Two Higgs Doublet Type III Seesaw with mu-tau symmetry at LHC

TL;DR

This paper introduces a two Higgs doublet Type III seesaw model with mu-tau symmetry, predicting observable heavy fermion decays and displaced vertices at the LHC, offering a natural explanation for small neutrino masses.

Contribution

It presents a novel two Higgs doublet Type III seesaw model with mu-tau symmetry, detailing its collider signatures and flavor structure implications at the LHC.

Findings

Heavy fermions can be produced at LHC with large cross-sections.

Heavy fermion decays exhibit distinctive flavor patterns due to mu-tau symmetry.

Displaced decay vertices of light Higgs bosons are predicted at LHC.

Abstract

We propose a two Higgs doublet Type III seesaw model with $\mu$-$\tau$ flavor symmetry. We add an additional SU(2) Higgs doublet and three SU(2) fermion triplets in our model. The presence of two Higgs doublets allows for natural explanation of small neutrino masses with triplet fermions in the 100 GeV mass range, without fine tuning of the Yukawa couplings to extremely small values. The triplet fermions couple to the gauge bosons and can be thus produced at the LHC. We study in detail the effective cross-sections for the production and subsequent decays of these heavy exotic fermions. We show for the first time that the $\mu$-$\tau$ flavor symmetry in the low energy neutrino mass matrix results in mixing matrices for the neutral and charged heavy fermions that are not unity and which carry the flavor symmetry pattern. This flavor structure can be observed in the decays of the heavy fermions at LHC. The large Yukawa couplings in our model result in the decay of the heavy fermions into lighter leptons and Higgs with a decay rate which is about $10^{11}$ times larger than what is expected for the one Higgs Type III seesaw model with 100 GeV triplet fermions. The smallness of neutrino masses constrains the neutral Higgs mixing angle $\sin\alpha$ in our model in such a way that the heavy fermions decay into the lighter neutral CP even Higgs $h^0$, CP odd Higgs $A^0$ and the charged Higgs $H^\pm$, but almost never to the heavier neutral CP even Higgs $H^0$. The small value for $\sin\alpha$ also results in a very long lifetime for $h^0$. This displaced decay vertex should be visible at LHC. We provide an exhaustive list of collider signature channels for our model and identify those that have very large effective cross-sections at LHC and almost no standard model background.