In the Standard Model, each elementary particle appears in three “replica” (also referred to as “flavours”) which differ only by their mass. Why we have three copies of elementary particles, and why they differ only by their mass, is one of the big open question in particle physics. One of the heavier partner of the electron is the muon (μ). The theory predicts that processes where electron or muons are produced have identical properties, once their different masses are taken into account. This is known as Lepton Universality and is a fundamental property of the Standard Model.
The analysis just presented by the LHCb collaboration measures the ratio (RK∗) between the probabilities that a Bomeson decays into a K∗ meson accompanied by a muon-anitmuon pair (μ+μ-) or an electron-antielectron pair (e+e-). This measurement deviates from Standard Model predictions, as shown in the Figure.
Prof. Nicola Serra