The decay $K^{-} to mu^{-} bar{ u} pi^{0}$ has been studied using in-flight decays detected with ``ISTRA+ setup operating in the 25 GeV negative secondary beam of the U-70 PS. About 112K events were used for the analysis. The $lambda_{+}$ and $lambda_{0}$ slope parameters of the decay formfactors $f_{+}(t)$, $f_{0}(t)$ have been measured : $lambda_{+}= 0.0321 pm 0.004$(stat) $pm 0.002$(syst) $lambda_{0}= 0.0209 pm 0.004$(stat) $pm 0.002$(syst); the correlation $d lambda_{0}/d lambda_{+}=-0.46$ The limits on the possible tensor and scalar couplings have been derived: $f_{T}/f_{+}(0)=-0.021 pm 0.028$(stat) $pm 0.014$(theory); $f_{S}/f_{+}(0)=0.004 pm 0.005$(stat) $pm 0.005$(theory)
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