Physicists are reporting significant progress in the search for evidence of a rare particle decay that could illuminate some of the Universe’s most profound mysteries. During a seminar on November 3, the LHCb Collaboration showcased findings that point to an exceptionally elusive type of decay occurring in what is referred to as the “zeptoverse.” This discovery could provide insight into the moments immediately following the Big Bang, the nature of dark matter, and the existence of normal matter.
After more than 40 years of endeavor, researchers at CERN’s Large Hadron Collider (LHC) have identified experimental evidence for the decay of a Bd meson into a muon and an anti-muon. This process is believed to occur only once in ten billion events, making its detection a significant achievement. Niladri Sahoo, a particle physicist at the University of Birmingham and convener of the LHCb Very Rare Decays working group, described this finding as “a powerful new test of the Standard Model and a window into potential new physics.”
The LHCb experiment has become more adept at detecting such rare phenomena due to recent upgrades that have enhanced its particle collision detection capabilities. Leveraging machine learning techniques has helped improve the signal-to-noise ratio, allowing for a more precise observation of weak signals in particle physics. The LHCb’s VELO sub-detector plays a crucial role in this work, positioned just a short distance from the particle beam to capture the rare events resulting from near-light-speed collisions.
The Bd meson, which consists of a beauty quark and a down antiquark, is formed when protons are collided at nearly the speed of light. The decay process is extremely brief, lasting approximately one and a half trillionths of a second. Among the myriad particles produced from these collisions, only one out of ten billion yields the desired muon and anti-muon, which emerge under the laws of quantum physics.
To validate their findings, the LHCb team aims for a significance level of 5 sigma, the gold standard for determining the presence of a new discovery, whereas their current evidence stands at 3.2 sigma. This level indicates a 0.3 percent chance that the observation may be a statistical fluke. The latest results combine data from 2024 and 2025 with earlier data collected from 2011 to 2018.
Future upgrades to the LHC planned for the 2030s are expected to further increase collision counts and data collection rates, which may help solidify these findings. The ongoing investigation into rare particle decays may not only contribute to testing established theories but also pave the way for discoveries beyond the current understanding of physics.
Harry Cliff, a particle physicist at the University of Cambridge, expressed excitement about the implications of this evidence, stating, “Seeing clear evidence for this rarest decay for the first time was thrilling. We’re knocking on the door of the zeptoverse now.”
The preprint paper detailing these findings is available on arXiv, awaiting further peer review and analysis.
