In general, the rarer a decay, the more sensitive it is to the influence of new particles. Among the rarest is when a beauty quark annihilates with an oppositely charged antiquark to produce a muon and an antimuon. In the Standard Model of particle physics, this process occurs through a complicated set of interactions, making the decay extraordinarily unlikely: less than a few times per billion beauty particles.

However, in the, new particles could provide an additional route for the decay to follow, altering how often it occurs. For decades, these decays have been seen as ‘golden channels’ in the search for new physics. ‘Physicists have been looking for this rare type of decay since 1984, which was before I was born,’ said Cliff.

In 2024, a major upgrade to the LHCb experiment made it possible that this rarest of decays – the Bd to mu mu decay – might be found at last. An enormous new sample of beauty decays from 2024 and 2025 harnessed the ability of the upgraded LHCb experiment to tell different types of particles apart. By using machine learning techniques, the researchers were able to suppress the huge amounts of background noise that would otherwise bury these very rare signals, allowing them to obtain the first evidence for the Bd to mu mu decay.

The team, working at the upgraded LHCb experiment at CERN’s Large Hadron Collider (LHC) near Geneva, were studying beauty quarks, exotic cousins of the up and down quarks that make up all atomic matter in the universe.

When these beauty particles are created, they live for just one and a half trillionths of a second before they decay. The way they decay can tell us about physics that happens at very short distance scales: trillionths of billionths of a metre.

“What we found in these latest results is an incredibly rare decay of one of these beauty particles, where it turns into two even smaller particles called muons,” said Dr Harry Cliff from the University of Cambridge’s Cavendish Laboratory, who led the analysis team. “This process only happens for one in every ten billion beauty particles, so it’s incredibly difficult to find.”