University of Warwick physicists are part of a UK-led team that has found the first evidence of an extremely rare 'beauty' particle decay at CERN's Large Hadron Collider (LHC), after a hunt that began in 1984.
The result, from the upgraded LHCb experiment and supported by the Science and Technology Facilities Council (STFC), is the rarest decay of a beauty particle ever seen at the LHC.
Beauty particles are produced by the LHC in huge numbers, but each one lasts only around one and a half trillionths of a second before it decays. How they break apart can reveal what is happening in a realm physicists call the 'Zeptoverse', a world a billionth of a trillionth of a metre across, where new particles and forces may be hiding that could help explain mysteries such as dark matter.
The rarer the decay, the more sensitive it is to Zeptoverse influence, and this newly evidenced decay in which a beauty particle turns into two particles called muons, known as 'Bd to mu mu', is predicted to happen in only around one in every ten billion beauty particles.
Finding it meant picking out a faint signal from a vast amount of background noise. The team combined new data from 2024 and 2025, collected by a detector that can now record collisions at up to five times its previous rate, with earlier runs from 2011 to 2018, and used machine learning to sharpen the search.
Professor Tim Gershon, from the Department of Physics at the University of Warwick, who leads the LHCb experiment, said: "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."
The result has a statistical significance of 3.2 sigma. That is enough to count as evidence, but short of the five-sigma gold standard for a discovery. It also agrees with the Standard Model, the leading theory of fundamental particles, which helps narrow the search for signs of new physics.
Dr Harry Cliff, an Ernest Rutherford Fellow at the University of Cambridge, who led the analysis team, said: "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."
The UK has made the largest contribution to the upgraded detector, including the silicon Vertex Locator, which sits just five millimetres from the LHC beams, and the RICH system that identifies particles. Another major upgrade, planned for the 2030s, could increase the rate of data collection by a further factor of five.
