## Lepton universality is verified (no new physics)

In my December 27, 2015 post titled “Lepton Universality” I reported about the early experimental signs pointing to a possible violation of the lepton universality. More data and analysis from the ATLAS experiment at CERN showed however that the lepton universality is exactly true: electrons, muons and taus are produced equally often in weak decays.

“Within the present accuracy, the W boson couplings to all leptons are consistent with the Standard Model prediction, and with lepton flavor universality in particular. Some tensions appearing in earlier global fits are all gone. The Standard Model wins again, nothing to see here, we can move on to the next anomaly. ” – Adam Falkowski

Lepton universality (also known as lepton flavor universality) should not be confused with “charged lepton flavor violation“. In the comments section of Falkowski’s blogpost there is a clarification:

“Lepton flavor violation in the B-meson sector is still an unresolved issue, and it’ll be interesting to see how it plays out.” – Adam Falkowski

Speaking of B decays, there is an excellent review article titled “Review of Lepton Universality in B decays“.

Lepton universality

Ignoring the decays of the $W^{+}$, and ignoring the decay modes involving quarks and gluons, $W^{-}$ decay modes are:

$W^{-} \rightarrow$ electron + electron type antineutrino

$W^{-} \rightarrow$ muon+ muon type antineutrino

$W^{-} \rightarrow$ tau + tau type antineutrino

Muon is 206.85 times heavier than electron and tau is 16.8 times heavier than muon. The rates (probabilities) (also known as branching ratios) of the decays mentioned above are the same which is 10.8%. This is known as lepton universality or lepton flavor universality.

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