Search for scalar leptoquarks in the b$\tau\tau$ final state in $pp$ collisions at $\sqrt{s} =$~13~TeV with the ATLAS detector

A search for singly-produced scalar leptoquarks decaying to $b\tau$ in proton-proton collisions is performed using Run 2 data from the Large Hadron Collider corresponding to an integrated luminosity of 139~fb$^{-1}$ at $\sqrt{s} = 13$~TeV recorded by the ATLAS detector. The signal benchmark model considered is a scalar leptoquark with an electric charge of $4/3e$ and quantum numbers $3B + L = -2$, which decays exclusively into a b-quark and a $\tau$-lepton. The mass range explored is from 0.4~TeV to 2.5~TeV. No significant excess above the Standard Model prediction is observed, and 95\% confidence-level upper limits are set on the production cross-section times branching fraction of leptoquarks decaying to $b\tau$. Considering both single and pair leptoquark production processes, under the assumption of exclusive decays of scalar leptoquarks to $b \tau$, leptoquark masses below 1.26~TeV, 1.30~TeV and 1.41~TeV are excluded for a scalar leptoquark Yukawa coupling to $b$-quark and $\tau$-lepton of 1.0, 1.7 and 2.5, respectively.