Physicists at CERN have detected evidence of an extraordinarily rare particle decay that occurs only once in every 10 billion cases. This finding, achieved using the upgraded Large Hadron Collider beauty (LHCb) experiment, provides a first glimpse into the zeptoverse—a subatomic scale at a billionth of a trillionth of a metre.
LHCb researchers observe rare beauty quark decay
The research identifies the decay of a particle containing a beauty quark and an oppositely charged antiquark, known as Bd to mu, into a pair of muons. In these specific decays, a beauty quark and an oppositely charged antiquark annihilate, releasing energy that transforms into a muon and its antiparticle.
Muons are particles similar to the electrons found in atoms, but they are approximately 200 times heavier. These muons themselves only live for about two millionths of a second before they decay into particles such as electrons and positrons. The analysis yielded a statistical significance of 3.2 sigma, which counts as the first evidence of the decay. While this meets the threshold for evidence, it remains below the five-sigma gold standard required to officially declare a discovery. The results also provided the most precise measurements to date of another rare decay, Bs to mu, which was first observed by LHCb in 2015.
Marco Santimaria, who co-led the international analysis team, noted the long history of this search.
Physicists have been chasing this decay for more than forty years.
Upgraded technology enables detection at zeptometer scales
The ability to observe this phenomenon was made possible by a major upgrade to the LHCb experiment in 2024. The team combined this new data from 2024 and 2025 with previous data gathered between 2011 and 2018. The upgrade enabled faster data collection and helped the team home in on such rare physics, which has been sought since the 1980s.
To successfully identify the decay, researchers used improved particle identification and machine-learning techniques. These tools were used to suppress the vast background of processes that would have otherwise obscured the signals from these rare beauty-particle decays.
Extreme precision is required to pick the muons out of the overwhelming background.
Marco Santimaria
Probing the zeptoverse for new physics
The discovery offers a way to study the zeptoverse, a scale where particles may be more sensitive to dark matter. The zeptoverse is a realm consisting of distances as small as a single zeptometre—one trillionth of a billionth of a metre. While scientists do not yet know if anything truly exists at that level, it is believed that physics as we know it may completely break down on that scale. Because dark matter and dark energy together account for approximately 95% of the universe, detecting activity in this tiny world could help scientists detect dark matter. Accessing this micro world could also help researchers understand the first infinitesimal fraction of a second following the Big Bang.
While the current results are consistent with predictions that align with the Standard Model of particle physics, they provide a powerful new test and a window into potential new physics. Dr. Harry Cliff said that this subatomic dance provided researchers with a ringside seat to the behaviour of possible new particles.
Future research aims to reach higher levels of sensitivity. Future upgrades in the 2030s will boost the number of LHC collisions and again increase the data collection rate, by an order of magnitude.
The future LHCb upgrade will allow us to observe many more of these decays, providing even greater sensitivity to undiscovered particles.
Harry Cliff
Professor Brian Cox, particle physicist at the University of Manchester, described the LHCb experiment as a major scientific success. He added, This is an important result and a beautiful example of how science advances
Earlier coverage: CERN Researchers Detect Rarest Beauty Particle Decay