Physicists have been chasing this decay for more than 40 years,
Marco Santimaria, who co-led the international analysis team, said in a press release from CERN.
After a search spanning more than four decades, researchers working at the Large Hadron Collider have captured the first evidence of one of the rarest particle decays ever recorded. Announced by the international LHCb collaboration, the breakthrough involves the decay of a neutral B meson—specifically a beauty particle known as a neutral B0 meson—into a muon and an antimuon. This process is predicted to occur only once in every 10 billion such decays.
The achievement offers a fresh method to test foundational theories while opening a pathway toward exploring scales of distance previously beyond reach.
Measuring Tiny Distances to Test Fundamental Physics Theories
The observation brings scientists closer to exploring what physicists term the “zeptoverse,” a scale of extraordinarily small distances measuring around a billionth of a trillionth of a meter, or a zeptometer.
Within this scale, unknown particles and forces of nature may remain hidden. The new measurements align with the predictions of the Standard Model, the leading theory describing fundamental particles and their interactions. Studying such improbable processes with greater precision could eventually reveal where the model begins to break down. According to The University of Manchester, researchers found that the results are consistent with current physics predictions, yet future observations could expose unique phenomena residing beyond our present knowledge.
The decay process, often written as B0 to mu, involves a beauty quark. These heavy quarks are produced in massive quantities during proton collisions at CERN’s collider but survive for only a fraction of a second before transforming into other particles. Because the decay probability is strongly suppressed—an effect linked to CKM matrix elements—finding the signal requires immense precision. The recent analysis reached a statistical significance of 3.2 sigma. This constitutes the first evidence of the decay in a single experiment, though it falls short of the five-sigma threshold required to officially claim a full discovery.
Along with the neutral B meson decay, the team also produced the most precise measurements yet of another rare decay involving a Bs0 meson, which was first observed by LHCb in 2015.
Upgrading Collider Detectors to Isolate Rare Particle Collisions
Observing an event that occurs only once in 10 billion required overcoming immense background interference from other collision products. Upgrades made to the collider and the LHCb experiment enabled the discovery.
At the center of the instrumentation is the Vertex Locator, a sophisticated silicon pixel detector assembled by researchers at the University of Liverpool and The University of Manchester. Sitting just five millimeters from the particle beams, the detector tracks particle paths with extreme accuracy, allowing scientists to pinpoint precisely where beauty particles are created and where they decay.
The hardware enhancements enabled researchers to collect data at up to five times the previous rate. Coupled with advanced machine-learning techniques used to suppress vast backgrounds of unrelated collision processes, the collaboration successfully extracted the elusive signals.
Professor Chris Parkes of The University of Manchester noted that the upgraded experiment culminated from a 15-year design and construction effort involving a 1,900-member international collaboration, with the United Kingdom providing the largest national contribution.
Planning Future Data Collection
The upgraded experiment is scheduled to continue collecting a much larger dataset into the early 2030s, which will allow physicists to study the rare decay with steadily increasing precision. Further upgrades are also planned that could increase data collection rates by another factor of ten.
Another rare decay involving a Bs0 meson was first observed by LHCb in 2015.
Earlier coverage: Physicists Detect Evidence of 1 in 10 Billion Particle Decay