Direct measurement of the Higgs boson mass, natural width, and cross section times branching ratio to four leptons using a per-event lineshape in the Higgs to ZZ to four lepton decay channel with the ATLAS detector

The discovery of the Higgs boson by the ATLAS and CMS Collaborations in 2012 remains the crowning achievement of the Large Hadron Collider (LHC) physics programme. Five years since its discovery, Run 2 at the LHC is underway and producing more data than ever before, allowing measurements of the Higgs boson beyond the reach of Run 1. Precise measurement of the Higgs boson’s properties help guide particle physicists in understanding the Standard Model, and what lays beyond. This thesis presents a measurement of the Higgs boson mass, natural width, and cross section times branching ratio in the H → ZZ(∗) → 4l decay channel using the full 2015+2016 combined dataset from Run 2 at the LHC, totaling 36.1/fb of p-p collisions at centre-of-mass energy √s = 13 TeV. The analysis is performed using a technique developed by the author, called the per-event response method. The technique is designed to produce a more precise, accurate, and model-independent measurement of the Higgs boson properties grounded directly on the performance of the ATLAS detector. Using this technique, the Higgs boson mass was measured to be mH = 124.61 +/-0.44(stat.) +/-0.06(syst.) GeV. The total Higgs boson production cross section times branching ratio to four electrons or muons in p-p collisions at √s = 13 TeV was measured to be σ × BR(H → 4l) = 8.55 +/-1.23(stat.) +/-0.63(syst.) fb. An upper limit was set on the natural width of the Higgs as ΓH < 3.65(stat.) ⊕ 0.19(syst.) GeV at 95% confidence. These measurements are consistent with previous measurements and with the Standard Model. This result is the first ever measurement of Higgs boson mass using ATLAS at √s = 13 TeV, exceeding the precision of any H → 4l mH measurement made to this point. Also presented are the author’s contributions to the precision assembly of the small-strip Thin Gap Chambers to be installed in ATLAS in 2019, and to the ATLAS reconstruction software, both essential to continued detector operation and the many physics analyses yet to come from ATLAS.