Recent findings from research carried out at the Large Hadron Collider (LHC) at CERN in Geneva suggest we may be closing in on signs of physics beyond the Standard Model.
If confirmed, these hints would challenge the Standard Model, the theory that has dominated particle physics for 50 years. Our measurements indicate that the behaviour of specific subatomic particles produced in the LHC disagrees with Standard Model predictions.
Fundamental particles are the most basic building blocks of matter. The four fundamental forces—gravity, electromagnetism, the weak force and the strong force—govern how these particles interact. The LHC, a 27 km circular particle accelerator built under the French–Swiss border, was designed in part to look for cracks in the Standard Model. That theory is our best description of known particles and forces, but it cannot account for gravity or dark matter, the invisible matter that comprises roughly 25% of the universe.
The new results come from LHCb, one of the LHC experiments that analyses collisions. We studied the decays of B mesons and found that the way they transform into other particles disagrees with Standard Model expectations. Specifically, we examined a rare process called an electroweak “penguin” decay, in which a B meson decays into a kaon, a pion and two muons. With some imagination the arrangement of outgoing particles resembles a penguin, hence the name.
Penguin decays let us probe how a beauty (b) quark converts into a strange (s) quark. In the Standard Model this process is extremely rare—about one in a million B mesons decays this way. We measured the angles, energies and rates of these decays with high precision and found a tension of four standard deviations from Standard Model predictions. After accounting for experimental and theoretical uncertainties, there is roughly a one-in-16,000 chance that a fluctuation this large would occur if the Standard Model were correct.
This does not reach the five-sigma threshold (about one in 1.7 million) commonly required to claim a discovery, but the evidence is accumulating. Results from an independent LHC experiment, CMS, published earlier in 2025, agree with the LHCb findings, although with lower precision, strengthening the overall case.
Penguin processes are particularly sensitive to potential effects of very heavy new particles that cannot be produced directly at the LHC but can influence rare decays indirectly. Historically, indirect observations have pointed to new physics before direct detection—radioactivity was observed long before the W bosons responsible for weak interactions were seen directly.
There are various theoretical interpretations that could explain the anomalies. Some models invoke new particles called leptoquarks, which would link leptons and quarks. Others propose heavier partners of Standard Model particles. Our measurements therefore constrain these models and focus future searches.
However, important theoretical uncertainties remain. A central concern is the contribution of so-called “charming penguins”—Standard Model processes involving charm quarks whose effects are difficult to calculate precisely. Recent estimates suggest charming penguins are unlikely to fully explain the observed discrepancies, and combined fits of theory models to LHCb data indicate the Standard Model has difficulty accommodating the anomalies.
We analysed roughly 650 billion B meson decays recorded between 2011 and 2018 to isolate these rare penguin decays. Since then, LHCb has recorded about three times as many B mesons, and additional data already collected will allow us to test the current results in the coming years. Planned upgrades to the LHC in the 2030s (the High-Luminosity LHC) aim to increase the dataset by another factor of about 15, which should permit definitive statements and could open a new understanding of the fundamental workings of the universe.
William Barter is a UKRI Future Leaders fellow, School of Physics and Astronomy, University of Edinburgh. Mark Smith is a research fellow in Collider Physics, Faculty of Natural Sciences, Imperial College London.
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