New analyses from the Large Hadron Collider (LHC) at CERN are producing results that may indicate physics beyond the Standard Model. If confirmed, these anomalies would challenge a framework that has guided particle physics for roughly fifty years.
Researchers working with the LHCb experiment examined rare decays of B mesons and found behaviour that conflicts with Standard Model predictions. The team focused on an electroweak “penguin” decay in which a B meson turns into a kaon, a pion and two muons (B → K π μ+ μ−). In the Standard Model, the underlying process—where a beauty (b) quark converts into a strange (s) quark—is extremely rare, occurring in roughly one out of a million B meson decays. Because such decays are so infrequent, even small contributions from new, heavy particles can leave measurable traces.
LHCb measured angles, energies and decay rates for these events with high precision and found a discrepancy of about four standard deviations from Standard Model expectations. Taking experimental and theoretical uncertainties into account, the probability of seeing a fluctuation this large if the Standard Model were correct is on the order of one in 16,000. That falls short of the five-sigma discovery threshold—the roughly one-in-1.7-million chance usually required to claim a discovery—but it is a statistically significant tension. An independent analysis from the CMS experiment, published earlier in 2025 with lower precision, shows compatible trends and strengthens the overall picture.
Penguin decays are particularly sensitive probes of new physics because very heavy particles that the LHC cannot produce directly can still influence these rare processes through quantum effects. Historically, indirect signs have sometimes preceded direct discoveries: for example, anomalous effects were observed long before the W bosons responsible for the weak interaction were detected directly.
Theoretical explanations under consideration include new particles such as leptoquarks, which would connect leptons and quarks, or heavier partners of known Standard Model particles. The new measurements restrict the parameters of these models and help guide what to search for next.
Important theoretical uncertainty remains, however. Standard Model processes involving charm quarks—sometimes called “charming penguins”—are difficult to calculate precisely and could potentially affect these observables. Recent estimates indicate charming-penguin contributions are unlikely to fully account for the observed tension, and global fits of theory to the LHCb data suggest the Standard Model struggles to accommodate the anomalies.
The LHCb analysis used roughly 650 billion B meson decays recorded between 2011 and 2018 to isolate the rare penguin events. Since that dataset was collected, LHCb has recorded about three times as many B mesons, and those additional data will allow the collaboration to test whether the current tension persists. Longer term, the High-Luminosity LHC upgrades planned for the 2030s aim to increase the available dataset by roughly another factor of 15, which should enable much more decisive tests and could open a new window on the fundamental workings of the universe.
The results are intriguing but not yet definitive. Continued analysis of the larger existing dataset, upcoming measurements, and refined theoretical work will be needed to determine whether these hints point to new particles or to subtleties within the Standard Model.
Article authors: William Barter (UKRI Future Leaders Fellow, School of Physics and Astronomy, University of Edinburgh) and Mark Smith (Research Fellow in Collider Physics, Faculty of Natural Sciences, Imperial College London). Originally published in The Conversation under a Creative Commons license.
