Electroweak and Higgs Measurements Using Tau Final States

Spin correlations for $\tau$ lepton decays are included in the PYTHIA 8 event generation software with a framework which can be expanded to include the decays of particles other than the $\tau$ lepton. The spin correlations for the decays of $\tau$ leptons produced from electroweak and Higgs bosons are calculated. Decays of the $\tau$ lepton using sophisticated resonance models are included in PYTHIA 8 for all channels with experimentally observed branching fractions greater than $0.04\%$. The mass distributions for the decay products of these channels calculated with PYTHIA 8 are validated against the equivalent distributions from the HERWIG++ and TAUOLA event generators. The technical implementation of the $\tau$ lepton spin correlations and decays in PYTHIA 8 is described. A measurement of the inclusive ${Z \rightarrow \tau\tau}$ cross-section using ${1.0~\mathrm{fb}^{-1}}$ of data from $pp$ collisions at $\sqrt{s} = 7~\mathrm{Te\kern -0.1em V}$ collected with the LHCb detector is presented. Reconstructed final states containing two muons, a muon and an electron, a muon and a charged hadron, or an electron and a charged hadron are selected as ${Z \rightarrow \tau\tau}$ candidates. The cross-section for $Z$ bosons with a mass between 60 and 120 GeV decaying into $\tau$ leptons with pseudo-rapidities between 2.0 and 4.5 and transverse momenta greater than 20 GeV is measured to be 72.3 $\pm$ 3.5 $\pm$ 2.9 $\pm$ 2.5 pb. The first uncertainty is statistical, the second uncertainty is systematic, and the third is to due the integrated luminosity uncertainty. The ${Z \rightarrow \tau\tau}$ to ${Z \rightarrow \mu\mu}$ cross-section ratio is found to be 0.94 $\pm$ 0.09 and the ${Z \rightarrow \tau\tau}$ to ${Z \rightarrow ee}$ cross-section ratio is found to be 0.95 $\pm$ 0.07. The uncertainty on these ratios is the combined statistical, systematic, and luminosity uncertainties. Limits on the production of neutral Higgs bosons decaying into $\tau$ lepton pairs with pseudo-rapidities between 2.0 and 4.5 are set at a 95% confidence level using the same LHCb dataset. A model independent upper limit on the production of neutral Higgs bosons decaying into $\tau$ leptons is set and ranges between 8.6 pb for a Higgs boson mass of 90 GeV to 0.7 pb for a Higgs boson mass of 250 GeV. This limit is compared to the expected standard model cross-section. An upper limit on $\tan\beta$ in the $m_{A^0}$ and $\tan\beta$ plane is set for the $m_{h^0}^\mathrm{max}$ scenario of the minimal supersymmetric model and varies from 34 for a $CP$-odd Higgs boson mass of 90 GeV to 70 for a $CP$-odd Higgs boson mass of 140 GeV.