RENORMALIZATION OF THE NEUTRINO MASS OPERATOR

A small neutrino Majorana mass can arise in the Standard Model as an e ffective dimension 5 operator. We calculate the renormalization of thi s operator in the minimal Standard Model and in its two-Higgs-doublet and supersymmetric extensions. Renormalization from the scale of lepto n number violation (e.g., the Planck scale or a GUT scale) to the weak scale decreases the strength of this operator by an order of magnitud e or more if the top quark and Higgs boson masses are large. Neutrino mixing angles also run with momentum. We show instances where small mi xing at a high scale becomes large at the weak scale, and vice versa.