After more than forty years of research, the first evidence of an extremely rare subatomic particle decay—occurring just once in every ten billion cases—has been found at CERN in Geneva.
The result, achieved within the LHCb experiment, could open the door to the so-called “zeptoverse”: the world of physical interactions on the scale of a zeptometer, equal to one trillionth of a billionth of a millimeter.
Hiding in this realm could be particles capable of shedding light on fundamental mysteries such as dark matter and the origin of matter during the Big Bang. The study, which has not yet undergone peer review for publication, was presented during a seminar at CERN.
The decay in question involves a neutral B meson—a particle composed of a “beauty” quark and a “down” antiquark—converting into a pair of even smaller particles: muons. Quantum physics dictates how this process is influenced by other particles; therefore, a precise measurement of this decay could reveal traces of hitherto unknown particles. Generally speaking, the rarer a decay is, the greater its sensitivity to the influence of new physics.
This is the first time a single experiment has obtained evidence of this rare type of decay. The research team, comprising around 20 physicists from the UK, Germany, Italy, Spain, and the Netherlands, used the LHCb experiment to push beyond the level of detail directly accessible by the LHC. By combining data from 2024 and 2025 with earlier datasets collected between 2011 and 2018, the signal intensity surpassed the threshold of statistical significance required to indicate a new phenomenon. The new results align with predictions from the Standard Model—the reference theory in physics—and provide guidance on which particles might be detected when the successor to the LHC comes online in the second half of the century.
The data analysis was conducted by an international team with a strong Italian presence. “Physicists have been chasing this decay for over forty years; extreme precision is needed to isolate the muons from the immense background noise,” commented Marco Santimaria, researcher at the INFN Frascati National Laboratories and co-coordinator of the group.
“Seeing evidence of this rare decay is truly thril and opens up exciting possibilities to explore physics beyond the direct reach of the LHC,” noted Harry Cliff, coordinator of the LHCb rare decays physics. He added: “The future upgrade of LHCb will allow us to observe many more of these decays, with even greater sensitivity to as-yet undiscovered particles.”
“The Italian LHCb community is already working on the experiment’s upgrade, which, over the next decade, will increase the rate of data collection tenfold, opening exciting prospects for studying rare decays and searching for new physics,” added Pasquale Di Nezza, national head of LHCb Italy.
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