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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 fine-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 fine-tuning problem. As a result, we find only two such mechanisms that are not yet excluded and in which large transition magnetic moments can be realized for Majorana neutrinos only.