Modular-symmetry-protected seesaw

Abstract

In the presence of a finite modular flavour symmetry, fermion mass hierarchies may be generated by a slight deviation of the modulus from a symmetric point. We point out that this small parameter governing charged-lepton mass hierarchies may also be responsible for the breaking of lepton number in a symmetry-protected low-scale seesaw, sourcing active neutrino masses and the mass splitting of a pseudo-Dirac pair of heavy neutrinos. We discuss the phenomenological implications of this mechanism, including the possibility to test the considered models at future planned and proposed heavy neutral lepton searches.

30 pages, 4 figures, 5 tables; typos corrected; reference updated; matches version published in JHEP