Search for low-mass resonances decaying to $ \tau\tau $ and measurement of the $ \Upsilon\to\tau\tau $ decay in proton-proton collisions at $ \sqrt{s} = $ 13.6 TeV

An inclusive search is presented for spin-zero bosons decaying to $ \tau\tau $ in a previously unexplored mass range between 20 and 60 GeV using proton-proton collision data at $ \sqrt{s}= $ 13.6 TeV, corresponding to an integrated luminosity of 61.9 fb$ ^{-1} $ recorded by CMS in 2022--2023. A high-rate trigger stream in combination with a novel low-momentum hadronic tau reconstruction algorithm have enabled a measurement of the $ \Upsilon(1S,2S,3S)\to\tau^{+}\tau^{-} $ process in the challenging environment of a hadron collider, with a 5.8 $ \sigma $ significance above background and production cross section of 3.5 $ \pm $ 0.7 (stat) $ \pm $ 0.7 (syst) nb for visible rapidity $ |y_{\text{vis}}| < $ 1.2 and visible transverse momentum $ p_{\mathrm{T}}^\text{vis} > $ 15 GeV. No significant excess above the standard model background is observed. Upper limits on the product of the spin-zero resonance production cross section and branching fraction to $ \tau\tau $ are set at 95% confidence level between 40 and 400 pb.