Discovery and Measurement of the Higgs Boson in the WW Decay Channel

Abstract

In the Standard Model of particle physics, the non-zero masses of the W and Z bosons and the fermions are generated through interactions with the Higgs field, excitations of which correspond to Higgs bosons. Thus, the experimental discovery of the Higgs boson is of prime importance to physics, and would confirm our understanding of fundamental mass generation.This thesis describes a search for the gg → H → W W → ν ν process of Higgs boson production and decay. It uses the LHC Run I dataset of pp collisions recorded by the ATLAS detector, which corresponds to an integrated luminosity of 4.5 fb −1 at √ s = 7 TeV and 20.3 fbTeV. An excess of events is observed with a significance of 4.8 standard deviations, which is consistent with Higgs boson production. The significance is extended to 6.1 standard deviations when the vector boson fusion production process is included. The measured signal strength is 1.11−0.21 at m H = 125 GeV. A cross section measurement of W W production, a major background to this search, is also presented using the √ s = 7 TeV dataset only.iii PrefaceAs a DPhil research topic, the H → W W analysis has proven to be a baptism of fire. It is the most complicated of the three "discovery channels", 1 as it involves a variety of physics objects and requires a good understanding of many difficult backgrounds. As such, the analysis took huge effort from a large number of individuals. My role focussed on theoretical aspects of the signal and background modelling, and these parts shall be emphasised. I contributed to multiple iterations of the analysis [1][2][3][4][5][6][7][8], though the version presented here is unpublished at the time of writing [9]. I also co-authored the third Yellow Report produced by the LHC Higgs Cross Section Working Group [10].When I began the degree in October 2010, there was no direct evidence for a Higgs boson. This thesis is written from a personal perspective and motivates a low mass search by electroweak fits, when in fact this aspect was motivated later by observations of a resonance in the γγ and ZZ channels.2 Also, an advanced search strategy is described, though the discovery of H → W W was actually a gradual process with multiple iterations of blinding, optimising and unblinding the analysis. As more data were recorded and the analysis was enhanced, the results improved.Early on, I gained relevant insight by performing multiple W W cross section measurements [12][13][14][15]. My main contribution was a jet veto correction factor applied to the W W signal, which reduces the dominant uncertainty in the analysis. This measurement shall be described when considering the W W background to the H → W W search.To qualify for authorship within the ATLAS collaboration, I performed Run Control shifts. I also worked within the Versatile Link project [16] to investigate radiation hardened optical components for the HL-LHC. As this research does not easily relate to the Higgs boson, it is excluded from this thesis. However, I have published articles on the radiation toler...