Searches for New Physics using Missing Energy at the Belle II Experiment

Sumitted to PubDB: 2025-05-02

Category: Phd Thesis

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Principal Authors Huw Haigh, Gianluca Inguglia, Christoph Schwanda
Date 2024-08-07
Belle II Number BELLE2-PTHESIS-2025-012
Abstract This thesis presents a search for new physics processes using 362fb−1 of collected data produced at a centre-of-mass energy of 10.58 GeV at the Belle II experiment in Tsukuba, Japan. The studies described herein focus on events that feature missing energy, which could indicate the presence of an as yet undiscovered particle. Such an observation could shed light on gaps in current understanding of the universe, such as in the nature of dark matter. This work specifically investigates events in which a pair of muons are produced along with some undetected particle that induces a recoil in the muon pair. Searches are conducted for invisibly decaying Z′ and muonphilic scalar particles, both of which are expected to appear as resonant structures in the recoil mass distribution (Mrecoil). Data are interpreted under multiple theoretical models including various couplings to dark matter. Upper limits are calculated on the cross section and coupling strength for each model that is investigated, providing results that are competitive with, and expand on, previous experimental efforts. The study makes use of a neural network trained using the novel Punzi-loss function for rejection of background processes, the development and implementation of which is outlined. Furthermore a phenomenological reinterpretation of recent measurements related to the branching fraction of the B→K𝜈𝜈 ̄ decay process is conducted. Previous results from multiple sources are studied in conjunction under a light new physics model with an associated two body B→K𝑋 decay process, and constraints are thereby set on such a model.

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