Some events in nature are so rare that finding one takes decades of work and some of the largest machines ever built. Physicists at CERN, the European laboratory for particle physics near Geneva, say they have now found evidence of one of the rarest: a particle decay expected to happen about once in every 10 billion tries.
The result comes from LHCb, one of the four big experiments at the Large Hadron Collider. It was announced at a seminar at CERN in early November.
What they found
The particle in question is called the B0 meson, often written Bd. It is made of two quarks, one of them a heavy “beauty” quark. B0 mesons live for a tiny fraction of a second before breaking apart into lighter particles.
Almost always, they break apart in common ways. Very rarely, the Standard Model, physicists’ best theory of particles and forces, predicts that one will turn into just two muons, which are heavier cousins of the electron. LHCb now reports evidence that this really happens.
“Physicists have been chasing this decay for more than forty years,” said Marco Santimaria, co-leader of the 19-member analysis team, as quoted by ScienceDaily. “Extreme precision is required to pick the muons out of the overwhelming background.”
Why such a rare event matters
A decay this rare is useful precisely because it is so delicate. Through quantum effects, particles too heavy for the collider to make directly can still nudge how often such decays happen. If the measured rate ever differs from the Standard Model’s prediction, that could be the first sign of unknown particles or forces.
So far, the new measurement agrees with the Standard Model. That still counts as progress, because it rules out some ideas about new physics and narrows where scientists should look next.
Evidence is not yet a discovery
Particle physicists use a measure called standard deviations, or sigma, to judge whether a signal is real or a statistical fluke.
| Level | What it means |
|---|---|
| 3 sigma | “Evidence.” About a 0.3% chance that random noise would look this convincing. |
| 5 sigma | “Observation,” the standard for a confirmed discovery. |
Using data collected in 2024 and 2025, LHCb found the decay at 3.0 sigma. Combined with older data from 2011 to 2018, the signal reached 3.2 sigma. That clears the bar for evidence but falls short of a discovery.
If future data reaches 5 sigma, this would become the rarest decay of a beauty particle ever observed.
Why it took more than 40 years
A close relative of this process was confirmed in 2015, when LHCb and the CMS experiment combined their data to observe a different particle, the Bs meson, decaying into two muons. That decay happens about three times in every billion. The B0 version is predicted to be about 30 times rarer, which makes it far harder to separate from the flood of ordinary collisions.
The 2015 analysis also hinted at the B0 decay, but later studies using data through 2018 could not confirm it.
What changed was the detector. After a major upgrade, LHCb has gathered data since 2024 at five times its earlier rate, and it got better at telling different kinds of particles apart. The team also used machine learning to pick out the handful of genuine events from enormous amounts of background noise. The same study produced the most precise measurement yet of the Bs decay.
What comes next
More data should settle the question. The collider is due for a major upgrade called the High-Luminosity LHC, scheduled to begin operating in 2030, which will produce far more collisions. With that, physicists expect to measure rare decays like this one with much greater precision, and to see whether the Standard Model keeps holding up.
