Ochoa de Castro, MIAJ; (2015) Searching for the Higgs boson in the bb decay channel with the ATLAS experiment. Doctoral thesis , UCL (University College London).

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Abstract

The discovery of the Higgs boson by the ATLAS and CMS experiments is one of the main results of Run 1 of the Large Hadron Collider. However, clear evidence for the Higgs boson decay to a pair of b-quarks has not been observed and is crucial to establish the nature of the new found particle. The work presented in this thesis focuses on the search for the Higgs boson in the VH(bb) channel, where it is produced in association with a leptonically decaying vector boson (W, Z), and decays to a pair of b-quarks. Prior to the start of LHC operations, the challenges posed by a pp collider to a H→bb search motivated the development of jet substructure techniques. The boosted regime plays a vital role in the sensitivity of a VH(bb) search and the topologies where the decay products merge can be recovered by implementing a substructure-based selection. The sensitivity of such an approach in a VH(bb) search is studied using ATLAS pp collision data, at a centre-of-mass energy of √s = 7 TeV. It was found that the sensitivity in the boosted region of the VH(bb) channel in Run 1 is already fully exploited by the resolved approach. The mass of the Higgs boson, the energies and luminosities delivered and the good performance of anti-kt jets resulted in little or no gain, at this stage, from performing a jet substructure analysis. The final ATLAS VH(bb) Run 1 result is presented. The systematic uncertainties related to the W+bb process are estimated and discussed. As an irreducible background to this search, the description of W+bb events plays an important role on the final obtained sensitivity. Finally, in preparation for Run 2 and future colliders, the potential benefits from jet substructure techniques are reviewed at different centre-of-mass energies in the context of a boosted WH(bb) search. A detailed study of the signal significance as a function of the boost of the system reveals that the region of highest sensitivity is already fully exploited by the resolved reconstruction. A substructure approach is only beneficial in events with boosts greater than 600 GeV, outside the phase-space region of maximum significance.

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