Physicists at CERN's LHCb experiment have found the first evidence of a particle decay that happens roughly once in every 10 billion cases, closing a search that began in 1984.
A needle in 10 billion haystacks
The decay is written B0 to two muons: a B0 meson, a short-lived particle containing a beauty quark, turns into a pair of muons, the heavier cousins of the electron. The Standard Model predicts this should happen about once in 10 billion B0 decays.
The CLEO experiment set the first upper limit on the rate in 1984. Generations of detectors have looked since. Beauty quarks are produced in huge numbers in LHC collisions but survive only about 1.5 trillionths of a second, so the signal has to be pulled out of an enormous background.
Three sigma, not five
The result has a statistical significance of 3.2 sigma. That counts as evidence but falls short of the five sigma that particle physics requires before calling something a discovery.
In the new data alone, the team measured the branching fraction at about 1.26 per 10 billion decays, with a statistical uncertainty of roughly 0.5. The collaboration also reported improved measurements of the related B0s to two muon decay. The paper is in press at Physical Review X.
Why it matters
Rare decays are sensitive to particles nobody has seen. New physics that couples to beauty quarks would shift the rate away from the Standard Model prediction. So far the measurement agrees with the prediction, which is useful in itself: it constrains models that expected a larger effect.
One member of the collaboration described the precision as knocking on the door of the Zeptoverse. More data from the High-Luminosity LHC upgrade should push the measurement past five sigma and tighten the error bars.
The result also shows how far the field has moved since the search started. In 1984 the question was whether such a decay existed at all. Now the question is whether its rate matches the theory to the second decimal place, which is where signs of new particles would show up.