UK physicists have found the first evidence of an extremely rare particle decay at CERN's Large Hadron Collider (LHC); the discovery, supported by the Science and Technology Facilities Council (STFC), provides evidence for the rarest decay of a 'beauty' particle ever seen at the LHC.
The finding was made by the upgraded Large Hadron Collider beauty experiment (LHCb) in which Oxford plays a leading role. LHCb, one of the four large experiments at the LHC, can now collect data at up to five times its previous rate. This enhanced capability will help scientists study rare particle processes in greater detail, improving our understanding of fundamental physics and potentially offering new insights into questions such as the nature of dark matter.
Precise measurements of a fleeting beauty
LHCb studies short-lived particles containing beauty quarks, heavier relatives of the up and down quarks that make up much of the matter around us. Beauty quarks are produced in huge numbers in LHC collisions but exist only fleetingly, decaying after around one and a half trillionths of a second.
The LHCb team has found evidence of an extremely rare decay known as 'Bd to mu mu', in which a beauty particle decays into two particles called muons (heavier relatives of the electron). This is predicted to occur only around once in every ten billion beauty particles. The result has a statistical significance of 3.2 sigma, enough to count as the first evidence of the decay, but below the five-sigma level, the gold standard for claiming a discovery in particle physics. The analysis also provided the most precise measurements yet of another rare decay involving a beauty quark, known as ‘Bs to mu mu’, first observed by LHCb in 2015.
More data, sharper analysis
Since the upgraded LHCb detector began collecting data in 2024, it has been able to record collisions at up to five times its previous rate, giving scientists a much larger sample of particles to study. The team combined new data collected in 2024 and 2025 with data from earlier runs between 2011 and 2018. Using improved particle identification and machine-learning techniques, they were able to distinguish this rare decay from the vast number of other particle collisions recorded. Together, the results enable scientists to test the Standard Model, the leading theory describing fundamental particles and their interactions, with greater precision.
UK physicists led the analysis behind this result, building on the country's long-standing role at the heart of the LHCb experiment. Funded by STFC, as part of UK Research and Innovation, the UK has also made the largest contribution to the upgraded LHCb detector, with teams designing and building crucial detector components through more than a decade of work.
Precision tracking: silicon detector
A major UK-built component, in which Oxford played a central role in both the construction and operation, is a silicon pixel detector that tracks the paths of particles created in collisions at the LHC with extreme precision. The Vertex Locator sits just five millimetres from the LHC beams and forms the first layer of the experiment. Crucially, this system enables scientists to pinpoint where particles are produced and how they decay.
UK researchers also contributed to the Ring Imaging Cherenkov (RICH) system, which identifies different types of particles. It detects light produced when particles travel through a gas faster than light can. By measuring this light, researchers can determine which particles are created when a beauty particle decays.
These capabilities give scientists a powerful new way to test our best understanding of the universe, potentially revealing cracks in the Standard Model that could point towards undiscovered particles or forces. The new results agree with the predictions of the Standard Model, however they provide clues about what new particles or forces might also be hiding in the Zeptoverse - a nickname used to describe the smallest subatomic realm of nature ever explored at the scale of a zeptometre. This information is particularly important at a time when particle physicists are considering what machine could succeed the LHC.
Dr Harry Cliff, an Ernest Rutherford Fellow at the University of Cambridge, who led the analysis team, said: ‘When we saw the result for the first time, it was a really thrilling moment. To see something that people had been searching for since before I was born was a real privilege. We’re knocking on the door of the Zeptoverse now. I can’t wait to see what we find as we explore deeper.’
‘LHCb is a great success story of CERN, and one in which Oxford has been proud to play a leading role, dating back to the inception of the project in 1995,' adds Professor Guy Wilkinson from the Department of Physics at the University of Oxford. 'Today’s announcement ushers in a new era of discovery for the experiment, and vividly demonstrates the increased sensitivity we have attained in our quest to probe beyond our current understanding of physics at the most fundamental level.’
Professor Tim Gershon, from the University of Warwick, who leads the LHCb experiment, concludes: ‘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.’