Probing TeV scale origin of neutrino mass at future lepton colliders via neutral and doubly-charged scalars

Abstract

We point out how future lepton colliders can provide unique insight into the scalar sector of TeV scale models for neutrino masses with local $B-L$ symmetry. Our specific focus is on the TeV scale left-right model, which naturally embeds this $B-L$ symmetry. In particular, we make a detailed study of the lepton collider implications of the neutral ($H_3$) and doubly-charged ($H^{\pm\pm}$) scalars from the right-handed triplet Higgs that is responsible for the spontaneous breaking of the $B-L$ symmetry and implementing the seesaw mechanism. Due to mixing with other scalars, the neutral scalar $H_3$ could acquire sizable flavor violating couplings to the charged leptons. Produced on-shell or off-shell at the planned $e^+e^-$ colliders, it would induce distinct lepton flavor violating signals like $e^+e^- \to \mu^\pm \tau^\mp ~ (+H_3)$, with the couplings probed up to $\sim 10^{-4}$ for a wide range of neutral scalar mass, which is well beyond the reach of current searches for charged lepton flavor violation. The Yukawa couplings of the doubly-charged scalar $H^{\pm\pm}$ to the charged leptons might also be flavor-violating, which is correlated to the heavy right-handed neutrino masses and mixings. With a combination of the pair, single and off-shell production of $H^{\pm\pm}$ like $e^+e^- \to H^{++} H^{--},\, H^{\pm\pm} e^\mp \mu^\mp,\, \mu^\pm \tau^\mp$, the Yukawa couplings can be probed up to $10^{-3}$ at future lepton colliders, which is allowed by current lepton flavor data in a large region of parameter space. For both the neutral and doubly-charged cases, the scalar masses could be probed up to the few-TeV range in the off-shell channel.