Single and double seesaw for quark-lepton and neutrino masses

Abstract

A left-right model of quarks and leptons based on the gauge group $SU(3{)}_c\times SU(2{)}_L\times SU(2{)}_R\times U(1{)}_{B-L}$ is studied. Here the scalar sector consists of only two doublets namely (1,2,1,1) and (1,1,2,1) but familiar bidoublet (1,2,2,0) is removed. Quarks and charged leptons get mass from a single see-saw mechanism but neutrinos get mass from a double see-saw mechanism. In this type of models the heaviest right handed neutrino can be of order $10^{13}$ GeVs or less in a natural way, depending on the size of related Yukawa couplings.

Comments: Latex. Invited talk in ICFP-2003, Seoul, Korea, Oct 6-11, 2003. To appear in proceedings