Improved measurement of the $π\rightarrow \mbox{e} ν$ branching ratio

A new measurement of the branching ratio, $R_{e/μ} =Γ(π^+ \rightarrow \mbox{e}^+ ν+ π^+ \rightarrow \mbox{e}^+ νγ)/ Γ(π^+ \rightarrow μ^+ ν+ π^+ \rightarrow μ^+ νγ)$, resulted in $R_{e/μ}^{exp} = (1.2344 \pm 0.0023 (stat) \pm 0.0019 (syst)) \times 10^{-4}$. This is in agreement with the standard model prediction and improves the test of electron-muon universality to the level of 0.1 %.

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