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Events on Monday, May 8th, 2023

Academic Calendar
Final exams
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Graduate Program Event
PhD Final Defense
Higgs production with a top quark pair in the diphoton channel, di-Higgs production in the bb-diphoton channel, and Inner Tracker upgrade with the ATLAS detector at the Large Hadron Collider
Time: 2:00 pm - 3:30 pm
Place: 4274 Chamberlin
Speaker: Alex Zeng Wang, Physics Graduate Student
Abstract: Since the discovery of the Higgs boson in 2012, the diphoton (γγ) decay channel of the Higgs boson has been one of the most potent channels due to the contrast between the smoothly falling background and sharply peaked signal in the diphoton invariant mass.

In 2018, with up to 80 fb−1 of data at √s = 13 TeV, ATLAS observed Higgs boson production in association with a pair of top quarks (ttH) with a significance of 5.8σ by combining measurements in the γγ, bb, ZZ, and multi-lepton Higgs decay channels. The ttH production cross-section was measured to be 670 ± 90 (stat) +110 -100 (syst) fb. The diphoton channel was one of the main contributors to this result, alone providing a significance of 4.1σ.

With 140 fb−1, a search for non-resonant Higgs pair production in the bbγγ final state was performed. No significant signal was observed and upper limits at 95% confidence level were set. The observed limit on the SM cross-section was 130fb, or 4.2 times the predicted value. The observed Higgs trilinear coupling modifier was constrained to be between [-1.5, 6.7].

Both the ttH (H → γγ) and HH → bbγγ analyses will benefit tremendously from the increased statistics expected from the High-Luminosity LHC (HL-LHC). To ensure the continued efficiency of the detector in the harsh HL-LHC environment, ATLAS will install a new Inner Tracker (ITk) consisting of silicon pixel sensors in its innermost layer. At SLAC National Accelerator Laboratory, a variety of electrical tests are performed for the construction of a prototype integrated pixel system, in order to provide early feedback and validation of the ITk design.
Host: Professor Sau Lan Wu
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Graduate Program Event
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
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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