After more than 40 years of searching, CERN physicists have reached a major milestone in the search for one of the rarest known particle decays. Researchers working with the LHCb experiment have detected evidence of a process that is expected to occur only once in about 10 billion beautiful particle decays. The finding, announced during a seminar at CERN, marks the first time that a single experiment has produced sufficient evidence for this extraordinarily rare event. Beyond its rarity, the decay is particularly valuable because it could help scientists investigate particles that are too massive to be created directly at the Large Hadron Collider. At the center of the discovery is a particle called Bd, which contains a beauty quark and a down antiquark. The scientists found evidence that this particle can decay into a pair of muons (μ+μ-). Muons are similar to electrons but have greater mass. The results agree with the predictions of the Standard Model, the leading theory describing fundamental particles and their interactions, and help narrow the possibilities of new particles beyond it. Why rare particle decays could reveal new physics One reason physicists are so interested in these unusual decays is that quantum effects allow other particles to influence how they occur. Even particles too heavy to be produced at the Large Hadron Collider can leave subtle traces on the speed of these processes. By measuring extremely rare decays with high precision, researchers can look for differences between experimental results and theoretical predictions. These discrepancies could provide indirect evidence for previously unknown particles. Generally, the rarer a cavity is, the more sensitive it may be to these hidden influences. The decay known as Bd → μ+μ belongs to a special category that physicists have long considered “golden channels” for investigating physics beyond the Standard Model. During this process, a beauty quark and an antiquark with opposite electrical charge annihilate each other. Its energy is transformed into a muon and its antiparticle. Scientists have previously observed a related process involving a different beauty particle, called Bs. In 2015, a joint analysis by the LHCb and CMS collaborations reported the first confirmed observation of Bs → μ+μ-, along with initial evidence for the decay of the much rarer Bd. However, subsequent analyzes incorporating data collected through 2018 failed to confirm the earlier evidence of Bd → μ+μ−. The difference in rarity is substantial. Bs decay occurs about three times per billion beautiful particles, while the Bd process is expected to occur about 30 times less frequently. Improved CERN detector makes breakthrough possible A significant improvement to the LHCb experiment helped researchers overcome the enormous difficulty of detecting such a rare event. Following a major upgrade, the experiment began collecting data in 2024 at a rate five times faster than that achieved between 2010 and 2018. The upgraded detector also improved at identifying different types of particles, giving researchers a clearer picture of the activity produced during the collisions. The team combined these improvements with machine learning techniques to distinguish the exceptionally rare signals from the plethora of other particle processes. Without these advances, the events that scientists wanted to study would have been overwhelmed by the background. “Physicists have been chasing this decline for more than forty years,” said Marco Santimaria, who co-led the international analysis team of 19 scientists. “Extreme precision is required to distinguish muons from the overwhelming background.” Four decades of searching finally produce solid evidence In particle physics, scientists use a statistical measure called standard deviations to assess the likelihood that an apparent signal results from random fluctuations rather than a genuine physical process. A result must reach three standard deviations to qualify as evidence of a new phenomenon. At that level, the probability that random fluctuations will produce a signal at least as statistically extreme is about 0.3%. Using data collected during 2024 and 2025, LHCb researchers identified evidence of Bd → μ+μ- with a significance of 3.0 standard deviations. When they combined those measurements with previous data collected between 2011 and 2018, statistical significance rose to 3.2 standard deviations, exceeding the widely accepted evidence threshold. The same research also produced the most precise measurements ever obtained for the related Bs → μ+μ decay, providing additional information on these exceptionally rare processes. Although the new findings represent a major achievement, physicists have not yet reached the threshold needed to declare a confirmed observation. This milestone requires a significance of five standard deviations. If future measurements reach that level of certainty, Bd → μ+μ- would become the rarest beauty particle decay ever observed. Future upgrades of the Large Hadron Collider could uncover hidden particles. Researchers hope that upcoming improvements to the Large Hadron Collider will make these measurements even more powerful. The high-luminosity LHC, also known as HiLumi LHC, is scheduled to begin operation in 2030. Another major upgrade of the LHCb experiment is planned for the middle of the next decade. Together, these advances could eventually increase the experiment’s data collection rate by another factor of ten. Collecting much larger data sets would allow physicists to detect more of these extremely rare decays and measure their properties more precisely. That greater sensitivity could make it possible to identify subtle effects associated with particles that remain outside the direct reach of existing accelerators. “Seeing evidence of this rare decay is incredibly exciting, as it opens up new possibilities to explore beyond the direct reach of the LHC,” explained Harry Cliff, rare decay coordinator and co-director of analysis at LHCb. “The future upgrade of LHCb will allow us to observe many more of these decays, providing even greater sensitivity to undiscovered particles.” For physicists who have chased this elusive signal for more than four decades, the new evidence represents a long-awaited breakthrough. It also offers a promising way to investigate whether the known laws of particle physics are part of a much larger picture.
