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PhD Final Defense
Investigation of Higgs Boson Decaying to Di-muon and Dark Matter Produced in Association with a Higgs Boson decaying to Di-b-quark
Date: Monday, May 8th
Time: 3:30 pm - 5:00 pm
Place: 4274 Chamberlin
Speaker: Jay Chan, Physics Graduate Student
Abstract: The discovery of the Standard Model (SM) Higgs boson by ATLAS and CMS at the LHC in 2012 marked a major milestone in particle physics. However, many questions remain unanswered, which has led to an active research program to search for either rare SM phenomena or Beyond Standard Model (BSM) physics that involve the Higgs boson. In this dissertation, I present two example searches involving the Higgs boson, using proton-proton (pp) collision data collected by the ATLAS detector.

The first search tackles the problem of how the SM Higgs couples to the second-generation fermions. It searches for the dimuon decay of the SM Higgs boson (H →μμ) using data corresponding to an integrated luminosity of 139 fb−1 collected by the ATLAS detector in pp collisions at √s = 13 TeV at the LHC. To identify this rare decay, we train boosted decision trees to separate signal and background. We obtain an observed (expected) significance over the background-only hypothesis for a Higgs boson with a mass of 125.09 GeV of 2.0σ (1.7σ). The observed upper limit on the cross-section times branching ratio for pp →H →μμ is 2.2 times the SM prediction at 95% confidence level, while the expected limit on a H → μμ signal assuming the absence (presence) of a SM signal is 1.1 (2.0). The best-fit value of the signal strength parameter, defined as the ratio of the observed signal yield to the one expected in the SM, is μ = 1.2 ±0.6.

In the second search, we look for Dark Matter produced in association with a Higgs boson decaying to b-quarks. This search uses the same dataset as the H → μμ search and targets events that contain large missing transverse momentum and either two b-tagged small-radius jets or a single large-radius jet associated with two b-tagged subjets. We split events into multiple categories that target different phase spaces of the Dark Matter signals. We do not observe a significant excess from the SM prediction. We interpret the results using two benchmark models with two Higgs doublets extended by either a heavy vector boson Z′ (Z′−2HDM) or a pseudoscalar singlet a (2HDM+a) that provide a Dark Matter candidate χ. For Z′−2HDM, the observed limits extend up to a Z′ mass of 3.1 TeV at 95% confidence level for a mass of 100 GeV for the Dark Matter candidate. For 2HDM+a, we exclude masses of a up to 520 GeV and 240 GeV for tan β = 1 and tan β = 10, respectively, and for a Dark Matter mass of 10 GeV. Additionally, we set limits on the visible cross sections, which range from 0.05 fb to 3.26 fb, depending on the regions of missing transverse momentum and b-quark jet multiplicity.

In addition to the two physics analyses, I present a new method to correct data for the detector effect, referred to as unfolding, which is a key procedure in the high energy experiments. This new unfolding method allows to unfold data without having any artificial binning and is also able to profile nuisance parameters simultaneously, which provides much higher flexibility and increases the reusability for different downstream tasks. It will benifit any future analyses including Higgs physics and Dark Matter searches.
Host: Sau Lan Wu
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