Is the Belle II $B\,\mathrm{to}\,K\,\nu\,\nu$ excess real?
In plain words
Belle II found that B mesons decay to a kaon plus invisible particles more often than expected, about four times the predicted rate. It could be new particles or a fluctuation.
Precise statement
Belle II (2023) measured BR(B+ -> K+ nu nubar) = (2.3 +- 0.7) x $10^{-5}$, $2.7\,\sigma$ above the SM prediction ($\sim 0.5 \times 1e-5$). Determine whether the excess persists at $\ge 5\,\sigma$ and whether it shows a peak in invisible mass from a new light particle.
What would settle it
Belle II with about 5 to 10 $\mathrm{ab}^{-1}$ and NA62-style rare kaon decays for related b to s and s to d invisible channels.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
The $2.7\sigma$ excess is still the latest measurement as of 2026.
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