Cracks in the Standard Model
Scientists at CERN’s Large Hadron Collider have detected particle behavior that contradicts predictions from the Standard Model, the 50-year-old theory that has governed understanding of fundamental physics. The findings, accepted for publication in Physical Review Letters, show a four-sigma deviation from theoretical expectations, suggesting the possible existence of undiscovered particles or forces.
The Large Hadron Collider, a 27-kilometer circular particle accelerator buried beneath the French-Swiss border, was built specifically to find limitations in the Standard Model. While the theory successfully explains fundamental particles and three of the four fundamental forces, it cannot account for gravity or dark matter, which comprises approximately 25% of the universe.
Researchers studying the decay of subatomic particles called B mesons found that the transformation process disagreed with Standard Model predictions. The results come from the LHCb experiment, one of four major detectors at the facility.
Four-Sigma Evidence
The measurement shows a tension of four standard deviations from Standard Model expectations. Statistically, this means there is only a one-in-16,000 probability that such an extreme result would occur from random data fluctuation if the Standard Model were correct.
While this falls short of the five-sigma gold standard required for a formal discovery, which corresponds to roughly a one-in-1.7-million chance of statistical fluke, the evidence is accumulating. Independent results from the CMS experiment at the LHC, published earlier in 2025, support the findings despite lower precision.
Physicists can compare measurements from facilities like the LHC against Standard Model predictions to rigorously test the theory. Despite five decades of increasingly precise testing, researchers had found no definitive cracks until recent observations began mounting.
Penguin Decays Reveal Anomalies
The anomalous results emerged from studying a rare process called electroweak penguin decay. The term “penguin” refers to the theoretical diagram shape representing how particles transform. In this case, researchers examined how B mesons decay into four other subatomic particles: a kaon, a pion, and two muons.
This decay process is extraordinarily rare. According to Standard Model calculations, only one in every million B mesons undergoes this transformation. Scientists carefully analyzed the angles and energies at which the resulting particles emerged, precisely measuring how frequently the process occurs.
The measurements disagreed with theoretical predictions. Penguin processes are uniquely sensitive to effects from potentially heavy new particles that cannot be created directly at the LHC. Such particles may still exert measurable influence on these decays despite their small contribution within the Standard Model framework.
Potential New Physics
Several theoretical frameworks could explain the findings. Many proposed models include new particles called leptoquarks, which would unite two different types of matter: leptons and quarks. Other theories suggest heavier analogues of particles already identified in the Standard Model.
The new results constrain the form of these models and will direct future searches. Studies of rare processes allow researchers to explore regions of nature that might otherwise only become accessible using particle colliders planned for the 2070s.
Despite excitement among physicists, open theoretical questions prevent definitive claims that physics beyond the Standard Model has been observed. The most serious concern involves so-called “charming penguins,” a set of Standard Model processes whose contributions are extremely difficult to predict accurately.
Recent estimates suggest charming penguin effects are insufficient to explain the anomalous data. Furthermore, combinations of theoretical models and LHCb experimental data indicate that the Standard Model struggles to account for the observed results.
Future Confirmation Expected
New data already collected should allow confirmation within the coming years. The current study analyzed approximately 650 billion B meson decays recorded between 2011 and 2018. Since then, the LHCb experiment has recorded three times as many B mesons.
Further advances are planned for the 2030s to exploit upgrades to the LHC, which will enable collection of a dataset 15 times larger. This ultimate phase will allow definitive claims, potentially unlocking new understanding of how the universe functions at the most elementary level.
The research represents decades of international scientific collaboration at CERN, the European Organization for Nuclear Research. The facility has hosted thousands of physicists from member nations, making it one of the largest and most complex scientific enterprises in human history.