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| Dissertation / PhD Thesis | PUBDB-2022-00933 |
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2021
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Please use a persistent id in citations: urn:nbn:de:gbv:18-ediss-97146 doi:10.3204/PUBDB-2022-00933
Report No.: CERN-THESIS-2021-232
Abstract: This thesis presents the first evidence of Higgs boson decay to two leptons and a photon. Rare Higgsboson decays predicted in the Standard Model (SM) presently come within experimental reach thanksto the large amount of data collected by the experiments at the Large Hadron Collider (LHC) atCERN. Given a central role played by the Higgs boson in the SM, studying its rare decays opens up apossibility for more stringent tests of the SM.A very rare decay of Higgs boson to a low mass electron or muon pair and a photon is exploredin this thesis using 139 fb−1of pp collision data at a center-of-mass energy √s = 13 TeV recordedwith the ATLAS detector at the LHC during Run 2 (2015-2018). In the phase space with lepton pairinvariant mass m`` < 30 GeV (l = e, μ) the expected SM branching ratio is B(H → ``γ) ≈ 10−4. Thelow branching ratio presents an experimental challenge, with the expected number of backgroundevents vastly exceeding the number of signal H → ``γ events.Another experimental challenge of the analysis is that in the low lepton pair mass regime the twoleptons tend to be highly collimated. For the electrons, their energy deposits in the calorimeter of theATLAS detector often remain unresolved by the standard reconstruction algorithms. To overcome this,a new identification algorithm is used, based on multivariate discriminant. The new algorithm morethan doubles the number of selected eeγ events compared to the standard reconstruction algorithms.In addition, collimated leptons often do not satisfy standard criteria on additional activity in theirvicinity (isolation), which is imposed to reduce backgrounds. Isolation criteria are therefore correctedtaking into account contribution of leptons to each other’s isolation.A combined statistical model is constructed using parametric functions describing signal andbackground m``γ distributions in each analysis category. The observed signal yield is extracted fromthe fit of this model to data. An excess is observed over the background-only hypothesis with asignificance of 3.2 standard deviations. The best-fit ratio of the observed event yield to the SMexpectation is 1.5 ± 0.5. The fiducial cross-section times branching ratio in the m`` < 30 GeV regionis measured at σ(H) × B(H → ``γ) = 8.7+2.8−2.7 fb. The analysis is still limited by the small numberof expected events and the systematic uncertainties constitute only 35% of the statistical uncertainty.Among systematic uncertainties, the uncertainty associated with a bias in background function choiceis dominating. The impact of systematic uncertainties on the results is studied in detail and the resultsare additionally verified using pseudo-experiments.
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