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Signal proxy and radiative Bhabha background studies : contributions to the search for invisibly decaying dark photons at Belle II Crook, Daniel Weston Mckay


The nature of dark matter is one of the greatest extant mysteries in physics. Many hypotheses have been proposed to explain dark matter, but none have ever been substantiated. Some of these hypotheses propose dark sectors, i.e., entire families of dark matter particles. One such hypothesis is that of the dark photon, a massive, gauge boson which would be the mediator of the dark electromagnetic force. The dark photon would couple feebly to the photon, and thereby be producible through e⁺e⁻ annihilation at a particle collider. Described here are contributions toward a search for the dark photon at Belle II, an electron-positron collider experiment in Japan. This search focuses on invisible decays of the dark photon, with a single initial-state radiation photon serving as the only evidence of the production and decay. Due to the delicate nature of this event signature, the dark photon search requires a carefully constructed set of event requirements and vetoes that remove as much background as possible without removing a large amount of the signal. To further this goal, two control samples were designed. One control sample is of a clean radiative dimuon sample, used as a signal proxy, while the other is of the radiative Bhabha background. These control samples are then used to design vetoes and selection criteria for the search for the dark photon at Belle II, termed the single-photon analysis.

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