Symmetry Breaking and Phenomenological Consequences

We study the inverse seesaw mechanism for neutrino masses and
phenomenological consequences in the context of conformal electro-weak symmetry
breaking. The main difference to the usual case is that all explicit fermion
mass terms including Majorana masses for neutrinos are forbidden. All fermion
mass terms arise therefore from vacuum expectation values of suitable scalars
times some Yukawa couplings. This leads to interesting consequences for model
building, neutrino mass phenomenology and the Dark Matter abundance. In the
context of the inverse seesaw we find a favoured scenario with heavy
pseudo-Dirac sterile neutrinos at the TeV scale, which in the conformal
framework conspire with the electro-weak scale to generate keV scale warm Dark
Matter. The mass scale relations provide naturally the correct relic abundance
due to a freeze-in mechanism. We demonstrate also how conformal symmetry
decouples the right-handed neutrino mass scale and effective lepton number
violation. We find that lepton flavour violating processes can be well within
the reach of modern experiments. Furthermore, interesting decay signatures are
expected at the LHC.Comment: 12 pages, 6 figures, new particles, Journal Version with minor
changes and new citation

### Minimal Radiative Neutrino Masses

We conduct a systematic search for neutrino mass models which only
radiatively produce the dimension-5 Weinberg operator. We thereby do not allow
for additional symmetries beyond the Standard Model gauge symmetry and we
restrict ourselves to minimal models. We also include stable fractionally
charged and coloured particles in our search. Additionally, we proof that there
is a unique model with three new fermionic representations where no new scalars
are required to generate neutrino masses at loop level. This model further has
a potential dark matter candidate and introduces a general mechanism for
loop-suppression of the neutrino mass via a fermionic ladderComment: final version as published in JHE

### Confronting Flavour Symmetries and extended Scalar Sectors with Lepton Flavour Violation Bounds

We discuss the tension between discrete flavour symmetries and extended
scalar sectors arising from lepton flavour violation experiments. The key point
is that extended scalar sectors will generically lead to flavour changing
neutral currents, which are strongly constrained by experiments. Due to the
large parameter space in the scalar sector such models will, however, usually
have no big problems with existing and future bounds (even though the models
might be constrained). This changes considerably once a flavour symmetry is
imposed in addition: Due to the symmetry, additional relations between the
different couplings arise and cancellations become impossible in certain cases.
The experimental bounds will then constrain the model severely and can easily
exclude it. We consider two examples which show how these considerations are
realized. The same logic should apply to a much wider class of models.Comment: 19 pages, 2 figures; Introduction extended, typos corrected, charged
lepton sector of model 2 corrected; matches journal versio

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