Discovery of a Higgs Boson with the ATLAS Detector

Details of the discovery of a Higgs boson and measurements of its properties with the ATLAS detector are presented, using up to 4.8 and 21 fb$^{-1}$ of data at $\sqrt{s} = 7$ and 8 TeV, respectively. The mass of the boson is measured to be $m_{H} = 125.5$ $\pm 0.2$ (stat) $^{+0.5}_{-0.6}$ (sys) GeV using a combination of the \hgg\ and \hfl\ channels. At 125.5 GeV, a combination of the $H\rightarrow WW^{(*)}\rightarrow\ell\nu\ell\nu$, $H\rightarrow\gamma\gamma$, $H\rightarrow ZZ^{(*)}\rightarrow\ell\ell\ell\ell$, $H\rightarrow b\bar{b}$, and $H\rightarrow\tau^{+}\tau^{-}$ channels yields a measured signal strength with respect to the Standard Model of $\hat{\mu} = 1.3\pm 0.2$. In this combination the couplings of the boson are explored under various assumptions motivated by Beyond Standard Model scenarios, with no significant deviations from the Standard Model observed. Strong evidence for the vector boson fusion production process is found with an observed (expected) significance of 3.1 (2.5) standard deviations. Among the individual channels, the search in the $H\rightarrow WW^{(*)}\rightarrow\ell\nu\ell\nu$ decay mode is outlined in detail. Combining the full 7 and 8 TeV datasets, the analysis yields strong evidence for this mode with an observed (expected) significance of 3.8 (3.7) standard deviations at $m_{H} = 125$ GeV. At this mass, the signal strength in the channel is measured to be $\hat{\mu} = 1.01\pm 0.31$. The statistical methods used in these measurements, and particle searches in general, are thoroughly detailed. Further, novel techniques are presented, including polynomial interpolation of nuisance parameters, procedures for asymptotically approximating quantiles of expected limits, uncapping of profile likelihood test statistics, and the applications of B-Splines for interpolating signal distributions in low resolution search channels.