Searching for the Higgs Boson in Pairs of Tau Leptons in Data from the ATLAS Experiment
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One of the key questions in particle physics today, is the origin of the electroweak symmetry breaking. The answer to this question will most likely be solved with the data provided by the Large Hadron Collider which started colliding protons in 2008. Many ideas have been posed to how particles gain their masses. The most promising of these ideas is the Higgs mechanism which predicts the existence of a new massive scalar boson, the Higgs boson.
Since the discovery of a new particle consistent with a Standard Model Higgs boson was made on July 4 by the ATLAS and CMS experiment, the solution for the puzzle of the electroweak symmetry breaking might be very near. However, in order to fully claim a discovery of the Standard Model Higgs boson, the new particle has to be proven to be a scalar boson and its decay has to be observed in both bosonic and fermionic final states with the corrected branching ratios predicted by the Standard Model. So far the new boson has only been seen in the bosonic γγ, ZZ and WW final states, and the new particle is yet to be observed in any fermionic final state. Among the fermionic final states, the largest sensitivity is expected in τ+τ− and b¯b.
In this thesis, a search for the Standard Model Higgs boson in the fully hadronic τ+τ− final state is presented. In the selection of Higgs candidate events, a technique for recreating the rest frame of the τ -pairs is shown to be a powerful tool for discriminating between Higgs and background events. Using the reconstructed rest frame, a new mass estimation technique is developed and compared to the existing methods. Finally, an updated limit on the Higgs production cross section is derived using the data recorded by ATLAS during 2011, and compared to the existing ATLAS limit.
The thesis is structured as follows: In the first part a brief introduction to the Standard Model including the Higgs mechanism is given, followed by a description on the production of Higgs bosons at the LHC in the Standard Model. The second part describes the ATLAS experiment in general, but a special emphasis is put on the SCT prompt calibration loop, which a great deal of the work of this thesis has gone into developing. The last part describes in the analysis of τ -pairs using the ATLAS detector. First, general description of the newly developed method for the rest frame reconstruction is given, followed by several chapters on the search for the Higgs boson in the τ+τ− final state. Here, the attention is put on mass estimation, background normalisation and evaluation of the theoretical uncertainties related to categorisation of events based on jet kinematics.