Effective theory of a doubly charged singlet scalar: complementarity of neutrino physics and the LHC

We consider a rather minimal extension of the Standard Model involving just one extra particle, namely a single SU(2)L singlet scalar Sil and its antiparticle S--. We propose a model independent effective operator, which yields an effective coupling of S++ to pairs of same sign weak gauge bosons, (WW +/-)-W-+/-. We also allow tree-level couplings of S++ to pairs of same sign right-handed charged leptons l(R)(+/-) l'(+/-)(R) of the same or different flavour. We calculate explicitly the resulting two-loop diagrams in the effective theory responsible for neutrino mass and mixing. We propose sets of benchmark points for various S++ masses and couplings which can yield successful neutrino masses and mixing, consistent with limits on charged lepton flavour violation (LFV) and neutrinoless double beta decay. We discuss the prospects for S++ discovery at the LHC, for these benchmark points, including single and pair production and decay into same sign leptons plus jets and missing energy. The model represents a minimal example of the complementarity between neutrino physics (including LFV) and the LHC, involving just one new particle, the S++.