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Search for bottom-squark pair production in $pp$ collision events at $\sqrt{s} = 13$ TeV with hadronically decaying $τ$-leptons, $b$-jets and missing transverse momentum using the ATLAS detector

A search for pair production of bottom squarks in events with hadronically decaying $τ$-leptons, $b$-tagged jets and large missing transverse momentum is presented. The analyzed dataset is based on proton-proton collisions at $\sqrt{s}$ = 13 TeV delivered by the Large Hadron Collider and recorded by the ATLAS detector from 2015 to 2018, and corresponds to an integrated luminosity of 139 fb$^{-1}$. The observed data are compatible with the expected Standard Model background. Results are interpreted in a simplified model where each bottom squark is assumed to decay into the second-lightest neutralino $\tilde χ_2^0$ and a bottom quark, with $\tilde χ_2^0$ decaying into a Higgs boson and the lightest neutralino $\tilde χ_1^0$. The search focuses on final states where at least one Higgs boson decays into a pair of hadronically decaying $τ$-leptons. This allows the acceptance and thus the sensitivity to be significantly improved relative to the previous results at low masses of the $\tilde χ_2^0$, where bottom-squark masses up to 850 GeV are excluded at the 95% confidence level, assuming a mass difference of 130 GeV between $\tilde χ_2^0$ and $\tilde χ_1^0$. Model-independent upper limits are also set on the cross section of processes beyond the Standard Model.