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.
We analyze rare charm baryon decays within the standard model and beyond. We identify all null test observables in unpolarized Λc→ pℓ+ℓ−, ℓ = e, μ decays, and study the new physics sensitivities. We find that the longitudinal dilepton polarization fraction FL is sensitive to electromagnetic dipole couplings $$ {C}_7^{\left(\prime \right)} $$ C 7 ′ , and to the right-handed 4-fermion vector coupling $$ {C}_9^{\left(\prime \right)} $$ C 9 ′ . The forward-backward asymmetry, AFB, due to the GIM-suppression a standard model null test already, probes the left-handed axialvector 4-fermion coupling C10; its CP–asymmetry, $$ {A}_{\mathrm{FB}}^{\mathrm{CP}} $$ A FB CP probes CP-violating phases in C10. Physics beyond the standard model can induce branching ratios of dineutrino modes Λc→ pν$$ \overline{\nu} $$ ν ¯ up to a few times 10−5, and one order of magnitude smaller if lepton flavor universality is assumed, while standard model rates are negligible. Charged lepton flavor violation allows for striking signals into e±μ∓ final states, up to 10−6 branching ratios model-independently, and up to order 10−8 in leptoquark models. Related three-body baryon decays Ξc→ Σℓℓ, Ξc→ Λℓℓ and Ωc→ Ξℓℓ offer similar opportunities to test the standard model with |∆c| = |∆u| = 1 transitions.
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