# Direct searches of Type III seesaw triplet fermions at high energy $e^+e^-$ collider

## TL;DR

This paper investigates the potential of high-energy electron-positron colliders to directly detect Type III seesaw fermionic triplets, focusing on their production mechanisms, mixing parameters, and mass reach capabilities.

## Contribution

It demonstrates that a 1 TeV $e^+e^-$ collider can probe triplet fermions up to 980 GeV with specific mixing, and pair production at 2 TeV extends this reach, providing new insights into collider-based detection of these particles.

## Key findings

- Single production can probe triplet masses up to 980 GeV at 1 TeV collider.
- Pair production at 2 TeV can detect triplets up to about 1 TeV.
- The collider's sensitivity depends on the mixing parameter and luminosity.

## Abstract

The signatures of heavy fermionic triplets ($\Sigma$) arising in scenarios like Type III seesaw model are probed through their direct production and subsequent decay at high energy electron-positron collider. Unlike the case of LHC, the production process has strong dependence on the mixing parameter ($V_{e,\mu}$), making the leptonic collider unique to probe such mixing. We have established that with suitably chosen kinematic cuts, a 1 TeV $e^+e^-$ collider could probe the presence of $\Sigma$ of mass in the range of 500 GeV having $V_e=0.05$ with a few inverse femto barn luminosity through single production. The cross section is found to be not sufficient to probe the case of triplet-muon mixing through single triplet production. On the other hand, the pair production considered at 2 TeV centre of mass energy is capable of probing both the mixing scenarios efficiently. Studying the mass reach, presence of charged fermionic triplets upto a mass of about 980 GeV could be established at $3\sigma$ level through single production at a 1 TeV $e^+e^-$ collider with moderate luminosity of 100 fb$^{-1}$, assuming $V_e = 0.05$ . The pair production case requires larger luminosity, as the cross section is smaller in this case. With an integrated luminosity of 300 fb$^{-1}$, the mass reach in this case is close to 1 TeV with triplet-muon mixing, while it is slightly lower at about 950 GeV in the case of $V_\mu = 0.05$.