New opportunities for rare charm from Z→ cc̄ decays

We analyze the potential of rare charm decays as probes of new physics at a high-luminosity flavor facility operating at the Z pole, such as the FCC-ee or CEPC. In particular, we identify clean null-test observables in D⁰ → π⁺ π⁻ νν and in polarized Λc⁺ → p ⁺ ⁻ decays with =e, μ. Complementarity with the LHC and HL-LHC flavor programs arises from the characteristic features of a Tera-Z environment: the capability to study missing-energy modes and charm production with significant polarization. We improve the theoretical description of D⁰ → π⁺ π⁻ νν decays and work out the phenomenology of polarization-induced null-test observables in Λc⁺ → p ⁺ ⁻ decays. In regions of dilepton mass near the $ϕ$ resonance, polarization asymmetries can reach $O(5 %)$ for muons and $O(14 %)$ for electrons times the Λc⁺ polarization. We also point out synergies between the dineutrino and the dilepton modes using the SMEFT framework of heavy new physics. Using the IDEA detector concept at FCC-ee, we find in simulation studies that dineutrino branching fractions as low as ~ 2 × 10⁻⁷ can be probed, which reaches well into the parameter space of new physics, and also allows for discrimination of lepton flavor structures. Furthermore, the measurement of asymmetries in Λc⁺ → p μ⁺ μ⁻ at $O(1 %)$ will be possible. Similar sensitivities are expected for dielectron final states, although robust predictions will require further dedicated studies.