the transverse mass $\mT(\ell, \ptvecmiss)$, calculated using the electron or muon momentum vector and , to be between 20 and 60or above 120. A significant background from DY+jets events is reduced by requiring the invariant mass of the electron or muon and the , $m_{\ell \tauh}$ to be above 50. To reduce background from QCD multijet events, we require $2.0< \Delta R(\ell, \tauh) <3.5$.

=800 With these preselection criteria in place, we train several BDTs corresponding to different signal hypotheses to classify signal and background events. The input variables are the of the electron or muon, the of the candidate, , $\mT(\ell, \ptvecmiss)$, $\Delta \eta(\ell,\tauh)$, $\Delta \phi(\ell, \ptvecmiss)$, $\Delta \phi(\tauh, \ptvecmiss)$, $\Delta R(\ell, \tauh)$, $m(\ell \tauh)$, and $\mT^{\text{tot}}\equiv\sqrt{\smash[b]{\mT^2(\ell, \ptvecmiss)+\mT^2(\tauh, \ptvecmiss)}}$. We also include and the contransverse mass ($m_{\mathrm{CT}}$) [@Tovey:2008ui; @Polesello:2009rn], computed from the visible decay products and defined as $$m_{\mathrm{CT}} \equiv \sqrt{2 \pt^{\ell} \pt^{\tauh} [1 + \cos\Delta\phi(\ell,\tauh)]}.$$ For signal events, $m_{\mathrm{CT}}$ is expected to have an endpoint near ${(m(\PSGt)^2-m(\PSGczDo)^2)}/m(\PSGt)$. Finally, we include the variable ${\ensuremath{D_{\zeta}}\xspace}=\ptvecmiss \cdot \vec \zeta - 0.85 (\ptvec^{\ell}+\ptvec^{\tauh}) \cdot \vec \zeta$, with $\vec{\zeta}$ being the bisector of the directions of the transverse momenta of the electron or muon and the candidate [@CuencaAlmenar:2008zza; @Khachatryan:2014wca]. The value of 0.85 