Physicists working on the LHCb experiment at CERN have found evidence for a particle decay so rare that it happens roughly once in 10 billion decays, closing a search that has run for more than 40 years.
The result concerns a beauty particle — a short-lived hadron containing a bottom quark — produced in high-energy proton collisions inside the Large Hadron Collider. The collaboration reported the observation this week.
Why a rare decay is worth chasing
Rare decays are useful precisely because they are rare. In the Standard Model, the probability of a given decay can be calculated with high precision. If an experiment measures a rate that differs from that prediction, the gap would point to particles or forces that have not yet been observed.
Finding the decay at the expected rate is therefore a test that the standard picture survives in a place where it might have broken. It also rules out a range of proposed models that predicted a different rate.
How the measurement was made
The LHCb detector is designed to identify particles produced in collisions of protons and to track their paths over short distances. Beauty particles decay inside the detector within a fraction of a nanosecond, so the experiment relies on precise tracking and particle identification to reconstruct what was produced and how it fell apart.
Separating the signal from background required analyzing a very large dataset collected over several years of running. The collaboration said the result agrees with the Standard Model, and that the measurement now sets tighter limits on several classes of new-physics models.
What comes next
The Large Hadron Collider is entering a period of upgrades intended to increase the rate of collisions and the volume of data recorded. More data will allow LHCb to confirm the observation and to measure related decays with greater precision.
Physicists have long expected that new physics, if it exists at accessible energies, would show up first in processes that are heavily suppressed in the Standard Model. So far those searches have found agreement rather than deviation, which is itself a result: it tells theorists where new particles cannot be hiding.