Physicists from the University of Cambridge have announced the discovery of evidence for an incredibly rare decay of a subatomic ‘beauty’ particle, observed at CERN’s Large Hadron Collider. This finding could provide clues about phenomena such as dark matter and the origins of matter after the Big Bang.
The team, working at the upgraded LHCb experiment, studied beauty quarks, which are exotic cousins of the quarks that form atomic matter. These beauty particles decay within 1.5 trillionths of a second, offering insights into physics at extremely short distance scales.
The observed decay, known as ‘Bd to mu mu’, involves a beauty particle transforming into two smaller particles called muons. This process occurs in only one in ten billion beauty particles, making it exceptionally difficult to detect. This marks the first time a single experiment has gathered evidence for this specific rare decay.
Dr Harry Cliff from Cambridge’s Cavendish Laboratory, who led the analysis, stated: “What we found in these latest results is an incredibly rare decay of one of these beauty particles, where it turns into two even smaller particles called muons. This process only happens for one in every ten billion beauty particles, so it’s incredibly difficult to find.”
The research, which involved approximately 20 physicists from the UK, Germany, Italy, Spain, and the Netherlands, utilised the LHCb experiment to achieve higher precision than the LHC itself. By precisely measuring the decay patterns of beauty quarks, physicists can infer the quantum mechanical influence of new, unseen particles.
The rarity of a decay often correlates with its sensitivity to new particles. The ‘Bd to mu mu’ decay has been a key target for new physics searches since 1984. An upgrade to the LHCb experiment in 2024, combined with data from 2024 and 2025, allowed researchers to isolate these rare signals from background noise using machine learning techniques.
The combined data, including earlier samples from 2011-2018, yielded a signal strength of 3.2 sigma, surpassing the 'three sigma' threshold for statistical significance. "Seeing clear evidence for this rarest decay for the first time was thrilling," said Cliff. "We’re knocking on the door of the zeptoverse now."
These findings align with predictions from the Standard Model. Future upgrades to the LHC, planned for the mid-2030s, aim to increase data collection rates significantly, enabling more precise searches for undiscovered particles and potential cracks in the Standard Model.
Professor Tim Gershon from the University of Warwick, who leads the LHCb experiment, commented: “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.” The UK's contribution to CERN is supported by the Science and Technology Facilities Council (STFC).
