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

The signatures of heavy fermionic isotriplets ($$\Sigma $$Σ) are probed through their direct production and subsequent decay at high energy electron–positron colliders. Unlike the case of LHC, the production process has strong dependence on the mixing of $$\Sigma $$Σ with electron ($$V_{e}$$Ve), making the leptonic collider unique to fingerprint the presence of such mixing. We establish that pair production considered at $$\sqrt{s}=2$$s=2 TeV can be employed to study both the cases of $$V_e=0$$Ve=0 and of $$V_e\ne 0$$Ve≠0 with $$\sim 100$$∼100 fb$$^{-1}$$-1 luminosity, while the single production can probe the latter case with a few inverse femto barn luminosity at $$\sqrt{s}=1$$s=1 TeV. Exploring the mass reach, both the single and pair productions are capable of probing $$\Sigma $$Σ of mass close to the kinematic limits through selected channels. Investigating simultaneous limits on the $$M_\Sigma -V_e$$MΣ-Ve parameter space, we identify suitable final states and their $$3\sigma $$3σ reach on $$V_e$$Ve and $$M_\Sigma $$MΣ at 300 (100) fb$$^{-1}$$-1 luminosity in the case of pair (single) $$\Sigma $$Σ production.