Promotionsvortrag von Piera Battista

Indirect searches for physics beyond the Standard Model (SM) provide a powerful probe of New Physics (NP) through virtual contributions to loop-level processes. The b → sℓ+ℓ− electroweak penguin transitions have been studied extensively by LHCb, which has revealed several tensions with SM predictions. To further investigate these effects, this thesis studies the rarer b → dℓ+ℓ− transitions, whose branching fractions are approximately 0.04 rarer than those of b → sℓ+ℓ− decays, thereby increasing their sensitivity to higher NP scales. The focus is on the baryonic decay modes Λ0 b → pπ−μ+μ− and Ξ0b → pK−μ+μ−. Baryonic transitions provide information complementary to mesonic decays, as their spin structure offers sensitivity to NP effects inaccessible in meson systems. Using the full LHCb Run 1 and Run 2 datasets, an updated measurement of the Λ0b → pπ−μ+μ− branching fraction is performed, together with the first measurement of its differential branching fraction. A feasibility study of the observation of the Ξ0b → pK−μ+μ− decay is also presented. This mode has not yet been observed due to both the suppression of the underlying electroweak penguin transition and the expected production fraction of Ξ0b → pK−μ+μ− baryons, which is approximately an order of magnitude smaller than that of Λ0b. The yield ratio R between the Λ0 b → pK−J/ψ and Ξ0 b → pK−J/ψ channels is estimated and used to predict the expected Ξ0b → pK−μ+μ− signal yield in the full Run 1 and Run 2 dataset. The predicted yield is found to be insufficient for a statistically significant observation. LHCb is expected to operate until at least 2041 and will undergo a major upgrade during Run 5 as part of the High-Luminosity LHC programme. The inner region of the current SciFi tracking detector will be replaced by a pixel detector, while the outer region (referred to as the Mighty-Scifi) will retain the scintillating fibre technology. This thesis also describes the recommissioning of a quality-assurance test bench for measuring the light yield of Mighty-SciFi fibre mats during production. The system is now fully operational and ready for the start of fibre mat production, scheduled after the summer of 2026.