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Once in ten billion: the decay physicists chased for 42 years has finally shown up

Qurexa Editorial Team4 November 20267 min read 0 0
Once in ten billion: the decay physicists chased for 42 years has finally shown up

What happened

On 3 November 2026, physicists working on the LHCb experiment at CERN's Large Hadron Collider announced they had found the first evidence of an extremely rare particle decay. They had been looking for it for 42 years. The decay is known as "Bd to mu mu". In plain terms: a particle containing a beauty quark turns into two muons, which are heavier relatives of the electron. It is predicted to happen only about once in every ten billion beauty particles. That makes it the rarest decay of a beauty particle ever seen at the Large Hadron Collider. UK physicists led the analysis. The work was supported by the Science and Technology Facilities Council (STFC), part of UK Research and Innovation. The result has a statistical significance of 3.2 sigma, which is enough to count as first evidence. It is below the five-sigma level that particle physicists require before claiming a discovery. CERN's own announcement describes a significance of 3.0 standard deviations from the 2024 to 2025 data alone, with 3.2 being the figure once earlier data is combined. The team combined new data collected in 2024 and 2025 with data from earlier runs between 2011 and 2018. 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. The same analysis also produced the most precise measurements yet of a related decay, "Bs to mu mu", which LHCb first observed in 2015. The result matches what the Standard Model predicts.

Why this matters

Beauty quarks are heavier relatives of the up and down quarks that make up most of the matter around us. They are produced in huge numbers in LHC collisions, but they exist for only about one and a half trillionths of a second before decaying. The reason physicists care so much about extremely rare decays is a little counter-intuitive. They are not hunting the decay for its own sake. They are using it as a test. The Standard Model is the best theory we have of fundamental particles and forces. It makes a precise prediction about how often this decay should happen. If the measurement had come out clearly different from the prediction, that would point to something the Standard Model does not include - a new particle, or a new force. Rare decays are especially good tests because there is so little ordinary activity to hide behind. In a common process, a small new effect is lost in the noise. In a process that happens once in ten billion times, even a modest extra contribution would stand out. UKRI describes this work as bringing physicists closer to exploring the "Zeptoverse", a world of extremely small distances, around a billionth of a trillionth of a metre, where new particles and forces may be hiding. Better measurements of rare decays also feed into questions like the nature of dark matter. There is a national angle worth noting too. UK teams led this analysis, and the UK made the largest contribution to the upgraded LHCb detector, designing and building crucial components over more than a decade. The UK LHCb collaboration spans Birmingham, Bristol, Cambridge, Edinburgh, Glasgow, Imperial College London, Liverpool, Manchester, Oxford and Warwick, along with STFC's Rutherford Appleton Laboratory.

What the evidence actually says

This is a good story for learning how physics results should be read, so it is worth being careful. "Evidence" is not "discovery". In particle physics those words have technical meanings. Three sigma is conventionally evidence. Five sigma is required for a discovery claim. At 3.2 sigma, the chance of a fluctuation producing this result is small but not negligible, and the history of the field includes three-sigma results that melted away with more data. The team themselves are clear about this, and UKRI states plainly that the result was "below the five-sigma level, the gold standard for claiming a discovery in particle physics". More data is expected through the coming years to establish whether it reaches that level. The slight difference in quoted significance, 3.0 versus 3.2, is not a contradiction. It reflects which data is included: the newer run alone, or the newer run combined with the 2011 to 2018 data. Both figures appear in official sources, and anyone citing one should say which. It is also worth saying what this result does not do. It does not break the Standard Model. The measurement agrees with the prediction. For physicists hoping for a crack in the theory, that is a mild disappointment, and an honest account should say so rather than dressing agreement up as revolution. What it does do is sharpen the test. A measurement that matches theory still narrows the space where new physics could hide, and precision measurements of the Bs decay are arguably as valuable as the headline. One methodological detail is worth flagging because it is increasingly common in modern physics: the team used improved particle identification and machine-learning techniques to separate this rare decay from the enormous number of other collisions recorded. Machine learning is now part of the instrument, not just the analysis.

Practical advice

This result will not change your week. But it is unusually useful as a teaching example, so here is how to get something out of it. If you have a teenager studying physics, this is a near-perfect illustration of what "statistical significance" means in practice. Three sigma is interesting, five sigma is believed. The same logic underlies medical trials and opinion polls. Understanding why scientists refuse to call 3.2 sigma a discovery is a genuinely transferable piece of thinking. It is also a good counter to the idea that science is about dramatic single moments. This took 42 years, ten UK universities, a national laboratory, a detector upgrade, and decades of engineering before anyone could even look. For anyone considering a career path, note how much of this is not theoretical physics. Someone built a silicon pixel detector that sits five millimetres from the beam. Someone wrote the machine-learning code that pulls one event in ten billion out of the noise. Those are engineering, computing and data science jobs. STFC and the UK universities involved publish public engagement material and run open days, and CERN's own website has free educational resources. If a young person is interested, those are better starting points than news coverage. And a general habit worth building from this story: whenever you read a scientific claim, ask how confident the researchers themselves say they are. In this case they said "evidence", not "discovery", and they were right to.

What to know

On 3 November 2026, the UK-led LHCb analysis at CERN announced first evidence of the "Bd to mu mu" decay, expected to occur about once in every ten billion beauty particles. The search had run for 42 years. The significance is 3.2 sigma when new and older data are combined, and 3.0 sigma from the 2024 to 2025 data alone. Both are below the five-sigma standard required to claim a discovery. The result agrees with the Standard Model. It does not overturn the theory; it tightens the test. The same analysis gave the most precise measurements yet of the related Bs decay, first seen by LHCb in 2015. The UK led the analysis, funded by STFC, and made the largest contribution to the upgraded LHCb detector, which has collected data at up to five times the previous rate since 2024. More data is needed to establish whether this reaches discovery level. "Evidence" and "discovery" are different words for a reason. Sources: UK Research and Innovation, "Scientists find evidence of rarest particle decay after 42-years", 3 November 2026, https://www.ukri.org/news/scientists-find-evidence-of-rarest-particle-decay-after-42-years/; CERN, "LHCb finds evidence of one-in-ten-billion particle decay", 3 November 2026, https://home.cern/lhcb-finds-evidence-of-one-ten-billion-particle-decay; University of Liverpool, "Liverpool physicists help uncover evidence of rarest particle decay after 42-year search", 3 November 2026, https://news.liverpool.ac.uk/2026/11/03/university-of-liverpool-detector-helps-scientists-uncover-evidence-of-rarest-particle-decay-after-42-year-search/ This article is for general information and is not legal, financial or medical advice. For questions about your own health or medicines, speak to a doctor, pharmacist or other qualified healthcare professional.

#particle physics#LHCb#CERN#STFC#Standard Model#UK research

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