Intriguing results at CERN show tension with the Standard Model of particle physics

The LHCb experiment at CERN. Credit: CERN

The new LHCb analysis still sees the previous intriguing result: the new analysis continues to find tensions with the Standard Model, but more data is needed to identify its cause.

At a seminar today at CERNthe LHCb collaboration presented new analysis of data from a specific transformation, or “decay”, that a particle called B0 meson can undergo. The analysis is based on twice as many B0 decays like previous LHCb analyses, which had revealed some tension with the Standard Model of particle physics. The tension is still present in the new analysis, but more data is needed to identify its nature.

The decay in question is the decay of a B0 meson, composed of a bottom quark and a down quark, into a K* meson (containing a strange quark and a down quark) and a pair of muons. This is a rare process: the Standard Model predicts only one such decay for every million B0 decomposes. In many theories that extend the Standard Model, new, unknown particles can also contribute to decay, causing a change in the rate at which decay is expected to occur. Moreover, the angle distribution of B0 decay products relative to parent B0 – ie muons, kaons and pions from the decay of K* – can also be affected by the presence of new particles.

In previous studies of this decay, the LHCb team analyzed data from the first cycle of the Large Hadron Collider and found a deviation from Standard Model predictions in a parameter calculated from the angular distributions, technically known as P’s name5. In the new study, the LHCb team has added LHC data from the second machine period to its analysis and still finds a deviation from the Standard Model calculations in P5 as well as other parameters. However, both old and new results have a statistical significance of about 3 standard deviations, whereas 5 standard deviations is the gold standard in particle physics. It is therefore too early to say whether the discrepancy is statistically significant and, if so, whether it is caused by a new particle or an unknown experimental or theoretical effect.

“It’s a very exciting time to do what we call flavor physics,” said Mat Charles, LHCb Physics Coordinator. “Here and in other related analyses, we continue to see moderate strains with the Standard Model. We still don’t know how this mystery will unfold – nothing has yet reached the level of solid evidence – but we look forward the next set of results using the full LHCb data, which will roughly double the number of new events.

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