This thesis presents the validation of the implementation of heavy neutral leptons (HNLs) within the framework of the Type I Seesaw mechanism in the Monte Carlo event generator Sherpa. The Seesaw mechanism is a widely studied extension of the Standard Model (SM). It explains the smallness of neutrino masses by introducing additional neutrino mass eigenstates, known
as heavy neutral leptons. The observation of HNLs would provide clear evidence for physics beyond the SM. Accurate Monte Carlo simulations of their production and decay at collider experiments are therefore essential for experimental searches and the interpretation of their results. Previous studies have investigated this scenario using the matrix element generator MadGraph5_aMC@NLO in combination with the parton shower Pythia8. In contrast, earlier studies of HNL simulations in Sherpa were limited and did not involve a systematic validation [6]. In this thesis, the implementation of HNL production and decay in Sherpa is validated by comparing its predictions to results obtained with MadGraph+Pythia. Good agreement between
the two simulation frameworks is found for kinematic distributions in both production and decay processes. In particular, both approaches reproduce characteristic differences in angular distributions between lepton number conserving (LNC) and lepton number violating (LNV) HNL decays. Furthermore, this thesis represents a significant advancement in the simulation of HNLs compared to previous works, as it demonstrates several advantages of the Sherpa simulation which are not available in MadGraph+Pythia. These include the consistent treatment of spin correlations in the semileptonic decay channel into a 𝜌-meson, such as 𝐻𝑁𝐿 → 𝜌^± 𝜇^∓ → 𝜋^± 𝜋⁰ 𝜇^∓ , as well as the implementation of HNL decays into 𝜏^± leptons.