Some measurements take a career. The one announced at CERN on Tuesday took several. Physicists working on the LHCb experiment reported the first solid evidence from a single detector that a particle called the Bd meson can break apart into two muons, the heavier cousins of electrons, something theory says should happen roughly once for every ten billion such particles.
Why chase anything that rare? A Bd meson pairs a beauty quark with a down-type antiquark and lives for only about one and a half trillionths of a second. In these ultra-rare decays the quark and antiquark annihilate, and any heavy, still-unknown particles lurking in the quantum background could nudge the rate up or down. That makes the channel a sensitive probe for physics beyond the Standard Model. Its sister process in the Bs meson is about thirty times more common and was observed in 2015.
The key was hardware plus software. An upgraded LHCb detector has, since 2024, been recording collisions five times faster and telling particle types apart more cleanly, while machine-learning tools strip away a flood of look-alike background events. The 2024–25 data alone gave a signal of 3.0 sigma; folding in the older 2011–18 runs nudged it to 3.2.
That clears the bar for “evidence” but not for a discovery, which needs five sigma. A joint LHCb and CMS hint from 2015 faded when more data came in, so the team is being careful. The paper has been submitted but not yet peer-reviewed.
For now the measured rate sits right where theorists expected, so no sign yet of exotic new particles, Nikhef physicist Mulder stressed. Cambridge’s Harry Cliff, whose group ran the analysis, described the signal as “incredibly difficult” to pin down. If five sigma comes, no beauty decay on record would be rarer.
