Neutrino Masses and Heavy Triplet Leptons at the LHC: Testability of Type III Seesaw

TL;DR

This paper investigates the potential to detect heavy triplet leptons predicted by the Type III seesaw mechanism at the LHC, focusing on their decay patterns, production channels, and the possibility of observing displaced vertices for small mixing scenarios.

Contribution

It provides a detailed analysis of LHC signatures of Type III seesaw triplet leptons, linking their decay properties to neutrino masses and exploring detection prospects for masses up to one TeV.

Findings

Heavy triplet leptons below 1 TeV can be produced at the LHC.

Displaced vertices can be observed for small lepton mixing.

Discovery potential is high with 100 fb^{-1} luminosity.

Abstract

We study LHC signatures of Type III seesaw in which SU(2)_L triplet leptons are introduced to supply the heavy seesaw masses. To detect the signals of these heavy triplet leptons, one needs to understand their decays to standard model particles which depend on how light and heavy leptons mix with each other. We concentrate on the usual solutions with small light and heavy lepton mixing of order the square root of the ratio of light and heavy masses, (m_\nu/M_{\nu_R})^{1/2}. This class of solutions can lead to a visible displaced vertex detectable at the LHC which can be used to distinguish small mixing and large mixing between light and heavy leptons. We show that, in this case, the couplings of light and heavy triplet leptons to gauge and Higgs bosons, which determine the decay widths and branching ratios, can be expressed in terms of light neutrino masses and their mixing. Using these relations, we study heavy triplet lepton decay patterns and production cross section at the LHC. If these heavy triplet leptons are below a TeV or so, they can be easily produced at the LHC due to their gauge interactions from being non-trivial representations of SU(2)_L. We consider two ideal production channels, 1) E^+E^- \to \ell^+\ell^+ \ell^-\ell^- jj (\ell=e,\mu,\tau) and 2) E^\pm N \to \ell^\pm \ell^\pm jjjj in detail. For case 1), we find that with one or two of the light leptons being \tau it can also be effectively studied. With judicious cuts at the LHC, the discovery of the heavy triplet leptons as high as a TeV can be achieved with 100 fb^{-1} integrated luminosity.