PhD defense by Lotte A. Thomsen – University of Copenhagen

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PhD defense by Lotte A. Thomsen

Title: A search for associated production of a SM Higgs decaying into tau leptons with the ATLAS experiment

Abstract: Two distinct topics are covered in this thesis: tau hadronic decay identification and the search for a Standard Model (SM) Higgs boson in associated production with a W boson decaying into tau leptons within the ATLAS experiment.

Prior to the work presented in this thesis, tau identification has mainly relied on charged $\pi^\pm$, and tracking information and global variables exploiting the energy  deposited in the calorimeter. Improvement of the tau identification for hadronically decaying taus is sought using new variables derived from a newly available algorithm capable of $\pi^0$ reconstruction. The added information about $\pi^0$'s together with a re-optimization of the algorithm results in gain of roughly 30\% in rejection of background over the full range of transverse momentum. Furthermore the threshold for tau identification can be lowered from 20 GeV to 15 GeV, which is potentially very valuable for physics analyses involving hadronic tau decays. 

The second part of this thesis covers the search for associated production of a SM Higgs boson decaying into tau leptons with the ATLAS experiment. The search procedure is made robust by developing a background estimation method relying on simulation for the irreducible background and a data-driven estimation for the reducible backgrounds. The data are still blinded internally in ATLAS. The result found is an expected upper limit on $\sigma_{VH}$ of $5 \times \sigma_{SM}$ for 20.3 $fb^{-1}$ 

which implies that, when combining all WH and ZH final states and including also ZH associated production one should be able to reach sensitivity to SM production of Higgs in association to vector bosons in Run 1 data of LHC. This also provides proof that this channel is important for Higgs property measurements in Run 2 and beyond.