Measurement of Tau Polarisation in Z/γ* → ττ Decays in Proton–Proton Collisions at ATLAS

This thesis presents a measurement of the tau polarisation in Z/gamma*->tautau decays at ATLAS using a dataset of proton–proton collisions recorded at a centre-of-mass energy of sqrt(s) = 8 TeV that amounts to an integrated luminosity of 20.2 fb^–1. A signal region enhanced with Z/gamma*->tautau decays is selected that are followed by one leptonic and one hadronic tau decay. Information about the tau spin states is gained from the hadronic decays. A tau polarisation of P_tau = –0.27 +/– 0.02 (stat) +/– 0.04 (syst) is measured in a fiducial region that resembles the signal region and is defined on generator level. An extrapolation to the full phasespace for ditau masses of 66–116 GeV yields P_tau = –0.14 +/– 0.02 (stat) +/– 0.04 (syst). The results agree with Standard Model predictions of P_tau = –0.270 +/– 0.006 for the fiducial region and P_tau = –0.1536 +/– 0.0014 for the ditau mass range. An approximate weak mixing angle of sin^2 theta_W ~ 0.232 +/– 0.005 is determined by neglecting the photon contribution and its interference with the dominant Z boson contribution. The measurement complements previous precision ones in electron–positron collisions at the Z boson pole. Furthermore, an upgraded reconstruction for hadronic tau decays is presented that allows for the determination of decay modes and of the four-momenta of the individual hadrons from these decays. Five decay modes are distinguished, and the classification is accurate for 75% of the hadronic tau decays in Z/gamma*->tautau events. Particle-flow methods are used to determine the visible tau four-momentum. In comparison with the previous reconstruction, the visible energy resolution is improved by up to a factor of two. The directional resolution is improved by up to a factor of five. Information about the decay modes and reconstructed neutral pions is provided to physics analyses for the first time at ATLAS. A planned measurement of the Higgs boson's CP properties in H->tautau decays, for example, relies on this type of information. The simulation reproduces the performance of the upgraded reconstruction in data well.