Physicists on the LHCb experiment at CERN's Large Hadron Collider say they have found the first evidence of one of the rarest particle transformations ever predicted, capping a search that has run for 42 years. The result, announced November 3 and led by analysts at the University of Cambridge, is being described by the team as a first knock on the door of the "zeptoverse" — the almost unimaginably small distance scale where the next layer of physics may be hiding.
The decay in question, called Bd → μμ, happens when a particle built from a beauty quark transforms into two muons. The Standard Model of particle physics predicts it should happen in only about one of every 10 billion beauty particles produced. Sifting through collision data recorded in 2024 and 2025 by LHCb's newly upgraded detector, combined with earlier runs from 2011 to 2018, the team found a signal with a statistical significance of 3.2 sigma — short of the five-sigma threshold physicists require to claim a firm discovery, but enough to count as first evidence, according to the UK Research and Innovation announcement.
Why such a small decay matters
The beauty quark behind the measurement exists for only about one and a half trillionths of a second before it decays — just long enough for LHCb's silicon Vertex Locator, mounted five millimeters from the particle beams, to trace its path. Physicists define the zeptoverse as a scale roughly a billionth of a trillionth of a meter across, smaller than a proton. Because any undiscovered particles or forces should show up as tiny deviations from the Standard Model's predictions in rare decays like this one — alongside the related Bs → μμ decay LHCb first observed in 2015 — a precise measurement gives physicists one of their most sensitive available tests for new physics, including the kind some theories say dark matter would require.
"We're knocking on the door of the Zeptoverse now," said Harry Cliff, the Cambridge physicist who led the analysis. Tara Shears, who heads the UK's LHCb program at the University of Liverpool, credited "the breathtaking precision and speed of the upgraded LHCb detectors," which can record collisions at up to five times their earlier rate. The UK supplied the single largest contribution to that upgrade, including the Vertex Locator and parts of the particle-identification system, funded through the Science and Technology Facilities Council.
The celebration is tempered by a funding dispute over what comes next. Reaching a five-sigma discovery will take considerably more data, which depends on a planned second upgrade — a roughly £150 million project due to begin construction in 2027. But UK Research and Innovation withdrew Britain's £49.4 million share of that upgrade in a letter delivered December 19, after around £5 million had already gone to preconstruction work, Physics World reported. Tim Gershon, the University of Warwick physicist set to become LHCb's international spokesperson, called the decision "a complete shock," adding: "It's like paying to heat your house but then sitting outside in the cold." Institute of Physics president-elect Paul Howarth said the pullback was "incredibly damaging to our international reputation as a science superpower." International partners say they remain committed to the project; whether UK physicists get to keep probing the zeptoverse they just reached may depend on whether the funding is restored.
