At the SuperKEKB accelerator, electrons collide with positrons, and among the debris, B-mesons are found. These heavy particles live briefly and break apart into lighter ones. Recently, the Belle II detector recorded a suspiciously high number of cases where a B-meson transforms into a K-meson (kaon) and something invisible—2.7 sigma more than expected. It's like a baker weighing loaves and consistently getting 30 grams over the norm — seems trivial, but it's systematic.
There's also a second, long-known anomaly: B-meson decays into pions and kaons behave differently from Standard Model predictions. The authors propose a unified explanation — a light axion-like particle (ALP) with a mass close to that of the neutral pion (about 135 MeV). This particle is like a pion's twin, but made of different 'dough': it doesn't feel the strong force, yet it can mix with the pion and change the rules of the game. As a result, the B-meson decay into a pion and a kaon gets a hidden boost, as if a pinch of a new spice were added to the recipe, altering the flavor.
The model doesn't just patch holes: it makes concrete predictions. For rare kaon decays (K⁺ → π⁺ + invisible) and for the muon's magnetic moment, the effect should be noticeable in upcoming experiments. But for the electron and b-quark decays into an electron-positron pair, there's almost no effect — everything stays as before. If measurements confirm these numbers, physicists will not only solve two puzzles but also find a trace of a new particle that was hiding right under their noses, disguised as a familiar pion.
The neutral pion lives only 8.5×10⁻¹⁷ seconds — in that time, light travels a distance a hundred times smaller than the thickness of a human hair. A hypothetical ALP with a similar mass could live much longer, traveling through a detector unnoticed.