Search for top squarks decaying to tau sleptons in $pp$ collisions at $\sqrt s$ = 13 TeV with the ATLAS detector

A search for direct pair production of top squarks in final states with two tau leptons, $b$-jets, and missing transverse momentum is presented, based on 36.1 fb$^{-1}$ of proton$-$proton collision data recorded at $\sqrt{s}$ = 13 TeV with the ATLAS detector at the Large Hadron Collider in 2015 and 2016. Two exclusive channels with either two hadronically decaying tau leptons or one hadronically and one leptonically decaying tau lepton are considered. No significant deviation from the Standard Model predictions is observed in the data. The analysis results are both interpreted in terms of model-independent limits and used to derive exclusion limits on the masses of the top squark $\tilde t_1$ and the tau slepton $\tilde\tau_1$ in a simplified model with a nearly-massless gravitino. In this model, masses up to $m(\tilde t_1)=$ 1.16 TeV and $m(\tilde\tau_1)$ = 1.00 TeV are excluded at 95$\,$% confidence level.