HEP In the literature: open

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 1e-5, $2.7\,\sigma$ above the SM prediction ($\sim 0.5 x 1e-5$). 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

The $2.7\sigma$ excess is still the latest measurement as of 2026.