Phenomenology in the minimal cascade seesaw mechanism for neutrino masses

Abstract

We make a comprehensive analysis of the phenomenology in the minimal version of the cascade seesaw mechanism for tiny neutrino masses. The seesaw mechanism induces, at tree level, a neutrino mass operator at dimension nine by introducing a quadruple scalar $\mathrm{\ensuremath{\Phi}}$ of hypercharge unity and a quintuple fermion $\mathrm{\ensuremath{\Sigma}}$ of hypercharge zero. We work in a framework that handles the complicated Yukawa couplings without losing generality. All mixing matrices are essentially expressed in terms of the vacuum expectation value of the quadruple scalar ${v}_{\mathrm{\ensuremath{\Phi}}}$, a free complex parameter $t$, and known neutrino parameters. We show that the low-energy lepton-flavor-violating transitions of the charged leptons set strong constraints on the free parameters. The constraints have a significant impact on collider physics and are incorporated in our signal analysis at the LHC. We investigate the signatures of new particles by surveying potentially important channels. We find that the $4j2{\ensuremath{\ell}}^{\ifmmode\pm\else\textpm\fi{}}$ signal is the most important for the detection of scalars, and the $2{\ensuremath{\ell}}^{\ifmmode\pm\else\textpm\fi{}}2{\ensuremath{\ell}}^{\ensuremath{\mp}}2j$, $3{\ensuremath{\ell}}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\ell}}^{\ensuremath{\mp}}2j$, and $3{\ensuremath{\ell}}^{\ifmmode\pm\else\textpm\fi{}}2{\ensuremath{\ell}}^{\ensuremath{\mp}}+\overline{){E}_{T}}$ signals are quite promising for the fermions.