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.
Highlights
Exploring the scale of new physics responsible for neutrino masses is one of the major topics under intense focus in particle physics today
If the scale is near the grand unification theory (GUT) scale, the only ways to explore this would be via rare decays of leptons and via proton decay in GUT models, providing only a limited window for their search
The situation changes drastically if the new physics scale is around a few TeV, which is theoretically quite plausible, opening up many more possible experimental probes
Summary
Exploring the scale of new physics responsible for neutrino masses is one of the major topics under intense focus in particle physics today. Positron Collider (CEPC) [64], International Linear Collider (ILC) [65], Future Circular Collider (FCC-ee) [66] and Compact Linear Collider (CLIC) [67] The reason these two scalars are interesting for studying the origin of neutrino masses is that in the LRSM, the couplings (denoted by fαβ) of the hadrophobic scalar and the doubly-charged scalar to leptons are the ones that are responsible for the seesaw masses of the RH neutrinos (RHNs) i.e., MN;αβ 1⁄4 2fαβvR. We show in this paper the interesting range of f values that can be measured in the planned lepton colliders such as CEPC and ILC and can provide new ways to test these models They will in any case provide complementary information to rare lepton decay constraints on the f couplings and make such studies interesting from the synergistic viewpoint of energy and intensity frontiers.
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