Search for B⁺ → K⁺ ν ν̄ at Belle II

{We use a 362 fb⁻¹ sample of e⁺ e⁻ collisions at Υ(4S) resonance collected with Belle II detector at SuperKEKB collider to search for the rare decay B⁺ → K⁺ ν ν̄. The main strategy consists in exploiting the inclusive properties of the other B to suppress the background. Another analysis is based on a conventional hadronic reconstruction of the accompanying B meson and is used to corroborate the first strategy. Both the procedures have been validated with several control samples. A maximum likelihood fit is used to extract the branching ratio, which results in [2.7± 0.5(stat) ± 0.5 (syst)] × 10⁻⁵ for the main analysis and [1.1+0.9-0.8(stat)+0.8-0.5 (syst)] × 10⁻⁵ for the support analysis. The combination of the two analyses gives a branching fraction of [2.3 ± 0.5(stat)+0.5-0.4 (syst)] × 10⁻⁵ which corresponds to the first evidence of the decay with 3.5 standard deviations and 2.7 standard deviations above the standard model expectation.