6,625 research outputs found
Systematic classification of three-loop realizations of the Weinberg operator
We study systematically the decomposition of the Weinberg operator at
three-loop order. There are more than four thousand connected topologies.
However, the vast majority of these are infinite corrections to lower order
neutrino mass diagrams and only a very small percentage yields models for which
the three-loop diagrams are the leading order contribution to the neutrino mass
matrix. We identify 73 topologies that can lead to genuine three-loop models
with fermions and scalars, i.e. models for which lower order diagrams are
automatically absent without the need to invoke additional symmetries. The 73
genuine topologies can be divided into two sub-classes: Normal genuine ones (44
cases) and special genuine topologies (29 cases). The latter are a special
class of topologies, which can lead to genuine diagrams only for very specific
choices of fields. The genuine topologies generate 374 diagrams in the weak
basis, which can be reduced to only 30 distinct diagrams in the mass eigenstate
basis. We also discuss how all the mass eigenstate diagrams can be described in
terms of only five master integrals. We present some concrete models and for
two of them we give numerical estimates for the typical size of neutrino masses
they generate. Our results can be readily applied to construct other
neutrino mass models with three loops.Comment: Erratum added, published version in JHE
Proton decay and light sterile neutrinos
Within the standard model, non-renormalizable operators at dimension six
() violate baryon and lepton number by one unit and thus lead to proton
decay. Here, we point out that the proton decay mode with a charged pion and
missing energy can be a characteristic signature of operators containing
a light sterile neutrino, if it is not accompanied by the standard
final state. We discuss this effect first at the level of effective operators
and then provide a concrete model with new physics at the TeV scale, in which
the lightness of the active neutrinos and the stability of the proton are
related.Comment: 7 pages, 2 figures, published versio
Proton decay at 1-loop
Proton decay is usually discussed in the context of grand unified theories.
However, as is well-known, in the standard model effective theory proton decay
appears in the form of higher dimensional non-renormalizable operators. Here,
we study systematically the 1-loop decomposition of the violating
operators. We exhaustively list the possible 1-loop ultra-violet completions of
these operators and discuss that, in general, two distinct classes of models
appear. Models in the first class need an additional symmetry in order to avoid
tree-level proton decay. These models necessarily contain a neutral particle,
which could act as a dark matter candidate. For models in the second class the
loop contribution dominates automatically over the tree-level proton decay,
without the need for additional symmetries. We also discuss possible
phenomenology of two example models, one from each class, and their possible
connections to neutrino masses, LHC searches and dark matter.Comment: 13 pages, 7 figure
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