Search for right-handed neutrinos with 79.8 fb$^{-1}$ of data collected at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for a heavy neutral lepton (HNL) of Majorana nature is performed using data collected by the ATLAS detector at $\sqrt{s}=13$ TeV which corresponds to an integrated luminosity of 79.8 fb$^{-1}$. The properties of the HNL are described in the $\nu$MSM which presents a possible extension of the Standard Model. The search is conducted in the prompt $W^\pm \rightarrow \mu^\pm \mu^\pm e^\mp \nu_e$ channel for which the mixing angle $V_{\mu N}$ is probed, where $V_{\mu N}$ is the matrix element describing the mixing between the Standard Model neutrino of flavor $\mu$ and the heavy neutral lepton $N$. In order to accurately estimate the background processes which contain fake or non-prompt leptons, the data-driven technique called the fake factor method is applied. The estimated background is compared to data in control regions which proves to be consistent. The sensitivity of the search is estimated for five mass points of the heavy neutral lepton, $M_N=5,$ 10, 20, 30 and 50 GeV. For the largest mass points, the expected sensitivity of the search is well below the current state-of-the-art limits set by the CMS and DELPHI experiments.