In this work we study the production of color-neutral and singly-charged heavy leptons at the proposed International Linear Collider. We use the optimal observable technique to determine the statistical accuracy to which the coupling of such fermions to the Z gauge boson (vector, axial or chiral) can be measured. We also consider a UV-complete model that contains these particles as well as a dark matter candidate, and consider some observable effects involving both; the correspondence to chargino production in supersymmetric models with heavy sleptons is briefly discussed.

We consider higher-dimensional effective (EFT) operators consisting of fermion dark matter (DM) connecting to Standard Model (SM) leptons upto dimension six. Considering all operators together and assuming the DM to undergo thermal freeze-out, we find out relic density allowed parameter space in terms of DM mass (mχ) and New Physics (NP) scale (Λ) with one loop direct search constraints from XENON1T experiment. Allowed parameter space of the model is probed at the proposed International Linear Collider (ILC) via monophoton signal for both Dirac and Majorana cases, limited by the centre-of-mass energy $$ \sqrt{s} $$ s =1 TeV, where DM mass can be probed within $$ {m}_{\chi }<\frac{\sqrt{s}}{2} $$ m χ < s 2 for the pair production to occur and Λ >$$ \sqrt{s} $$ s for the validity of EFT framework.

The accuracy to which new physics (NP) couplings can be determined using the optimal observable technique (OOT) depends on the strength of the Standard Model (SM) contribution. In this work we study this effect in two cases: one where the SM contribution is dominant and another where it is subdominant. For the first we consider t t production in presence of dimension six effective operators and determine the optimal statistical precision to which the Wilson coefficients can be extracted in an e + e − collider. As an example of NP dominance we consider the production of heavy charged scalars in several 2-Higgs multiplet models (with and without right handed neutrino contributions), and again determine the optimal range of statistical uncertainty for the NP couplings. We confirm the expectation that the optimal estimation of NP couplings with minimal SM interference is more precise than those where SM dominates, and discuss the effects of the efficiency of background reduction, luminosity and beam polarization.

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