  A search for pair production of heavy scalar leptoquarks (LQs), each decaying
into a top quark and a $\tau$ lepton, is presented. The search considers final
states with an electron or a muon, one or two $\tau$ leptons that decayed to
hadrons, and additional jets. The data were collected in 2016 in proton-proton
collisions at $\sqrt{s}=$ 13 TeV with the CMS detector at the LHC, and
correspond to an integrated luminosity of 35.9 fb$^{-1}$. No evidence for pair
production of LQs is found. Assuming a branching fraction of unity for the
decay LQ$ \to $t$\tau$, upper limits on the production cross section are set as
a function of LQ mass, excluding masses below 900 GeV at 95% confidence level.
These results provide the most stringent limits to date on the production of
scalar LQs that decay to a top quark and a $\tau$ lepton.
