CERN Detects Ultra-Rare B Meson Decay, Opening Path to Zeptometer-Scale Physics

Researchers at the Large Hadron Collider have recorded an exceptionally rare decay of a neutral B meson into a muon and an antimuon, a process predicted to occur only once in about 10 billion decays. The observation, made possible by upgrades to the collider in 2024 and advanced machine-learning analysis, marks the first experimental evidence of this phenomenon and offers a new probe of physics at distances measured in zeptometers, or one quadrillionth of a meter.

The decay was identified among the myriad particles produced when protons collide at near-light speed. While mesons routinely transform into other particles, the specific muon-antimuon channel is so unlikely that it has eluded detection for more than four decades.

"Physicists have been chasing this decay for more than 40 years," said Marco Santimaria, co-lead of the international analysis team, in a CERN press release. "Extreme precision is required to pick the muons out of the overwhelming background."

CERN noted that the 2024 upgrades increased data-collection speed and that new machine-learning techniques helped suppress background processes that would otherwise mask the rare signal. The results, currently consistent with the Standard Model, are detailed in a preprint posted to arXiv.

The discovery opens a pathway to explore the so-called “zeptoverse,” where distances are measured in zeptometers—about 100,000 million zeptometers across a typical atom. While the Planck length lies far beyond this scale, accessing the zeptometer regime could eventually shed light on conditions moments after the Big Bang.

Further measurements will test whether any deviations from existing theories emerge at these unprecedentedly small scales.