Search for Minimal Supersymmetric Standard Model Higgs bosons $H/A$ and for a $Z^{\prime}$ boson in the $\tau \tau$ final state produced in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS Detector

A search for neutral Higgs bosons of the Minimal Supersymmetric Standard Model (MSSM) and for a heavy neutral $Z^{\prime}$ boson is performed using a data sample corresponding to an integrated luminosity of 3.2 fb$^{-1}$ from proton--proton collisions at $\sqrt{s} = 13$ TeV recorded by the ATLAS detector at the LHC. The heavy resonance is assumed to decay to a $\tau^+ \tau^-$ pair with at least one $\tau$ lepton decaying to final states with hadrons and a neutrino. The search is performed in the mass range of 0.2--1.2 TeV for the MSSM neutral Higgs bosons and 0.5--2.5 TeV for the heavy neutral $Z^{\prime}$ boson. The data are in good agreement with the background predicted by the Standard Model and hence results are given as upper limits on the production cross section times branching fraction of the boson decay to $\tau^+\tau^-$ as a function of the boson mass. The results are interpreted in MSSM and $Z^{\prime}$ benchmark scenarios. The most stringent MSSM parameter space constraints for the Higgs boson search exclude at 95\% confidence level (CL) $\tan\beta > 7.6$ for $m_A = 200$ GeV in the $m_{h}^{\textrm{mod+}}$ MSSM scenario. This analysis extends the MSSM limits from previous searches for the mass range $m_A > 500$ GeV. For the Sequential Standard Model, a $Z^{\prime}_\mathrm{SSM}$ mass up to 1.90 TeV is excluded at 95% CL and masses up to 1.82--2.17 TeV are excluded for a $Z^{\prime}_{\mathrm{SFM}}$ of the Strong Flavour Model.