Improved Limit on the Branching Ratio ofμ→eConversion on Lead

The SINDRUM II spectrometer at Paul Scherrer Institute is used in a search for coherent $\mu \to e$ conversion in muonic atoms. We report on a measurement on lead, which resulted in $\gamma ({\mu }^{-}\mathrm{Pb}\to e^{-}{\mathrm{Pb}}^{\mathrm{g}.\mathrm{s}.\mathrm{}})/\gamma \left({\mu }^{-}\mathrm{Pb}\mathrm{capture}\right)<4.6\mathrm{}\times {10}^{-11\mathrm{}}$ (90% C.L.). This upper limit improves on the previous value by an order of magnitude. Combined with the limit on titanium, it is interpreted in terms of phenomenological couplings between the lepton current and the isoscalar and isovector quark currents.