What does "Lepton Flavour Universality" mean?

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Lepton Flavour Universality (LFU) is a concept in particle physics that suggests different types of leptons, such as electrons, muons, and tau particles, should behave similarly when involved in certain processes. This idea is based on the Standard Model, which is our best theory explaining how particles and forces interact.

In recent experiments, scientists have looked closely at how certain particles decay, particularly focusing on $b$-hadrons. These are particles that contain a bottom quark. When these particles decay, they can produce different types of leptons. By comparing the behavior of these decays for different leptons, researchers test if they behave as expected under LFU.

So far, measurements have shown some surprising results. The ratios of decay rates involving different leptons indicate that there might be differences that go against what the Standard Model predicts. This suggests the possibility of new physics beyond our current understanding. While some measurements align with the predictions, others highlight a gap, sparking curiosity and further investigation into the rules governing particle interactions.