Lepton flavour universality is a principle in particle physics that concerns how all leptons (electrons, muons and taons) should interact with the fundamental forces of nature. The only difference between these interactions should be due to the different masses of the three particles.

This idea is a crucial testable prediction of the Standard Model and any deviations might suggest new physics beyond it.

Although many experimental results have generally supported this claim, some recent experimental results have shown tensions with its predictions.

Therefore the CMS collaboration at CERN set out to analyse data from proton-proton collisions, this time using a special high-rate data stream, designed for collecting around 10 billion proton decays.

They looked for signs of the decay of B mesons (a bottom quark and an up antiquark) into electron-positron or muon-antimuon pairs.

If lepton flavour universality is true, the likelihood of these two outcomes should be almost equal.

The authors found exactly that. To within their experimental uncertainty, there was no evidence of one decay being more likely than the other.

These results provide further support for this principle and suggest that different avenues ought to be studied to seek physics beyond the Standard Model.

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