British scientists have utilized the Large Hadron Collider (LHC) to probe the "zeptoverse," a realm of particles existing at scales as small as one zeptometre—one trillionth of a billionth of a metre. The LHC can currently measure phenomena down to 100 zeptometres, but researchers believe even smaller particles may exist.

After a search spanning 42 years, the team has observed a one-in-10-billion event: the decay of a beauty quark into two muons. This transformation, known as the Bd to mu mu decay, is exceptionally rare and could provide crucial insights into the nature of the zeptoverse.

"Seeing clear evidence for this rarest decay for the first time was thrilling," said Dr. Harry Cliff, who led the analysis team at the University of Cambridge's Cavendish Laboratory. "Physicists have been looking for this rare type of decay since 1984, which was before I was born."

Beauty quarks are exotic relatives of the up and down quarks that constitute atomic matter. While these particles do not typically exist in nature, they can be generated in large quantities by smashing protons together at nearly the speed of light within the LHC. Beauty quarks exist for less than two trillionths of a second before decaying, and the extreme rarity of the Bd to mu mu decay makes it notoriously difficult to observe.

Particle physicists are particularly interested in this decay's rarity because rarer decays are more sensitive to the influence of new, undiscovered particles. The decay of a beauty quark into two muons involves a complex series of reactions, and the presence of new particles in the zeptoverse could offer alternative pathways for this decay, subtly altering its frequency. By analyzing minute variations in these decay events, researchers hope to study these incredibly small particles indirectly.

Scientists meticulously examined ten years of collision data from the Large Hadron Collider beauty (LHCb) experiment to identify this elusive decay. Following a significant upgrade in 2024, enhanced particle identification capabilities and machine learning algorithms were employed, leading to the identification of approximately a dozen potential candidates.

The evidence gathered has been classified at a "3.2 sigma" level of statistical significance, indicating a notable finding, though it falls short of the "five sigma" threshold required to claim a definitive discovery.

Future advancements are expected to shed further light on the zeptoverse. The LHC is slated for a major upgrade by the mid-2030s, which is anticipated to increase its data collection rate significantly. Furthermore, the successor to the LHC, the Future Circular Collider, is projected to come online around 2045, offering even greater potential for exploration.

Professor Brian Cox, a physicist and broadcaster not involved in the experiment, called the result "an important result and a beautiful example of how science advances. We upgrade our instruments, and we see nature in greater detail."

The research is also being highlighted as a success for British science, with academics from the Universities of Cambridge and Manchester leading the effort. However, the future role of the UK in international scientific collaborations has faced uncertainty. Despite the UK's continued membership in CERN, funding cuts from the UK Research and Innovation (UKRI) agency to particle physics projects have raised questions about future contributions and discoveries.