Extraction of kaon formfactors from K−→μ−νγ decay at ISTRA+ setup

Abstract The radiative decay K − → μ − ν μ γ has been studied at ISTRA+ setup in a new kinematic region. About 22K events of K − → μ − ν μ γ have been observed. The sign and value of F V − F A have been measured for the first time. The result is F V − F A = 0.21 ± 0.04 ( stat ) ± 0.04 ( syst ) .