Emma Ideal

On July 4, 2012, the discovery of the Higgs boson was simultaneously announced by the ATLAS and CMS collaborations. Since then, evidence for its decay to tau leptons has been claimed. As of now, there have been no Higgs discoveries in any of its associated production modes. For this thesis, a search for the Higgs boson produced in association with a vector boson V = W^(+-), Z and decaying to a tau lepton pair was conducted using 2012 ATLAS data. The data corresponds to 20.3 fb^(-1) of 8 TeV center-of-mass energy proton-proton collisions delivered by the LHC. This analysis focuses on the search in the WH and ZH categories in which the Higgs boson decays to a pair of hadronically decaying taus. In order to better identify this signature in the midst of overwhelming hadronic activity, the vector bosons are required to decay leptonically. A data-driven multi-lepton fake factor method is used as the primary background estimation technique, modeling all reducible backgrounds where jets are misidentified as hadronic taus; Monte Carlo simulation is used to model the irreducible diboson backgrounds. There are insufficient statistics in the 2012 dataset to discover the Standard Model (SM) Higgs boson in these categories, so 95% confidence-level upper limits on the SM Higgs associated production cross section times the ditau branching ratio are calculated. For a Higgs boson mass m_H = 125 GeV, an upper limit of 5.3 times the SM Higgs cross section is expected.