British-led researchers using CERN’s Large Hadron Collider have detected the rarest decay of a subatomic beauty particle ever seen, achieving evidence for the rare process known as Bd to mu mu and offering a first glimpse into a strange subatomic realm called the zeptoverse.
After a search lasting more than four decades, a Cambridge-led team of physicists has found evidence for the rarest decay of a subatomic beauty particle ever seen. Working at the upgraded LHCb experiment at CERN’s Large Hadron Collider near Geneva, the researchers studied beauty quarks, which are exotic cousins of the up and down quarks that make up all atomic matter in the universe.
When created in high-energy collisions at the LHC, beauty particles live for just one and a half trillionths of a second before decaying. The way they decay allows scientists to probe distance scales as small as trillionths of billionths of a metre.
Probing the Zeptoverse at CERN
The recent findings center on an incredibly rare decay where a beauty particle turns into two smaller particles called muons. According to Harry Cliff from the University of Cambridge’s Cavendish Laboratory, who led the analysis team, This process only happens for one in every ten billion beauty particles, so it’s incredibly difficult to find.
Physicists have been looking for this rare type of decay since 1984, which was before I was born.
Harry Cliff, University of Cambridge’s Cavendish Laboratory
This marks the first time evidence for the Bd to mu mu decay has been achieved by a single experiment. The results were presented at CERN, following decades of searching that began in 1984. By combining data collected between 2011 and 2018 with a massive new sample from 2024 and 2025, the researchers pushed the signal strength to 3.2 sigma, above the ‘three sigma’ level of statistical significance, the accepted evidence threshold for a new phenomenon.
Machine Learning and Future Upgrades
Detecting the signal required advanced technology. A major upgrade to the LHCb experiment in 2024 equipped the facility to distinguish different types of particles. Researchers used machine learning techniques to suppress background noise that otherwise would have buried the rare signals.
The LHC is powerful enough to study reality down to a hundred zeptometres in search of undiscovered particles hiding in the zeptoverse, a region where particles might help explain mysteries like dark matter and the origins of matter during the Big Bang. While the new measurements agree with Standard Model predictions, future upgrades could offer even greater precision. Tim Gershon from the University of Warwick, who leads the LHCb experiment, emphasized the path forward.
This fantastic result showcases the extraordinary capabilities of LHCb to probe nature at the fundamental level. This project, driven and led by UK physicists, is producing beautiful science. My focus is now on delivering the upgrade of LHCb planned for the 2030s, which will be essential if we are to continue on this journey of discovery.
Tim Gershon, University of Warwick