New analyses from the Large Hadron Collider (LHC) at CERN in Geneva have produced results that may be the strongest hint yet of physics not described by the Standard Model. If confirmed, these findings would require revisions to a framework that has governed particle physics for roughly half a century.
The Standard Model summarises our best understanding of fundamental particles and three of the four fundamental forces (electromagnetism, the weak force and the strong force). It does not incorporate gravity and gives no explanation for dark matter, so physicists expect it to be incomplete. The LHC—built in a 27 km circular tunnel under the French–Swiss border—collides beams of protons so researchers can probe tiny discrepancies between measurements and the Standard Model’s predictions.
The new result comes from the LHCb experiment, which specialises in precise studies of heavy-flavoured particles. LHCb examined a rare transformation of B mesons — an electroweak “penguin” decay in which a B meson turns into a kaon, a pion and two muons. These decays are extremely uncommon under the Standard Model (about one such decay per million B mesons), but they are especially sensitive to virtual effects from heavy particles that cannot be produced directly at the LHC.
By measuring the angles, energies and rate of this process with high precision, the LHCb team found a disagreement with the Standard Model prediction at the level of four standard deviations. After combining experimental and theoretical uncertainties, that corresponds to roughly a one-in-16,000 probability that such a discrepancy would arise from a random fluctuation if the Standard Model were correct. This is short of the five-sigma (about one in 1.7 million) threshold usually required to claim a discovery, but it is a notable tension.
An independent LHC experiment, CMS, published results earlier in 2025 that, while less precise, are consistent with LHCb’s findings, strengthening the case that the anomaly is real rather than a statistical fluke.
Why this matters
Rare “penguin” processes provide an indirect window onto potential new physics. Heavy new particles—for example, hypothesised leptoquarks that connect leptons and quarks, or heavier cousins of known Standard Model particles—could influence these decays through quantum effects and produce measurable deviations from Standard Model expectations. Historically, indirect effects have often pointed the way to new particles before direct detection; radioactivity was observed decades before the W boson was directly seen.
Caveats and open questions
There remain important theoretical uncertainties. Certain Standard Model contributions, colloquially called “charming penguins,” arise from hadronic effects that are difficult to compute precisely. Some have proposed that underestimated hadronic effects could explain anomalies of this type. Recent theoretical estimates, however, indicate these contributions are unlikely to account fully for the observed discrepancy. A combined analysis of theory inputs and LHCb data also suggests the Standard Model struggles to reproduce the pattern of results.
What’s next
The current study used about 650 billion B meson decays recorded between 2011 and 2018. Since then, LHCb has accumulated roughly three times that many B mesons, and further upgrades planned for the 2030s aim to expand the dataset by another factor of about 15. These larger datasets and improved analyses will clarify whether the anomaly strengthens into a discovery or fades away with more data and better theoretical control.
If the effect persists, it will constrain and guide model building—pointing toward or ruling out classes of new particles and interactions—and could open a new chapter in our understanding of particle physics.
This rewritten summary is based on work by William Barter (University of Edinburgh) and Mark Smith (Imperial College London) originally published in The Conversation and republished under a Creative Commons licence.
