Phenomenology of neutrino magnetic moments

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.