Scalar interaction inτdecay

Study of resonant and nonresonant production of πην and KK¯ν in τ decay leads us to a model-independent inequality between their branching ratios which is based on approximate flavor-SU(3) symmetry: B(${\mathrm{\tau }}^{\mathrm{-}}$→${\mathrm{\pi }}^{\mathrm{-}}$ην)${\mathrm{\tau }}^{\mathrm{-}}$→${\mathrm{K}}^{\mathrm{-}}$ ${\mathrm{K}}^0$ν). It is shown that an accurate determination of an upper limit on B(${\tau }^{\mathrm{-}}$→$K^{\mathrm{-}}$ $K^0$ν) is the most effective way to constrain experimentally the magnitude of a possible scalar interaction in τ decay.