BEGIN:VCALENDAR VERSION:2.0 CALSCALE:GREGORIAN PRODID:UW-Madison-Physics-Events BEGIN:VEVENT SEQUENCE:1 UID:UW-Physics-Event-8264 DTSTART:20230508T203000Z DTEND:20230508T220000Z DTSTAMP:20260409T005309Z LAST-MODIFIED:20230417T144937Z LOCATION:4274 Chamberlin SUMMARY:Investigation of Higgs Boson Decaying to Di-muon and Dark Matt er Produced in Association with a Higgs Boson decaying to Di-b-quark\, Graduate Program Event\, Jay Chan\, Physics Graduate Student DESCRIPTION:The discovery of the Standard Model (SM) Higgs boson by AT LAS and CMS at the LHC in 2012 marked a major milestone in particle ph ysics. However\, many questions remain unanswered\, which has led to a n active research program to search for either rare SM phenomena or Be yond Standard Model (BSM) physics that involve the Higgs boson. In thi s dissertation\, I present two example searches involving the Higgs bo son\, using proton-proton (pp) collision data collected by the ATLAS d etector.
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\nThe first search tackles the problem of how the SM Higgs couples to the second-generation fermions. It searches for th e dimuon decay of the SM Higgs boson (H →μμ) using data correspond ing to an integrated luminosity of 139 fb−1 collected by the ATLAS d etector in pp collisions at √s = 13 TeV at the LHC. To identify this rare decay\, we train boosted decision trees to separate signal and b ackground. We obtain an observed (expected) significance over the back ground-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 bra nching 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 as suming the absence (presence) of a SM signal is 1.1 (2.0). The best-fi t 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.
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\nIn the second search\, we look for Dark Matter produc ed in association with a Higgs boson decaying to b-quarks. This search uses the same dataset as the H → μμ search and targets events tha t contain large missing transverse momentum and either two b-tagged sm all-radius jets or a single large-radius jet associated with two b-tag ged subjets. We split events into multiple categories that target diff erent phase spaces of the Dark Matter signals. We do not observe a sig nificant excess from the SM prediction. We interpret the results using two benchmark models with two Higgs doublets extended by either a hea vy vector boson Z′ (Z′−2HDM) or a pseudoscalar singlet a (2HDM+a ) that provide a Dark Matter candidate χ. For Z′−2HDM\, the obser ved 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 Î ² = 10\, respectively\, and for a Dark Matter mass of 10 GeV. Addition ally\, 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 mo mentum and b-quark jet multiplicity.
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\nIn addition to the t wo physics analyses\, I present a new method to correct data for the d etector effect\, referred to as unfolding\, which is a key procedure i n the high energy experiments. This new unfolding method allows to unf old data without having any artificial binning and is also able to pro file nuisance parameters simultaneously\, which provides much higher f lexibility and increases the reusability for different downstream task s. It will benifit any future analyses including Higgs physics and Dar k Matter searches. URL:https://www.physics.wisc.edu/events/?id=8264 END:VEVENT END:VCALENDAR