The first limit on invisible decays of $B_s$ mesons comes from LEP

Abstract

Motivated by the recent evidence for $B^{+}\to K^{+}\bar{\nu }\nu$ decays at Belle II, we point out that fully invisible $B_d$ and $B_s$ meson decays are strongly constrained by LEP. A reinterpretation of an old inclusive ALEPH search for $b$-hadron decays with large missing energy allows us to place the limits $\mathrm{Br}(B_d\to \mathrm{invisible})<1.4\times 10^{-4}$ and $\mathrm{Br}(B_s\to \mathrm{invisible})<5.6\times 10^{-4}$, both at $90\%$ CL. The $B_d$ limit is only a factor of 6 looser than the world-leading one provided by the BaBar collaboration, while the $B_s$ one is the first limit in the literature on this decay mode. These results are relevant in the context of new light states coupled to quarks and exemplify the power of a future Tera-$Z$ factory at FCC-ee to look for $B$ meson decays containing missing energy.

arXiv comment3.5 pages + references. v2: minor improvement on the analysis by using mean PDG value of B fragmentation ratios. Added appendix with recasted ALEPH bounds on partially invisible flavor-violating B decays such as B -> tau a and B -> rho a, with a being an axion-like particle. v3: matches published version. Added clarifications about the analysis. We now use the CLs method to derive the bounds

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