213 research outputs found
New Leptoquark Mechanism of Neutrinoless Double Beta Decay
A new mechanism for neutrinoless double beta (\znbb) decay based on
leptoquark exchange is discussed. Due to the specific helicity structure of the
effective four-fermion interaction this contribution is strongly enhanced
compared to the well-known mass mechanism of \znbb decay. As a result the
corresponding leptoquark parameters are severely constrained from
non-observation of \znbb-decay. These constraints are more stringent than
those derived from other experiments.Comment: LaTeX, 6 pages, 1 figur
Neutrino oscillation constraints on neutrinoless double beta decay
We have studied the constraints imposed by the results of neutrino
oscillation experiments on the effective Majorana mass || that characterizes
the contribution of Majorana neutrino masses to the matrix element of
neutrinoless double-beta decay. We have shown that in a general scheme with
three Majorana neutrinos and a hierarchy of neutrino masses (which can be
explained by the see-saw mechanism), the results of neutrino oscillation
experiments imply rather strong constraints on the parameter ||. From the
results of the first reactor long-baseline experiment CHOOZ and the Bugey
experiment it follows that || < 3x10^{-2} eV if the largest mass-squared
difference is smaller than 2 eV^2. Hence, we conclude that the observation of
neutrinoless double-beta decay with a probability that corresponds to || >
10^{-1} eV would be a signal for a non-hierarchical neutrino mass spectrum
and/or non-standard mechanisms of lepton number violation.Comment: 20 pages, including 4 figure
Neutrino masses beyond the tree level
Models for Majorana neutrino masses can be classified according to the level
in perturbation theory at which the effective dimension five operator is
realized. The possibilities range from the tree-level up to the three-loop
level realizations. We discuss some general aspects of this approach and
speculate about a model independent classification of the possible cases. Among
all the realizations, those in which the effective operator is induced by
radiative corrections open the possibility for lepton number violation near -or
at- the electroweak scale. We discuss some phenomenological aspects of two
generic radiative realizations: the Babu-Zee model and supersymmetric models
with bilinear R-parity violation.Comment: 7 pages, 3 figures. Contribution to the workshop "30 years of strong
interactions", Spa, Belgium, 6-8 April 201
Neutrino masses from higher than d=5 effective operators
We discuss the generation of small neutrino masses from effective operators
higher than dimension five, which open new possibilities for low scale see-saw
mechanisms. In order to forbid the radiative generation of neutrino mass by
lower dimensional operators, extra fields are required, which are charged under
a new symmetry. We discuss this mechanism in the framework of a two Higgs
doublet model. We demonstrate that the tree level generation of neutrino mass
from higher dimensional operators often leads to inverse see-saw scenarios in
which small lepton number violating terms are naturally suppressed by the new
physics scale. Furthermore, we systematically discuss tree level
generalizations of the standard see-saw scenarios from higher dimensional
operators. Finally, we point out that higher dimensional operators can also be
generated at the loop level. In this case, we obtain the TeV scale as new
physics scale even with order one couplings.Comment: 22 pages, 3 figures, 2 tables. Some references adde
Implications of a charged-current anomaly at HERA
We demonstrate that in the presence of mixing between different scalar
leptoquark multiplets it is possible to simultaneously account for the HERA
high- neutral current anomaly, and produce a charged current anomaly of
comparable magnitude. The reduced branching ratio to electrons and jets of the
lightest leptoquark state results in a significant weakening of the CDF/D0
limits on scalar leptoquarks; masses consistent with the HERA neutral current
excess are comfortably within the allowed range. We show that the possibilities
for such a successful mixed leptoquark scenario are quite limited, and we
investigate some aspects of their phenomenology.Comment: 10 pages Latex. Corrected 3-body decay width
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