These exotic particles could break physics

Physicists know that their elegant theoretical description of forces and particles — the standard model of particle physics — must be incomplete, because there are a host of phenomena it cannot explain, such as the existence of dark matter.

But observations continue to confirm the model’s accuracy with ever greater precision. Even measurements that seemed to break the mould, such as a discrepancy in the mass of a particle called the W boson, have evaporated under further investigation.

Now, an analysis from an experiment at the Large Hadron Collider (LHC) at CERN, Europe’s particle physics laboratory near Geneva, Switzerland, suggests that evidence for one result that deviates from the standard model has grown. It concerns the decay of particles called B mesons into other particles. The result, which has been accepted for publication in Physical Review Letters, is one of the last remaining anomalies for particle physicists, who look for new physics in the debris from proton–proton collisions that turn energy into matter.

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Nature explores the latest findings from CERN’s LHC beauty (LHCb) experiment, and the exotic and heavy particles that could explain them.

What did the experiment find?

Rather than looking for new, heavy particles directly, LHCb looks for their subtle effects, including when they pop up fleetingly as ‘virtual particles’ that influence particle decay. To look for these effects, researchers analysed the frequency and angle at which particles emerge from decays, to check whether they match those predicted by the standard model. The new analysis looks at when a B meson — a particle composed of a bottom quark and another lighter quark — decays into another meson that contains a strange quark, known as a kaon, as well as two muons (heavier cousins to the electron). They found that the angles at which the final products emerge from the decay disagree with those predicted by the standard model. Evidence for this anomaly has been growing since 2015.

How does this point to new physics?

Physicists think that this B-meson decay — known as a penguin decay — should be particularly sensitive to as-yet undiscovered physics. (British theorist John Ellis coined the term in 1977, owing to the resemblance of a diagram of the decay to a penguin, after losing a bet which forced him to include the word in his next paper). The decay involves a quantum loop, in which a bottom quark changes into a strange quark, through a temporary transition into ‘virtual’ particles that pop in and out of existence. Quantum physics allows even heavy, non-standard-model particles, to fleetingly enter this loop and leave the final products with properties that are not possible from only known particles.

Because this decay is so rare — around one in one million B mesons decay in this way — the impact of new particles should be easier to spot than in other, more common decays, in which the signal would be drowned out.

Should we be excited?

The analysis includes around 650 billion decays amassed at the LHC during two runs between 2011 and 2018. Measurements of the angles of the particles emerging disagree with the standard model with a significance of around four sigma. This means that the chance that random noise from regular standard-model processes would produce this signal is around one in 16,000, says William Barter, a particle physicist at the University of Edinburgh, UK, who works on LHCb. “This is among the most significant results of the last few years at the LHC,” says Barter. Particularly exciting is that the finding seems to be tentatively corroborated by another LHC experiment, called the Compact Muon Solenoid or CMS, which has observed a discrepancy in this B-meson decay, albeit with lower statistical significance.

But excitement is tempered, he adds, because a rival decay involving particles called charm quarks can create the same products as does the bottom-to-strange transition, and it is hard for theorists to predict precisely how these ‘charming penguins’ would impact the angles of the final decay products. Theory suggests that this decay is unlikely to explain the full deviation from the standard model, but its existence gives room for caution.

If the signal is real, what new particles could explain it?

One possibility that could explain the discrepancy is whether a particle known as Z′ (pronounced Z prime) is a virtual particle involved in breaking up the B mesons as part of the bottom-to-strange quark transition. Physicists have suggested that this particle — which would be associated with a new, as-yet undiscovered force — would be similar to the Z boson, one of the two particles that mediates the weak nuclear force that is involved in radioactive decay. But Z′ would be heavier, and have a preference to interact with certain families of particles, says Ben Allanach, a theoretical physicist at the University of Cambridge, UK. The Z′ would mediate a force that discriminates between different ‘flavours’ of particle, he adds. This theory could also help to explain why masses of particles in the standard model can be so radically different.

Another possibility is the existence of a leptoquark, a short-lived particle that, at high energies, is suggested to take on the properties of two families of particles — leptons and quarks. Leptoquarks provide another way in which bottom quarks could transition to strange quarks, and could also cause the decay angles observed, says Barter.

What other anomalies might challenge the standard model?

There aren’t any others left. A long-standing and unexpected difference in the way that B mesons decayed into electrons and muons evaporated in 2022 with more data. And in 2024, physicists at the LHC quashed hopes of an apparent anomaly seen by another experiment, the Collider Detector at Fermilab (CDF), two years earlier. For decades physicists had also wondered whether the strange way in which muons behaved in a magnetic field could be explained by new physics, but revised predictions in 2023 suggested that there might be no discrepancy to explain.

Experiments at the LHC have observed other tensions between their results and the standard model — in findings related to B-meson decays and also to the Higgs boson, the particle associated with the field that gives everything mass. But they are all less significant than the latest result, says Allanach.

When will we know more?

LHCb physicists have yet to analyse the mountain of penguin-decay data accrued at the collider since 2018. This will happen quicker now that the initial analysis has been done, says Barter, but new results are still not expected until next year at the earliest. If the Z′ exists and is not too heavy, it might be possible for other LHC experiments to observe its decay directly, adds Allanach, especially with the upgraded high intensity machine planned from 2030.

This article is reproduced with permission and was first published on May 1, 2026.