A signal 40 years in the making
Physicists at CERN's LHCb experiment have reported the first evidence of a decay that theory says occurs about once in every 10 billion beauty particles. The result, from the upgraded detector at the Large Hadron Collider, follows a search that began more than four decades ago.
The process is known as Bd to mu mu: a neutral particle containing a beauty quark decays into two muons, heavier relatives of the electron. The measurement reached a statistical significance of 3.2 sigma, enough to call it evidence but short of the five sigma that particle physics requires for a discovery.
Why rare decays matter
Beauty quarks are produced in large numbers in collisions and last about 1.5 trillionths of a second. The rarer a decay, the more sensitive it can be to particles too heavy to be produced directly at the collider, because those particles can change how often the decay happens.
The measured rate agrees with the Standard Model, the current description of fundamental particles. That agreement is useful in itself: it narrows the room available for unknown physics in this channel and gives theorists a sharper benchmark for future data.
Probing the zeptoverse
The upgraded LHCb, a 15-year project involving about 1,900 researchers, can collect data up to five times faster than before. A British-built silicon pixel detector called the Vertex Locator sits five millimetres from the beam and pinpoints where particles are produced and where they decay.
Chris Parkes of the University of Manchester, who led the collaboration through the upgrade, compared the odds to finding a perfect match on a first date among everyone on Earth. Conor Fitzpatrick, deputy physics coordinator, called the decay a golden channel for spotting new physics.
The collaboration expects a much larger dataset by the early 2030s, and a further upgrade could raise collection rates tenfold. That would push measurements towards the zeptometre, a billionth of a trillionth of a metre, where physicists hope to find particles and forces that the Standard Model does not describe.