A Flavour Search for Invisible Particles at Belle

Sumitted to PubDB: 2026-10-29

Category: Phd Thesis

Tags: Belle

Principal Authors Daniel Marcantonio
Date 2025-03-28
Belle II Number BELLE2-PTHESIS-2026-028
Abstract This thesis presents searches for five $B$ meson decays of the form $B\to h\alp$, where $h$ is a Standard Model hadron ($h\in\pipm,\Kpm,p^\pm,\Dspm,\Dzb$) and $\alp$ is an invisible particle with a narrow decay width. There are several extensions to the Standard Model that predict this type of decay, and yet, to date, there have been few direct searches for such processes. Every fundamental particle predicted by the Standard Model since its inception has been found through a series of particle colliders with ever-increasing discovery power. From the 1950s to the 2010s, a new fundamental particle was discovered in every decade. The discovery of the Higgs boson in 2012 marked the end of this era, thus completing the Standard Model. However, the unknown particle nature of dark matter, the strong $CP$ problem, the baryon asymmetry of the universe, and the fact that neutrinos have mass constitute incredibly compelling evidence that our current understanding of the universe is incomplete. Therefore, the Standard Model is not a complete description of nature, and a new theory or an extension of the Standard Model is required. The discovery of any new particle not explained by the Standard Model would constitute evidence for new physics. Since the discovery of the Higgs boson there has been a proliferation of theories and opinions surrounding the most likely place to find the next fundamental particle, one that may bridge the gap between the sector of known particles and the unknown. This work is the first-ever search for most of these decays. This analysis finds no evidence for new particles in any of the five decay channels studied. Upper limits on the branching fractions of these decays have been established. When mapped onto the couplings between known particles and particles from new physics models, these branching fraction limits provide the most stringent constraints to date.
Institute Melbourne

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