Thesis BELLE2-MTHESIS-2021-003

Search for an invisibly decaying Z' boson and study of particle identification at the Belle II experiment

Paul Feichtinger ; Gianluca Inguglia ; Christoph Schwanda

Atominstitut (ATI); Institut für Hochenergiephysik (HEPHY) Vienna

Abstract: The subject of this thesis is the search for a hypothetical Z' vector boson in events with two muons plus missing energy in the final state (e+e− → μ+μ− (Z' → invisible)) at the Belle II detector, located at the SuperKEKB accelerator in Tsukuba, Japan. The Z' arises through a natural extension of the Standard Model gauge group with a gauged Lμ − Lτ number. The resulting abelian interaction only couples to leptons of the second and third generation, thus hiding from most experiments only sensitive to first-generation leptons. This new particle could explain the discrepancies concerning the anomalous magnetic dipole moment of the muon, the tensions in b → sμ+μ− transitions observed by LHCb, and act as a mediator between the interactions of the Standard Model and the Dark Sector. After an initial separation of signal from background, an artificial neural network (ANN) is used for additional classification. It is optimised directly on a figure of merit designed to maximise the sensitivity of the search. Based on these results, the expected 90% confidence intervals on the coupling constant for a Z' with a mass up to 9 GeV/c2 are calculated. Furthermore, the particle identification of Belle II is studied with pions originating from one of the τ leptons in e+e− → τ+τ− events. This pion sample is then used to estimate the probability that pions are misidentified as electrons or muons. This probability is compared between simulation and collision data, and a correction factor is calculated, which can be used to account for discrepancies in the simulation.

Note: Presented on 16 03 2021
Note: MSc

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 Record created 2021-03-18, last modified 2021-04-07

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