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.

Professor Chris Parkes, head of the Department of Physics and Astronomy at The University of Manchester, led the international LHCb collaboration through the construction of the upgrade and led the collaboration’s approval of the new result.

He said, “The upgraded LHCb was a major undertaking—a 15-year design and construction project from a 1,900-member international collaboration.

“The U.K. made the largest national contribution to the project, pioneering new technologies that have made today’s result possible where we probe nature at a new scale. But this is also just the start; we are planning the construction of an experiment that will exploit the capabilities of the next-generation High Luminosity LHC, probing physics at an even smaller scale.”

Professor Parkes, who was made a fellow of the Royal Society earlier this year for his work on LHCb, added, “This decay is remarkably unlikely; the chance of it happening is roughly like going on a romantic date and, on your very first try, finding the one perfect match for you among everyone on Earth.”

A major U.K.-built component of the upgraded LHCb experiment is a silicon pixel detector that tracks the paths of particles created in collisions at the LHC with extreme precision.

Known as the Vertex Locator, it sits just five millimeters from the LHC beams and enables scientists to pinpoint where particles are produced and where they decay.

Dr. Stefano De Capua, who led the construction of the detectors at The University of Manchester, added, “We worked with nearly 90 U.K. companies to construct this detector and trained the next generation of scientists and engineers on the novel technologies we are using. It is these detectors that we use to locate where the beauty meson was produced and pinpoint where it decayed into two muons.”

| Sadie Harley and Robert Egan, physics.org