Physicists know that their elegant theoretical description of forces and particles – the standard model of particle physics – must be incomplete, because it cannot explain a multitude of phenomena, such as the existence of dark matter.
But observations continue to confirm the accuracy of the model with ever greater precision. Even measures that seemed to be off the beaten track, such as that of a deviation in the mass of a particle called W bosonevaporated upon further investigation.
Now, an analysis of an experiment conducted at the Large Hadron Collider (LHC) at CERN, Europe’s particle physics laboratory near Geneva, Switzerland, suggests that evidence has grown for a result that deviates from the Standard Model. This is the disintegration of particles called B mesons into other particles. The result, which was accepted for publication in Physical Examination Lettersis one of the latest anomalies for particle physicists, who search for new physics in the debris from proton-proton collisions that transform energy into matter.
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Nature explores the latest discoveries from CERN’s LHC beauty experiment (LHCb) and the exotic, heavy particles that could explain them.
What did the experiment reveal?
Rather than directly searching for new heavy particles, LHCb looks for their subtle effects, particularly when they appear fleetingly as “virtual particles” that influence particle decay. To look for these effects, the researchers analyzed the frequency and angle at which particles emerge from the decays, to see if they match those predicted by the Standard Model. The new analysis examines when a B The meson – a particle composed of a bottom quark and another lighter quark – decays into another meson that contains a strange quark, called a kaon, as well as two muons (heavier cousins of the electron). They found that the angles at which the final products emerge from decay do not match those predicted by the Standard Model. Evidence of this anomaly has been growing since 2015.
How does this point to new physics?
Physicists think that this BMeson decay – known as penguin decay – is expected to be particularly sensitive to the still unknown physics. (British theorist John Ellis coined the term in 1977, due to the resemblance of a decomposition diagram to a penguinafter losing a bet which forced him to include the word in his next article). The decay involves a quantum loop, in which a bottom quark transforms into a strange quark, via a temporary transition to “virtual” particles that appear and disappear. Quantum physics allows even heavy particles, which do not conform to a standard model, to fleetingly enter this loop and leave the final products with properties that would not be possible with known particles alone.
Because this cavity is so rare – about one in a 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 enthusiastic?
The analysis includes around 650 billion decays accumulated at the LHC during two runs between 2011 and 2018. Measurements of the angles of the emerging particles disagree with the Standard Model with a significance of around four sigma. This means that the chance of random noise from regular Standard Model processes producing this signal is about one in 16,000, says William Barter, a particle physicist at the University of Edinburgh, UK, who works on LHCb. “This is one of the most significant results at the LHC in recent years,” says Barter. What is particularly interesting is that this finding appears to be tentatively corroborated by another LHC experiment, called Compact Muon Solenoid or CMS, which observed a divergence in this B-meson decay, although with less statistical significance.
But enthusiasm is tempered, he adds, because a rival decay involving particles called charmed quarks can create the same products as the background-to-strange transition, and it is difficult for theorists to predict precisely how these “charming penguins” would affect the angles of the final decay products. Theory suggests that this degradation is unlikely to explain the total deviation from the standard model, but its existence calls for caution.
If the signal is real, what new particles could explain it?
One possibility that could explain this discrepancy is whether a particle known as Z‘ (pronounced Z prime) is a virtual particle involved in the breakdown of the B mesons as part of the transition from low to strange quarks. Physicists suggested that this particle – which would be associated with a new, yet unknown force – would be similar to the Z the boson, one of two particles that mediate the weak nuclear force involved in radioactive decay. But Z′ would be heavier and would have a preference for interacting with certain families of particles, explains Ben Allanach, a theoretical physicist at the University of Cambridge, in the United Kingdom. THE Z” would mediate a force that distinguishes between different “flavors” of particles, he adds. This theory could also help explain why particle masses in the Standard Model can be so radically different.
Another possibility is the existence of a leptoquark, a short-lived particle which, at high energy, would adopt the properties of two families of particles: leptons and quarks. Leptoquarks provide another way that bottom quarks could transition to strange quarks and could also cause the observed decay angles, Barter says.
What other anomalies could call into question the standard model?
There are no others left. A long-standing and unexpected event difference in the way B mesons decay into electrons and muons evaporated in 2022 with more data. And in 2024, LHC physicists have dashed all hopes an apparent anomaly observed by another experiment, the Fermilab Collider Detector (CDF), two years ago. For decades, physicists have also wondered whether the new physics could explain the strange behavior of muons in a magnetic field. revised forecasts in 2023 suggested that perhaps there was no discrepancy to be explained.
The LHC experiments observed further tensions between their results and the Standard Model—in findings related to B-the decays of mesons as well as the Higgs boson, the particle associated with the field which gives mass to everything. But they are all less significant than the last result, says Allanach.
When will we know more?
Physicists at the LHCb still need to analyze the mountain of data on penguin decay accumulated at the collider since 2018. This will happen more quickly now that the initial analysis has been done, Barter says, but new results are still not expected until next year at the earliest. If the Z′ exists and is not too heavy, it would perhaps be possible for other LHC experiments to directly observe its decay, adds Allanach, in particular thanks to the improved high-intensity machine planned from 2030.
This article is reproduced with permission and has been published for the first time May 1, 2026.
