Physicists have found evidence of a particle decay so rare that it is expected to occur only around once in every 10 billion "beauty" particles. The result, from the upgraded LHCb experiment at CERN's Large Hadron Collider (LHC), follows a search spanning more than 40 years and brings physicists closer to exploring the "Zeptoverse"—an unexplored world that exists at distances of a billionth of a trillionth of a meter, or a "zeptometer," where new particles and forces of nature may be hiding.

Researchers from The University of Manchester have played key roles in the LHCb experiment and its major upgrade—which can now collect data at up to five times its previous rate—including developing detector technology that has helped make the new measurement possible.

The finding will allow physicists to test the Standard Model, our current best theory of fundamental particles and their interactions, with greater precision and search for signs of particles or forces that may lie beyond it.

Searching for signs of new physics
The LHCb studies particles containing beauty quarks, heavier relatives of the up and down quarks found in the ordinary matter around us.

Beauty quarks are produced in huge numbers in collisions at the LHC but exist for only around one and a half trillionths of a second before decaying.

By making precise measurements of the different ways beauty quarks decay, physicists can detect the influence of new particles that may be far too small to see at the LHC.

The rarer a decay is, the more sensitive it can be to the influence of new particles. These particles could alter how often the decay occurs.

The new result concerns a particularly rare decay known as 'Bd to mu mu,' in which a particle containing a beauty quark decays into two muons, heavier relatives of electrons.

Scientists predict that this happens only around once in every 10 billion beauty particles. Until now, no single experiment had found solid evidence for the process.

The new measurement reached a statistical significance of 3.2 sigma. This is sufficient for physicists to describe the result as the first evidence of the decay, although further data will be needed to reach the five-sigma threshold required to claim a discovery in particle physics...