Measurement of the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">π</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>→<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="italic">e</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>ν branching ratio

Abstract

A new measurement of the ${\mathrm{\ensuremath{\pi}}}^{+}$\ensuremath{\rightarrow}${\mathit{e}}^{+}$\ensuremath{\nu} branching ratio gives ${\mathit{R}}_{\mathrm{\ensuremath{\pi}}\mathit{e}\ensuremath{\nu}}$=\ensuremath{\Gamma}(\ensuremath{\pi}\ensuremath{\rightarrow}e\ensuremath{\nu}+\ensuremath{\pi}\ensuremath{\rightarrow}e\ensuremath{\nu}\ensuremath{\gamma})/\ensuremath{\Gamma} (\ensuremath{\pi}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\nu}+\ensuremath{\pi}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\nu}\ensuremath{\gamma}) =[1.2265\ifmmode\pm\else\textpm\fi{}0.0034(stat)\ifmmode\pm\else\textpm\fi{}0.0044(sys)]\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$. This result is in agreement with standard model calculations and confirms the hypothesis of electron-muon universality at the 0.2% level.