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Physicists have measured how a B meson transforms into a muon and a neutrino, setting new limits on elusive particles.
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Inside the accelerator, particles collide, and for a fleeting moment a B meson is born — a heavy particle that lives for less than a billionth of a second. Usually, it decays into a shower of debris, but sometimes it transforms into just two particles: a muon and a neutrino. This rare path is like a lottery where the winning ticket appears once in every two million draws.
Physicists at the Belle and Belle II facilities collected data for years to catch such events. They looked for muons flying out of the collision point and used their energy to infer how much energy the invisible neutrino carried away. The measured decay probability is (4.4 ± 1.9 ± 1.0) × 10⁻⁷ — meaning that out of ten million B mesons, only about four decay this way. The precision is still limited, but this is the most stringent result to date.
Neutrinos barely interact with matter: stopping one would require a lead slab a light-year thick.
The obtained number constrains theories that predict additional types of Higgs bosons — particles responsible for mass. In addition, the scientists searched for sterile neutrinos — hypothetical particles that do not participate in the weak interaction at all. Such particles could carry away energy, altering the decay pattern. No signal was found, but new limits were set on their properties: if sterile neutrinos with masses up to 1.5 GeV exist, they mix with ordinary neutrinos extremely weakly.
The study also refined another B meson decay, where instead of a muon, an electron or a muon is produced along with other particles. There, the probability is higher — about 2.7 × 10⁻⁴, and the measurement agrees with Standard Modelpredictions. All in all, it resembles sifting sand for gold nuggets: most events are ordinary background, but rare spikes reveal the deep laws of the microcosm.