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 ⁻ ν ν ̄ D 0 → π + π - ν ν ¯ and in polarized c⁺ → p ⁺ ⁻ Λ c + → p ℓ + ℓ - decays with =e, μ 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 → π ⁺ π ⁻ ν ν ̄ 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 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