One in Ten Billion: LHCb Catches First Glimpse of Rarest Beauty Particle Decay

Agreement, though, is not the end of the story. Rare decays like this one are sensitive places where undiscovered particles or forces could reveal themselves as small deviations. The researchers say the measurements offer clues about what might be hiding in what they call the 'Zeptoverse' — distances around a billionth of a trillionth of a metre, smaller than anything currently probed.

The timing matters too. Particle physicists are actively debating what machine should follow the LHC, and precision measurements like these help decide where the next instrument should point.

The same analysis also produced the most precise measurements yet of a related rare decay, 'Bs to mu mu', which LHCb first observed in 2015.

How did they catch it?

Since the upgraded LHCb detector started taking data in 2024, it records collisions at up to five times its previous rate, giving scientists a far larger sample to work with.

UK physicists led the analysis, funded by the Science and Technology Facilities Council (STFC), part of UK Research and Innovation, and the UK made the largest contribution to the upgraded detector itself. Two UK-built components did heavy lifting:

• The Vertex Locator: a silicon pixel detector sitting just five millimetres from the LHC beams, forming the experiment's first layer. It tracks particle paths precisely enough to pinpoint where particles are produced and decay.
• The Ring Imaging Cherenkov (RICH) system: this identifies particle types by detecting light emitted when particles travel through a gas faster than light can in that medium.

The UK LHCb collaboration spans the universities of Birmingham, Bristol, Cambridge, Edinburgh, Glasgow, Imperial College London, Liverpool, Manchester, Oxford and Warwick, plus STFC's Rutherford Appleton Laboratory.

What do the researchers say?

Dr Harry Cliff of the University of Cambridge, an Ernest Rutherford Fellow who led the analysis team, didn't hide his excitement: '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.'

Professor Tara Shears of the University of Liverpool, principal investigator of LHCb-UK, credited the hardware: 'That such a rare signature can be teased out of such huge volumes of LHCb data, and so precisely, is thanks to the breathtaking precision and speed of the upgraded LHCb detectors.'

Professor Tim Gershon of the University of Warwick, who leads the LHCb experiment, called it 'beautiful science' driven by UK physicists probing nature at the fundamental level.

What does this change for physics?

Nothing is overturned — yet. The results match the Standard Model, and 3.2 sigma is a hint, not a discovery. But with the upgraded detector now collecting data five times faster, the sample will grow, and hints either firm up or evaporate. Either outcome tells us something about the Zeptoverse. The door is being knocked on. Someone, eventually, will answer.