Physicists at CERN have announced the first evidence of a particle decay so rare it is expected to occur only once in every 10 billion events, ending a search that has lasted more than 40 years. The discovery, made using the upgraded LHCb experiment at the Large Hadron Collider, opens a new window to explore the “zeptoverse”—a realm of subatomic distances at a billionth of a trillionth of a meter.
The finding concerns the decay of a “beauty” particle, formally known as a Bd meson, which transforms into two muons—heavier relatives of the electron. This process, known as Bd to mu, was first predicted by the Standard Model of particle physics but has remained elusive due to its extreme improbability. The LHCb collaboration, which includes researchers from the University of Liverpool, reported a statistical significance of 3.2 sigma for the result, comfortably above the 3-sigma threshold required to claim evidence but below the 5-sigma gold standard for a formal discovery.
Physicists End 42-Year Search for Improbable Decay
Physicists have been searching for this decay since the 1980s. The challenge lies in distinguishing the signal from an overwhelming background of other particle events. Physicists have been chasing this decay for more than 40 years,
said Marco Santimaria, who co-led the international analysis team. Extreme precision is required to pick the muons out of the overwhelming background.
The breakthrough became possible following a major upgrade to the LHCb experiment in 2024. The revamp, which included the installation of a new Vertex Locator (VELO) detector assembled by the University of Liverpool team, increased the experiment’s data collection rate by a factor of five. This, combined with advanced machine-learning techniques, allowed researchers to isolate the faint signal of the rare decay from the noise.
That we can see such an incredibly rare decay, that has the potential to tell us so much about the universe, is testament to the skills, ingenuity and real craftsmanship of our talented staff who built the VELO. As this result shows, every member of LHCb can reap the rewards of the VELO’s exquisite precision.
Professor Tara Shears, from the University of Liverpool and the LHCb-UK spokesperson
Scientists Probe the Zeptoverse
The discovery is not a confirmation of new physics, as the results are consistent with the predictions of the Standard Model. However, the extreme rarity of the decay makes it a powerful new tool for testing the limits of current theory. Scientists believe such golden channels
can reveal the influence of undiscovered particles or forces that exist at scales too small or too energetic to be produced directly by the collider.
This is the gateway to the “zeptoverse,” a scale of reality measured in zeptometers (10-21 meters). At this scale, which is a billionth of a trillionth of a meter, physicists hope to find answers to some of the universe’s most profound questions, including the nature of dark matter and the events of the first moments after the Big Bang.
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,
explained Dr. Harry Cliff, a physicist at the University of Cambridge’s Cavendish Laboratory and leader of the analysis team. We’re knocking on the door of the zeptoverse now. I can’t wait to see what we find as we explore deeper.
UK Technology Paves the Way
The success of the experiment is closely tied to the technological advancements made by the UK team. The VELO detector, which sits just five millimeters from the LHC’s proton beams, was a critical component. The UK 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,
said Dr Stefano De Capua. We worked with nearly 90 UK companies to construct this detector and trained the next generation of scientists and engineers on the novel technologies we are using.
These detectors allow scientists to reconstruct particle paths with extraordinary precision. It is these detectors that we use to locate where the beauty meson was produced and pinpoint where it decayed into two muons,
De Capua added.
LHCb Aims for Discovery in the 2030s
While this result is a landmark, it is not the final word. To claim a formal discovery, physicists must reach a 5-sigma significance level, which would reduce the probability of the result being a statistical fluke to less than one in a very small number. The LHCb team plans to continue collecting data into the 2030s, with a planned upgrade that could increase the data collection rate by another factor of 10.
As Professor Guy Wilkinson, a physicist at the University of Oxford, noted, LHCb is a great success story of CERN, and one in which Oxford has been proud to play a leading role, dating back to the inception of the project in 1995. Today’s announcement ushers in a new era of discovery for the experiment, and vividly demonstrates the increased sensitivity we have attained in our quest to probe beyond our current understanding of physics at the most fundamental level.
The team’s findings have been submitted to the journal Physical Review X and are available as a preprint on the arXiv server (arXiv:2611.00230).
Earlier coverage: CERN scientists obtain first evidence of rare particle decay
