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enter image description here

The image shows experimental data for the ratio of cross sections $$R=\frac{\sigma(e^+e^-\rightarrow\text{hadrons})}{\sigma(e^+e^-\rightarrow\mu^+\mu^-)}.$$ I am wondering why $R$ is so large for the $Z$ resonance. I thought that it would only depend on the branching ratios of the $Z$ decay into hadrons and muons, but this gives $R\approx 20$ which is much too small.

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    $\begingroup$ Does this answer your question? Interpreting the Cross Section Ratio R $\endgroup$
    – TEH
    Commented May 11, 2021 at 21:07
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    $\begingroup$ Read the figure caption 5 times! This R, as normalized, excludes Z->μμ decays, including only a theoretical QED normalization, instead! The PDG perpetrators have been highly unfriendly. $\endgroup$
    – Cosmas Zachos
    Commented May 11, 2021 at 22:16

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