Measurement of structure dependent radiative $K^{+}\to e^{+}\nu \gamma$ decays using stopped positive kaons

Abstract

The structure dependent radiative $K^{+}\to e^{+}\nu \gamma$ ($K_{e2\gamma }^{\mathrm{S}{\mathrm{D}}^{+}}$) decay was investigated with stopped positive kaons. The $e^{+}$ momentum spectra containing 574$\pm$30 $K_{e2\gamma }^{\mathrm{S}{\mathrm{D}}^{+}}$ events with a $K^{+}\to {\mu }^{+}\nu$ ($K_{\mu 2}$) background of 28$\pm$19 events were measured with and without a photon in coincidence and analyzed with Monte Carlo simulations for acceptance and detector response to extract the ratio of the branching ratio of the $K_{e2\gamma }^{\mathrm{S}{\mathrm{D}}^{+}}$ decay and the $K^{+}\to e^{+}\nu$ decay including the internal bremsstrahlung process ($K_{e2(\gamma )}$). A value of $Br(K_{e2\gamma }^{\mathrm{S}{\mathrm{D}}^{+}})/Br(K_{e2(\gamma )})=1.12\pm 0.07_{\mathrm{stat}}\pm 0.04_{\mathrm{syst}}$ was obtained. This indicates a partial branching ratio, $Br(K_{e2\gamma }^{\mathrm{S}{\mathrm{D}}^{+}},p_e>200\mathrm{MeV}/c,E_{\gamma }>10\mathrm{MeV})/Br(K_{\mu 2})=(1.85\pm 0.11_{\mathrm{stat}}\pm 0.07_{\mathrm{syst}})\times 10^{-5}$, which is $25\%(\sim$2.5$\sigma )$ higher than the previous experimental result.

20 pages, 7 figures