Physicists at CERN have detected evidence of an exceptionally rare particle decay that occurs only once in approximately 10 billion beauty particle decays.
The international LHCb collaboration announced the finding during a seminar, marking the first time a single experiment has produced sufficient evidence for this specific process, according to CERN.
Probing the zeptoverse
The decay involves a neutral beauty meson, known as Bd, which transforms into a pair of muons. This process is so infrequent that researchers have been searching for it for more than 40 years, WIRED reported.
In these decays, a beauty quark and an oppositely charged antiquark annihilate, releasing energy that becomes a muon and its antiparticle. Muons are similar to electrons but are approximately 200 times heavier, WIRED reported. These muons themselves only exist for about one and a half trillionths of a second before breaking down.
Scientists hope these measurements allow them to explore the “zeptoverse,” a scale of reality involving distances around a billionth of a trillionth of a metre. At these extraordinarily small scales, undiscovered particles or forces of nature may be hiding, UKRI noted.
Exploring this scale could provide insight into the first infinitesimal fraction of a second following the Big Bang. The zeptoverse represents a scale of one zeptometre, which is one trillionth of a billionth of a metre.
While the new results align with the predictions of the Standard Model, the precision of the data could eventually reveal cracks in the theory, The Debrief reported.
The Standard Model explains many universal phenomena but does not account for dark matter or the matter-antimatter imbalance that prevented the universe from self-annihilating after the Big Bang. Scientists believe that discovering physics beyond current understanding might occur through indirect evidence in decay patterns, much like how the charm and top quarks were identified.
Upgraded detector technology
The breakthrough was made possible by the upgraded LHCb experiment, which began collecting data in 2024. This upgrade allowed the experiment to record collisions at up to five times the rate achieved between 2010 and 2018, ScienceDaily reported.
A central component of this upgrade is the Vertex Locator, or VELO, a silicon pixel detector. The VELO sits just five millimetres from the LHC beams to track particles with extreme precision, University of Liverpool reported.
This proximity allows for the precise observation of exotic particles created when protons collide at nearly the speed of light. The University of Liverpool played an important role in developing this upgraded technology, assembling the VELO detector which serves as the first layer of the experiment’s particle-tracking system.
The upgrade was a massive undertaking. Professor Chris Parkes, Head of the Department of Physics and Astronomy at The University of Manchester, who led the international LHCb collaboration through the construction of the upgrade, described the effort as a 15-year design and construction project from a 1,900-member international collaboration,
The Debrief reported.
Dr Stefano De Capua, who led the construction of the detectors at The University of Manchester, noted that nearly 90 UK companies worked together to build this detector.
Researchers utilized machine-learning techniques to distinguish the rare signals from the immense background of other particle processes. CERN stated that without these advances, the signals would have been swamped by background activity. UK researchers contributed to the Ring Imaging Cherenkov system, which identifies different particle types by measuring light produced when particles travel through gas faster than light can.
Statistical significance levels
The analysis yielded a statistical significance of 3.2 sigma when combining new data from 2024 and 2025 with earlier measurements gathered between 2011 and 2018. ScienceDaily reported that this level is sufficient to constitute evidence, though it remains below the five-sigma “gold standard” required to claim a formal discovery. In particle physics, a three-sigma result corresponds to a 0.3% chance that the signal is merely a random fluctuation.
| Comparison of Rare Decays | |
|---|---|
| Decay Type | Predicted Frequency |
| Bs to mu | Approximately 3 per billion beauty particle decays |
| Bd to mu | Approximately 1 per 10 billion beauty particle decays |
The analysis also provided the most precise measurements to date of a related process, the Bs to mu decay, which was first observed by LHCb in 2015. The difference in rarity between these two processes is substantial, as the Bd decay is predicted to happen roughly 30 times less frequently than the Bs decay, ScienceDaily reported.
Future research goals
The next phase of research involves reaching the five-sigma threshold to confirm the decay. Planned enhancements scheduled for the 2030s could multiply the number of collisions by an order of magnitude, potentially allowing the team to reach the required threshold for definitive confirmation, CERN reported.
Seeing evidence of this rare decay is incredibly exciting, opening up new possibilities to probe beyond the direct reach of the LHC,
Dr Harry Cliff
The findings offer a way to investigate whether known laws of physics are part of a larger picture. Niladri Sahoo, a particle physicist at the University of Birmingham and convener of the LHCb Very Rare Decays working group, explained that the results provide a powerful new test of the Standard Model and a window into potential new physics,
ScienceAlert reported.
Earlier coverage: Physicists Detect Evidence of 1 in 10 Billion Particle Decay