Measurement of the Higgs boson o -shell coupling to constrain the total width in the H → ZZ(*) → 4l channel and Level-1 Track muon isolation performance for the HL-LHC with the ATLAS detector

Abstract

Since the observation of a narrow mass resonance consistent with the Higgs boson by ATLAS and CMS collaborations in 2012, a number of important studies have been made in order to understand the properties of the newly discovered particle. The most fundamental precision measurements include the Higgs coupling to other particles and itself; properties that have a direct relation with the total decay width. The theoretical total width of the Standard Model (SM) Higgs boson is extremely small (4.2 MeV), making its direct measurement by the LHC experiments non feasible due to the finite experimental detector resolution. Recent publications have shown a novel way to set an indirect limit on the total Higgs boson width by using measurements of both off-shell and on-shell production. This thesis presents a determination of the off-shell Higgs boson coupling, and a further interpretation of the Higgs total width in the H → ZZ → 4l channel (l = e; μ). The results are based on pp collision data collected by the ATLAS experiment at the LHC, corresponding to an integrated luminosity of 20.3 fb-1 at a collision energy of √s = 8 TeV. Using the CLs statistical method and assuming the same higher-order QCD corrections applied for both signal and background processes, the observed 95% confidence level (CL) upper limit on the off-shell signal strength is 7.3 (with the yields normalized to the SM expectation). Similarly, the 95% CL upper limit on the Higgs total width is 24.7 MeV. The LHC will undergo its last big upgrade in 2021, in preparation for the high-luminosity LHC Run (HL-LHC), with a luminosity increase of approximately a factor of 5 beyond its nominal design rate. Raising the muon transverse momentum threshold becomes a necessity in order to maintain a low online selection rate with the existing trigger system, at the cost of a reduced efficiency for the electroweak scale physics. An alternative to this approach is a proposed design of a first-level hardware trigger that uses tracking information. Being able to use tracking information at the first level of the ATLAS trigger in the implementation of a muon isolation algorithm offers an extra handle for differentiating between signal and background. The second part of this thesis presents studies on the performance of tracking-based muon isolation designed for a first-level hardware trigger system. These studies demonstrate the improved trigger performance of the muon isolation algorithm when compared to an increase of the transverse momentum threshold of the muon candidates.