An unexpected behavior in a particle decay seems to disagree with the predictions of the Standard Model, despite its high reliability. It could betray the existence of still unknown forces or particles, beyond our current knowledge.
B mesons are unstable: they only live for a fraction of a second before transforming into other particles. By studying these transformations, researchers hope to detect the influence of new forces or particles that the Standard Model ignores.
Illustration image Pixabay
The LHCb experiment at the LHC is specially designed to capture these rare decays, recording billions of collisions to find the few events where "penguin" decays occur. In these cases, the B meson transforms into a kaon, a pion, and two muons – a signature that is both rare and rich in information. The angles at which these daughter particles move away from each other are clues to the underlying physics.
To carry out these investigations, the LHC accelerates protons to near-light speed and smashes them together. Among the detectors, LHCb has been operating since 1994. Between 2011 and 2018, the experiment recorded 650 billion B meson decays, from which scientists extracted the rare penguin-type events.
The analysis focused on an electroweak process where a B meson transforms into a kaon, pion, and two muons – a decay that occurs only once per million B mesons. By precisely measuring the angles and energies of the produced particles, the team found a clear disagreement with the predictions of the Standard Model.
The deviation from the Standard Model reaches four standard deviations. In practice, there is only a 1 in 16,000 chance that this result is due to random chance if the Standard Model is correct. The results, published in
Physical Review Letters, are consistent with those independently obtained by another LHC experiment, CMS. Although the so-called "five-sigma" threshold has not yet been reached, which would validate a scientific discovery, the combined evidence is already compelling.
At the LHC, magnets bend protons along a 17-mile ring (27 km), built under the Franco-Swiss border. Credit: Cern
Several theoretical models could explain the anomaly. One popular idea involves leptoquarks, hypothetical particles that bridge leptons and quarks – the two families of matter. Another possibility is the existence of more massive versions of known particles. The new data already constrain these models and will guide future research.
The LHCb collaboration has already begun analyzing new data collected since 2018. This set contains three times more B meson decays than the previous sample, offering a powerful tool to verify the anomaly. Initial analyses are underway, and results are expected in the next few years.
In parallel, physicists are refining theoretical calculations to better understand the contribution of charming penguins. If the discrepancy persists or increases, it will strengthen the hypothesis of physics beyond the Standard Model.
Looking further ahead, LHC upgrades in the 2030s will significantly increase the collision rate. The LHCb experiment plans to collect a data set 15 times larger than that used in the current study. With such statistics, the sensitivity will be sufficient to reach a significance of five sigma, the threshold for a discovery. If the anomaly is confirmed, a new era will open in our understanding of the infinitely small.