Cascade Seesaw for Tiny Neutrino Mass

Abstract

The accessibility to physics responsible for tiny neutrino mass suggests that the mass should better originate from certain higher dimensional operators. The conventional three types of seesaw operate at dimension five with the help of either a new fermion or scalar multiplet. Here we propose a seesaw that generates neutrino mass through a dimension-(5+4n) operator. The seesaw is functioned by a fermion of isospin n+1 and zero hypercharge and a sequence of scalar multiplets that share unity hypercharge but have isospin from 3/2 to n+1/2 at a step of unity. Only the scalar of the highest isospin can couple to the relevant fermions while only the scalar of the lowest isospin can directly develop a naturally small vacuum expectation value (VEV). The VEV is then transmitted to scalars of higher isospin through a cascading process. No global symmetry is required to forbid lower dimensional operators. A neutrino mass of desired order can thus be induced with a relatively low seesaw scale without demanding too small couplings.

10 pages, 2 figures. v2: corrected a trivial mistake in CG coefficients in eq(11), but none of other formulae or numbers affected. Thanks to G.Z. Ning and Y.B. Wu for drawing this to my attention. v3: proofread version for JHEP; modified last two paragraphs to clarify a few phenomenological points, added a few refs