Rare |∆c| = |∆u| = 1 transitions into dineutrinos are strongly GIM-suppressed and constitute excellent null tests of the standard model. While branching ratios of D → P ν ν, D → P + P − ν ν, P = π, K, baryonic Λ + c → p ν ν, and Ξ + c → Σ + ν ν and inclusive D → Xν ν decays are experimentally unconstrained, signals of new physics can be just around the corner. We provide model-independent upper limits on branching ratios reaching few ×10 −5 in the most general case of arbitrary lepton flavor structure, ∼ 10 −5 for scenarios with charged lepton conservation and few ×10 −6 assuming lepton universality. We also give upper limits in Z and leptoquark models. The presence of light right-handed neutrinos can affect these limits, a possibility that can occur for lepton number violation at a TeV, and that can be excluded with an improved bound on B(D 0 → invisibles) at the level of ∼ 10 −6 , about two orders of magnitude better than the present one. Signatures of c → uν ν modes contain missing energy and are suited for experimental searches at e + e − -facilities, such as BES III, Belle II and future e + e − -colliders, such as the FCC-ee running at the Z.

| c| = | u| = 1 processes are unique probes of flavor physics in the up-sector within and beyond the Standard Model (SM). SM tests with rare semileptonic charm meson decays are based on an approximate CP-symmetry, a superior GIM-mechanism, angular distributions, leptonuniversality and lepton flavor conservation. We analyze the complete set of null test observables in D → π ( ) and D s → K ( ) decays, ( ) = e, μ, and find large room for new physics safely above the SM contribution. We identify signatures of supersymmetry, leptoquarks and anomaly-free U (1) -models with generation-dependent charges, for which we provide explicit examples. Z -effects in c → u ( ) transitions can be sizable if both left-handed and right-handed couplings to quarks are present.

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Abstract$$U(1)^\prime $$ U ( 1 ) ′ extensions of the standard model with generation-dependent couplings to quarks and leptons are investigated as an explanation of anomalies in rare B-decays, with an emphasis on stability and predictivity up to the Planck scale. To these ends, we introduce three generations of vector-like standard model singlet fermions, an enlarged, flavorful scalar sector, and, possibly, right-handed neutrinos, all suitably charged under the $$U(1)^\prime $$ U ( 1 ) ′ gauge interaction. We identify several gauge-anomaly free benchmarks consistent with $$B_s$$ B s -mixing constraints, with hints for electron-muon universality violation, and the global $$b \rightarrow s$$ b → s fit. We further investigate the complete two-loop running of gauge, Yukawa and quartic couplings up to the Planck scale to constrain low-energy parameters and enhance the predictive power. A characteristic of models is that the $$Z^\prime $$ Z ′ with TeV-ish mass predominantly decays to invisibles, i.e. new fermions or neutrinos. $$Z^\prime $$ Z ′ -production can be studied at a future muon collider. While benchmarks feature predominantly left-handed couplings $$C_9^{\mu }$$ C 9 μ and $$C_{10}^{\mu }$$ C 10 μ , right-handed ones can be accommodated as well.