Are $b \text{ to } s \mu \mu$ tensions new physics or charm-loop effects?
In plain words
After a well-known hint was withdrawn in 2022, some B decays to muon pairs still occur less often, or with different angular patterns, than predicted. Whether hard-to-compute strong-force effects explain this is disputed.
Precise statement
Branching ratios of $B \to K \mu \mu$ and $B_{s} \to \phi \mu \mu$ at low $q^{2}$ and the angular observable $P5'$ in $B \to K* \mu \mu$ deviate from SM predictions by about $2 \text{ to } 4 \sigma$, while $R_{K}$ and $R_{K}*$ agree with the SM (LHCb, 2022). Identify whether a shift in the Wilson coefficient $C_{9}$ is short-distance new physics or long-distance c-cbar loops.
What would settle it
Data-driven determination of the $q^{2}$ dependence of non-local charm contributions from LHCb Run 3 and Belle II, combined with lattice form factors.
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