Measurement of charged kaon semileptonic decay branching fraction $$K^ - \to e^ - \bar \nu _e \pi ^0$$ using ISTRA+ detector

The ratio of branching fractions for \(K^ - \to e^ - \bar \nu _e \pi ^0\) andK − → π−π0 decays has been measured using the ISTRA+ spectrometer. The result of our measurement is the following: $$\mathcal{R}_{Ke_3 /K_{2\pi } } = 0.2423 \pm 0.0015(stat.) \pm 0.0037(syst.).$$ Using the current PDG value for the K 2π branching fraction, this result leads to the measured K e3 branching fraction of Br(K e3) = 0.0501 ± 0.0009 and to the value of |V us |f +(0) = 0.2115 ± 0.0021.