$W\rightarrow\tau\nu_{\tau}$ Cross Section Measurement and Search for the Standard Model Higgs Boson in the $\tau_{\text{lep}}\tau_{\text{had}}$ Final State with the ATLAS Detector

Two measurements based on proton-proton collisions recorded with the ATLAS experiment at the LHC with $\tau$ leptons and missing transverse energy in the final state are presented. The $W$ boson production cross section with subsequent $W\rightarrow\tau\nu_{\tau}$ decay is measured based on 2010 ATLAS data at a centre-of-mass energy of $\sqrt{s} = 7$ TeV corresponding to an integrated luminosity of 34 pb$^{-1}$. The product of the $W$ production cross section and branching ratio is measured to be $\sigma_W^{\text{tot}} \times \text{BR}(W\rightarrow \tau\nu_{\tau}) = (11.1\,\pm0.3\,\text{(stat.)}\,\pm1.7\,\text{(syst.)}\,\pm0.4\,\text{(lumi.)})\,$nb. This is the first measurement of the $W\rightarrow\tau\nu$ process at the LHC. It is one of the first analyses to successfully validate the reconstruction and identification techniques for hadronically decaying $\tau$ leptons and missing transverse energy. A search for the Standard Model Higgs boson decaying to a pair of $\tau$ leptons is performed with a leptonically and hadronically decaying $\tau$ lepton in the final state. The analysis is carried out on the full 2012 ATLAS data sample at a centre-of-mass energy of $\sqrt{s} = 8$ TeV corresponding to an integrated luminosity of 20.3 fb$^{-1}$. The selection of data events is based on the kinematic properties of a Higgs boson produced via vector-boson fusion or a boosted Higgs boson resulting mainly from gluon fusion. A deviation from the predicted background is found in data with an observed (expected) significance of 1 (1.7) $\sigma$ at $m_{H} = 125$ GeV. The measured ratio of the signal strength to the Standard Model expectation is $\mu=0.4\,\pm\,0.6$. In the combination of all $H\rightarrow\tau\tau$ decay modes an excess in data over the predicted background with a significance of 3.2 (2.5) $\sigma$ at $m_H = 125$ GeV is found with a relative signal strength of $\mu=1.4\,^{+0.6}_{-0.5}$. This result constitutes evidence that the recently discovered Higgs boson directly couples to fermions consistent with the expectation from the Standard Model. In particular the compatibility with a mass of $m_H = 125$ GeV is confirmed with this analysis.