CERN Finds First Hint of an Ultra-Rare Particle Decay That Could Expose New Physics

Physicists on the LHCb experiment at CERN's Large Hadron Collider have found the first evidence of one of the rarest decays ever searched for: a particle containing a beauty quark turning into two muons. The signal has a statistical significance of 3.2 sigma, short of the 5 sigma needed to claim a discovery, but it ends a search that has run for more than 40 years.

The particle is the B-zero meson, called Bd. The Standard Model of particle physics predicts it should decay into a pair of muons, the heavier cousins of the electron, only about once in every 10 billion beauty particles. A beauty quark lives for roughly 1.5 trillionths of a second, so catching this decay means sorting a handful of events out of a vast number of collisions.

The measured rate agrees with the Standard Model's prediction. That matters because the decay is a sensitive probe: if undiscovered particles or forces exist, they could nudge how often it happens. The physicists call the realm where such effects would show up the Zeptoverse, distances of about a zeptometer, a billionth of a trillionth of a meter.

Conor Fitzpatrick of the University of Manchester, the deputy physics coordinator of LHCb, called it "a golden channel to observe the effects of new physics from beyond our current knowledge." Brian Cox of Manchester, who was not part of the experiment, said: "This is an important result and a beautiful example of how science advances."

The result was possible because LHCb has been rebuilt. The upgrade took 15 years of design and construction by a collaboration of about 1,900 members, and it lets the detector collect data at up to five times its previous rate. Its Vertex Locator, a silicon pixel detector, sits about 5 millimeters from the LHC beams and was built with nearly 90 U.K. companies. Chris Parkes, head of physics and astronomy at Manchester, led the collaboration through the upgrade and approved the new result. Stefano De Capua led construction of the detectors.

The 3.2 sigma figure means the data are not yet strong enough for a confirmed discovery. The upgraded experiment is expected to keep collecting a much larger data set into the early 2030s, and a further planned upgrade could raise the collection rate by another factor of 10. Parkes said the team is already planning an experiment for the High-Luminosity LHC.

For now the finding is a null result for new physics, which is itself informative. The Standard Model has passed another stringent test, and the room for deviations is a little smaller. If the rate drifts from the prediction as the data grows, this decay is one of the places a crack would show first.