CERN Experiments Find Rare Particle Decays Drifting Slightly From Standard Model Predictions
CMS and LHCb both see small, consistent differences in a decay that happens about once in 10 million beauty-particle decays, though neither result is conclusive.
Two experiments at CERN's Large Hadron Collider are seeing small deviations from Standard Model predictions in one of the rarest decays physicists can measure, and the hints point the same way. The results were presented at the International Conference on High Energy Physics and summarized by CERN in a report by Emma Hattersley, published October 14.
The decay in question, written b→sμ⁺μ⁻, is one in which a beauty quark turns into a strange quark and emits a muon and an antimuon. It runs through the weak force and is extremely uncommon, occurring roughly once in every 10 million decays of particles containing beauty quarks. Because the process is so suppressed, any new particle or force that couples to it could leave a visible imprint, which is why physicists keep a close eye on it.
The CMS collaboration carried out the first simultaneous analysis of six decay types that all involve this transition and measured the probability of each, known as a branching fraction. The numbers differ slightly from Standard Model predictions, and the difference runs in a consistent direction across all six channels. The team used a much larger dataset from the LHC's Run 3 and a new trigger designed for the job, which gave it unprecedented statistical precision for this kind of study.
The LHCb collaboration looked at the same family of decays in a different way. Using data from Runs 1 and 2, it measured an angular quantity called P₅′ in the decay of a neutral B meson into a K* meson and two muons. The team applied a new, model-independent method for the measurement. Its result also shows a small difference from the Standard Model prediction. LHCb plans to fold in Run 3 data to sharpen the picture.
Anomalies in this corner of physics have a long and complicated history. P₅′ in particular has been a source of tension for years, and earlier hints have sometimes faded as measurements improved or as theorists revised how they calculate the Standard Model side. That is why the CERN summary is cautious. It describes the results as far from conclusive and says neither one alone establishes new physics. The report gives no specific values for the measured branching fractions or for P₅′, and no statistical significance for either deviation. Both are described only as small, and neither has been published in a peer-reviewed journal; they are conference presentations.
What makes the new data interesting is the agreement between independent measurements. CMS and LHCb use different detectors, different datasets and different analysis methods, and both drift away from prediction by modest amounts. If more data keep the pattern intact, the case for something beyond the Standard Model, such as a new heavy particle, would strengthen. If the differences shrink, they will join a list of near-misses that eventually dissolved.
The next step is more data. Run 3 is still collecting collisions, and LHCb's planned analysis of that sample, along with improved theory calculations, should show within the next few years whether this slight drift is a real signal or a statistical illusion.
