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Abstract:
The neutrino magnetic moment (NMM) in the Standard Model, minimally extended
allowing for massive neutrinos, is many orders of magnitude below current and expected
experimental sensitivities. A potential measurement would therefore strongly
hint to new physics beyond the Standard Model. It raises the question how a positive
NMM signal in future experiments could be explained in a theoretically consistent
way. After a brief theoretical introduction, we summarize existing experimental
bounds and systematically analyze the possibilities of model building for accommodating
large NMMs in beyond the Standard Model frameworks. As a by-product, we
derive new limits on millicharged particles from the non-observation of NMMs. The
tight connection of NMMs and neutrino masses generically leads to a ne-tuning
problem in typical models that predict sizable NMMs. We explicitly demonstrate
this problem using a model in which NMMs are proportional to neutrino masses.
Finally, we investigate mechanisms that provide large NMMs and at the same time
avoid the ne-tuning problem. As a result, we nd only two such mechanisms that
are not yet excluded and in which large transition magnetic moments can be realized
for Majorana neutrinos only.
