Determination of spin and parity of the Higgs boson in the $WW* \to ev\nu\rho$ decay channel with the ATLAS detector

So far, the Standard Model is the most successful theory to summarise our knowledge on Particle Physics. The fundamental particles, which are the constituents of our universe, as well as their interactions via the fundamental forces are elegantly explained in a concise way. The last building block, predicted by the Standard Model, the Higgs boson, was portrayed as the "Holy Grail" of particle physics, due to the unsuccessful experimental searches for it, going back five decades. At the Large Hadron Collider the collaboration of thousands of scientists and researchers across various fields, a significant breakthrough was achieved in its first data taking period: a new resonance was found in the searches for the predicted Standard Model Higgs boson. Precision measurements were employed to measure the mass of the resonance with the least possible uncertainty. This dissertation focuses on the H→WW^*→lvlv decay channel using the data collected with the ATLAS detector during 2012 at 8 TeV center of mass energy. This channel is one of the discovery channels, with quoted observed excess of 6.1 standard deviations over the background only hypothesis and with an expected of 5.8. The discovery of a new resonance compatible with the Higgs boson is not enough to validate that is indeed the well-sought boson. For this reason, additional studies have been employed to identify the properties of the new resonance. The studies of the Spin and CP properties of the new resonance with data from the ATLAS detector are presented in this thesis. The results show that the spin number is confidently found to be zero. Similarly, only small deviations from the SM are observed in the CP study, but are comfortably compatible with it within the uncertainties. The possibility of mixed CP states was also studied and according to the Run-I data it is not fully excluded. This is exciting news for the Run-II era of the LHC.