Physicists acknowledge that while the standard model of particle physics provides a sophisticated framework for understanding forces and particles, it has its limitations. Notably, it fails to account for certain phenomena, such as dark matter.
Despite its limitations, the model continues to be validated by increasingly precise observations. Even supposed anomalies, like the discrepancy in the mass of the W boson, have been resolved upon closer examination.
Recently, research conducted at the Large Hadron Collider (LHC) at CERN, near Geneva, Switzerland, has indicated a strengthening of evidence for a deviation from the standard model. This finding involves the decay of B mesons into other particles. The results, accepted for publication in Physical Review Letters, represent one of the few remaining anomalies that particle physicists hope could lead to new physics insights from proton–proton collision debris.
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Nature delves into the latest findings from CERN’s LHC beauty (LHCb) experiment and the potential exotic and heavy particles that could account for them.
What did the experiment find?
Instead of directly searching for new, heavy particles, the LHCb experiment detects their subtle effects, like when they briefly appear as ‘virtual particles’ influencing particle decay. Researchers examined the frequency and angles of particles emerging from decays to see if they aligned with standard model predictions. The recent analysis focused on B mesons, which consist of a bottom quark and another lighter quark, decaying into a kaon (a meson containing a strange quark) and two muons. The angles at which these decay products appear differ from standard model predictions, and this anomaly has been noted since 2015.
How does this point to new physics?
The decay of B-mesons, termed “penguin decay,” is thought to be particularly sensitive to new physics yet to be discovered. British theorist John Ellis introduced the term in 1977 due to a diagram’s resemblance to a penguin, following a lost bet. This decay involves a quantum loop where a bottom quark transforms into a strange quark through transient ‘virtual’ particles. Quantum physics allows even heavy particles not described by the standard model to momentarily enter this loop, altering the final products’ properties in ways not possible with known particles.
Since this decay is exceedingly rare — occurring in about one in a million B mesons — the impact of new particles is more discernible compared to more common decays where signals might be obscured.
Should we be excited?
The study encompasses roughly 650 billion decays recorded at the LHC from 2011 to 2018. The angles of the emerging particles differ from standard model expectations with a significance of about four sigma, indicating that the likelihood of the signal being random noise is about one in 16,000, according to William Barter, a particle physicist at the University of Edinburgh, UK, involved in LHCb. “This is among the most significant results of the last few years at the LHC,” Barter remarks. Furthermore, the finding appears to be tentatively supported by another LHC experiment, the Compact Muon Solenoid (CMS), which has also observed a discrepancy in this B-meson decay, albeit with lower statistical significance.
Yet, Barter cautions, enthusiasm is tempered by the fact that an alternative decay involving charm quarks can produce the same products as the bottom-to-strange transition. Predicting how these ‘charming penguins’ influence the angles of the final decay products is challenging for theorists. Although theory suggests this decay is unlikely to fully account for the deviation from the standard model, its presence necessitates caution.
If the signal is real, what new particles could explain it?
A possible explanation for the discrepancy is the presence of a particle known as Z′ (pronounced Z prime), a virtual particle implicated in disrupting B mesons during the bottom-to-strange quark transition. Scientists propose this particle, linked to a new, undiscovered force, would resemble the Z boson, which helps mediate the weak nuclear force involved in radioactive decay. However, Z′ would be heavier and preferentially interact with specific particle families, explains Ben Allanach, a theoretical physicist at the University of Cambridge, UK. The Z′ would facilitate a force that distinguishes between different ‘flavours’ of particles, Allanach adds. This theory might also clarify why particle masses in the standard model vary so dramatically.
Another theory suggests the existence of a leptoquark, a short-lived particle that at high energies exhibits properties of both leptons and quarks. Leptoquarks offer another mechanism for bottom quarks to transition into strange quarks, potentially accounting for the observed decay angles, says Barter.
What other anomalies might challenge the standard model?
No other anomalies remain. A long-standing unexpected difference in B meson decays into electrons and muons was resolved in 2022 with additional data. In 2024, LHC physicists dismissed hopes of another anomaly previously observed by the Collider Detector at Fermilab (CDF) two years prior. For decades, physicists speculated whether the unusual behavior of muons in a magnetic field could point to new physics, but revised predictions in 2023 indicated that there might not be any discrepancy to explore.
While other LHC experiments have noted tensions between their results and the standard model — in studies related to B-meson decays and the Higgs boson, which is tied to the field giving everything mass — they are less significant than the latest findings, according to Allanach.
When will we know more?
LHCb physicists have not yet analyzed the extensive penguin-decay data gathered at the collider since 2018. This process will be faster now that the initial analysis is complete, says Barter, but new results are unlikely before next year. If the Z′ particle exists and isn’t excessively heavy, it might be possible for other LHC experiments to directly observe its decay, adds Allanach, particularly with the planned high-intensity machine upgrade from 2030.
This article is reproduced with permission and was first published on May 1, 2026.
