Abstract 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 ^+ ^- D 0 → π + π - ν ν ¯ and in polarized c^+ p ^+ ^- Λ c + → p ℓ + ℓ - decays with =e, ℓ = 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 ^+ ^- D 0 → π + π - ν ν ¯ decays and work out the phenomenology of polarization-induced null-test observables in c^+ p ^+ ^- Λ c + → p ℓ + ℓ - decays. In regions of dilepton mass near the ϕ resonance, polarization asymmetries can reach O (5 \%) O (5 %) for muons and O (14 \%) O (14 %) for electrons times the c^+ Λ c + 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 ∼ 2 × <mml: m
Canto et al. (Mon,) studied this question.