Thesis defense of Marcel Golz

This thesis comprises a study of rare semileptonic |Δc|=|Δu|=1 transitions in the Standard Model and beyond. The Standard Model phenomenology of three-body meson decays D→ Pll, as well as three- and four-body charmed baryon decays B0 → B1 (→ B2 π) ll, is analyzed and branching ratios are found to be dominated by resonances. Physics effects beyond the Standard Model are studied model-independently as well as in the framework of leptoquark- and flavorful, anomaly-free Z’-models. Resonance dominance is shown to be overcome or even used as a catalyst in analyses of clean null-test observables, where any signal indicates new physics. Null tests are based on angular observables, CP-conservation, lepton flavor universality and charged lepton flavor conservation. Already a subset of four angular observables in three- and four-body rare charm baryon decays are sufficient to disentangle beyond Standard Model effects in dipole and (axial) vector Wilson coefficients, whereas (semi-)leptonic rare meson decays D→ (P)ll are shown to control (pseudo-)scalar and (pseudo-)tensor effects. A global view on connections between different flavor sectors is put forward in the context of flavor summed dineutrino branching ratios. Indirect tests of lepton flavor universality and charged lepton flavor conservation are presented for the charm sector, where dineutrino modes already pose null tests of the Standard Model. In |Δb|=|Δs|=1 transitions, the first evidence for the violation of lepton flavor universality is found in muon over electron ratios of B-meson branching fractions. Complementing the direct probes, dineutrino modes are shown to test charged lepton flavor indirectly in the correlation of B → P ν ν and B → V ν ν. Similarly, indirect flavor probes in other quark sectors are also commented on.