Decays mediated by flavor-changing neutral currents are suppressed in the standard model (SM) and provide an indirect route to search for new physics because beyond-the-SM particles may enter via loop diagrams. The 𝐵 0 → 𝐾 0𝜋 0 decays comprise an important example, which allow us to probe CP violation either directly in the decays (𝒜CP), or indirectly due to interference between decays with and without 𝐵 0–𝐵̅0 mixing (𝒮CP). The CKM and color suppression of the tree-level 𝑏 → 𝑢𝑠𝑢̅ transition implies that 𝐵 0 → 𝐾 0𝜋 0 decays are dominated by the top-quark mediated 𝑏 → 𝑠𝑑𝑑̅loop diagram, which carries a weak phase arg(𝑉𝑡𝑏𝑉𝑡𝑠 ∗ ); 𝑉𝑖𝑗 denote the CKM matrix elements. Neglecting subleading contributions to the decay amplitude, 𝒮CP is expected to be equal to sin 2𝛽 and 𝒜CP ≈ 0. Possible new physics contributions to the amplitude can introduce deviations from the expected values. Furthermore, 𝐵 0 → 𝐾 0𝜋 0 decays are the key in testing an isospin sum rule that relates all four 𝐵 → 𝐾𝜋 decays. We report measurements of branching fraction and direct CP-violating asymmetry 𝒜CP in the charmless decay 𝐵 0 → 𝐾𝑆 0𝜋 0 performed using 189.8 fb −1 of e +e − collision data recorded at the Υ(4𝑆) resonance by the Belle II experiment. This is the first measurement of 𝒜CP in 𝐵 0 → 𝐾 0𝜋 0 conducted at Belle II relying on a time-dependent analysis.
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