Search for top-squark pair production with the LHC experiment ATLAS in final states with b-quarks and tau leptons

A search for supersymmetry in final states with two b-quarks, two tau leptons and weakly-interacting particles is presented. The signal model assumes direct production of top-squark pairs decaying to tau sleptons. Proton–proton collisions provided by the Large Hadron Collider in the years 2015 and 2016 at 13 TeV centre-of-mass energy are recorded with the ATLAS experiment. The integrated luminosity of the dataset amounts to 36.1 fb−1. Depending on the decay of the tau leptons, two search channels are defined: the mixed channel, where one tau lepton decays hadronically and the other one leptonically, and the hadronic channel, where both tau leptons decay hadronically. Control regions to constrain the normalisation of the top-quark pair-production process are designed and corresponding validation regions defined. For the subleading processes of pair produc- tion of massive vector bosons and of the top-quark pair production with an associated Z boson, dedicated control regions are developed. No significant excess over the event yields expected from the Standard Model of particle physics is observed in either of the channels, therefore exclusion limits using the CLs prescription are set. For the top squark (tau slepton), masses up to 1.16 TeV (1.00 TeV) can be excluded at 95 % confidence level.