Measurement of the , → 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 τ-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 τ-lepton decaying hadronically ($\tau \to \mathrm{hadrons}+{\nu }_{\tau }$) are considered. For the vector bosons, only leptonic decay channels are considered: $Z\to \ell \ell $ and $W\to \ell {\nu }_{\ell }$, with $\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 τ-leptons. The ratio of the measured cross-section to the Standard Model prediction is ${\mu }_{\text{VH}}^{\tau \tau }=1.28\phantom{\rule{0.25em}{0ex}}{}_{-0.29}^{+0.30}\phantom{\rule{0.25em}{0ex}}\left(\mathrm{stat}.\right)\phantom{\rule{0.25em}{0ex}}{}_{-0.21}^{+0.25}\phantom{\rule{0.25em}{0ex}}\left(\mathrm{syst}.\right)$. This result represents the most accurate measurement of the $VH\left(\tau \tau \right)$ process achieved to date.