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 result brings physicists closer to exploring the ‘Zeptoverse’, a world of extremely small distances, around a billionth of a trillionth of a metre.
Here new particles and fundamental forces of nature may be hiding.
The finding was made by the upgraded Large Hadron Collider beauty experiment (LHCb), one of the four large experiments at the LHC, which 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.
In turn this improves our understanding of fundamental physics and potentially offering new insights into questions such as the nature of dark matter.
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.
First evidence
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.
However, it was 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.
This gives 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 leadership
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.
Our teams have designed and built crucial detector components through more than a decade of work.
The UK LHCb collaboration includes researchers from:
- University of Birmingham
- University of Bristol
- University of Cambridge
- The University of Edinburgh
- University of Glasgow
- Imperial College London
- University of Liverpool
- The University of Manchester
- University of Oxford
- University of Warwick
- STFC Rutherford Appleton Laboratory
Precision tracking: silicon detector
A major UK-built component 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.
Identifying particles: RICH system
UK researchers also contributed to the Ring Imaging Cherenkov 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.
Nowhere to hide
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 be hiding in the Zeptoverse.
This information is particularly important at a time when particle physicists are considering what machine could follow the LHC.
Knocking at the door of the Zeptoverse
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.
Breathtaking precision
Professor Tara Shears, from the University of Liverpool and principal investigator of the LHCb-UK collaboration, said:
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.
Our experiment has really moved to the next level when it comes to exploring the finest details of the universe.
Beautiful science
Professor Tim Gershon, from 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.
Read the full press release on the CERN website: LHCb finds evidence of one-in-ten-billion particle decay.