The structure dependent radiative $K^+ \rightarrow e^+ \nu \gamma$ ($K_{ e2\gamma}^{ \rm SD^+}$) decay was investigated with stopped positive kaons. The $e^+$ momentum spectra containing 574$\pm$30 $K_{ e2\gamma}^{ \rm SD^+}$ events with a $K^+ \rightarrow \mu^+ \nu$ ($K_{ \mu2}$) 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 $K_{ e2\gamma}^{ \rm SD^+}$ decay and the $K^+ \rightarrow e^+ \nu$ decay including the internal bremsstrahlung process ($K_{ e2(\gamma)}$). A value of $Br(K_{ e2\gamma}^{ \rm SD^+}) / Br(K_{ e2(\gamma)}) = 1.12\pm0.07_{ \rm stat} \pm 0.04_{ \rm syst}$ was obtained. This indicates a partial branching ratio, $Br(K_{ e2\gamma}^{ \rm SD^+}, ~{ p_e>200~{ \rm MeV}/c}, ~{ E_\gamma>10~{ \rm MeV}})/Br(K_{ \mu2}) =(1.85 \pm 0.11_{ \rm{ stat}} \pm 0.07_{ \rm{ syst}}) \times10^{ -5}$, which is $25\%(\sim$2.5$\sigma)$ higher than the previous experimental result.
Categories: hep-ex