Measurement of the $VH,H\to \tau \tau$ process with the ATLAS detector at 13 TeV

A measurement of the Standard Model Higgs boson produced in association with a $W$ or $Z$ boson and decaying into a pair of $\tau$-leptons is presented. This search is based on proton-proton collision data collected at $\sqrt{s}=13$ TeV by the ATLAS experiment at the LHC corresponding to an integrated luminosity of 140 fb${}^{-1}$. For the Higgs boson candidate, only final states with at least one $\tau$-lepton decaying hadronically ($\tau \to \mathrm{h}\mathrm{a}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{s}+{\nu }_{\tau }$) are considered. For the vector bosons, only leptonic decay channels are considered: $Z\to \mathrm{\ell }\mathrm{\ell }$ and $W\to \mathrm{\ell }{\nu }_{\mathrm{\ell }}$, with $\mathrm{\ell }=e,\mu$. An excess of events over the expected background is found with an observed (expected) significance of 4.2 (3.6) standard deviations, providing evidence of the Higgs boson produced in association with a vector boson and decaying into a pair of $\tau$-leptons. The ratio of the measured cross-section to the Standard Model prediction is ${\mu }_{\text{VH}}^{\tau \tau }=1.28{}_{-0.29}^{+0.30}(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}){}_{-0.21}^{+0.25}(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.})$. This result represents the most accurate measurement of the \vh process achieved to date.