Thesis BELLE2-PTHESIS-2023-003

Search for muonic dark force and performance studies of lepton identification at Belle II

Rajesh Kumar Maiti ; Gianluca Inguglia

Institut of High Energy Physics Vienna

Abstract: This thesis presents an advanced search strategy to find a new dark gauge boson 𝑍′ and performance studies of particle identification to assist the new physics searches, especially involving flavor-changing charge and neutral currents in semi- leptonic 𝐵 decays. The data used or planned for these measurements are gathered at the Belle II experiment, located at the SuperKEKB asymmetric 𝑒+𝑒− collider in Tsukuba, Japan. The existence of 𝑍′ has been predicted by extending the standard model lagrangian with a new abelian gauge symmetry, which couples mainly to the second and third generation of leptons. The decay of 𝑍′ into muons in the event topology 𝑒+𝑒− → 𝜇+𝜇−𝑍′, 𝑍′ → 𝜇+𝜇− has been investigated model dependently in the prompt decays and model independently in the displaced decays for the first time. The 𝑍′ mass phase space 0.212 – 9 GeV has been explored in both cases. A 90% CL on the coupling constant 𝑔′ has been determined with a targeted luminosity of 178.47 fb−1 for the prompt case. These bounds are similar to the BaBar measurement except for the low mass regions, with a ∼3 times less dataset. A 90% CL on the production cross-section for the displaced search is also provided at a level of below 0.1 fb for different lifetime scenarios under consideration with a targeted luminosity of 200 fb−1. The expected results are based on simulation only. This thesis also presents an advanced particle identification (PID) method using different machine learning techniques and comparative studies between them. The developed algorithm provided overwhelming improvements in PID, especially for the low momentum particle tracks where backgrounds are more severe. Finally, some trigger studies related to dark sector searches are also provided.

Note: Presented on 20 01 2023
Note: PhD

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 Record created 2023-02-01, last modified 2023-02-01

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