Precision Measurement of the Mass Differencemπ−−mπ0

Using the time-of-flight method to determine the velocity of the neutrons from the reaction ${\pi }^{-}+p\to {\pi }^0+n$ at rest we have measured the mass difference $m_{{\pi }^{-}}{m}_{{\pi }^0}$ to be 4.5930 ± 0.0013 ${\mathrm{M}\mathrm{e}\mathrm{V}/\mathit{c}}^2$. From the width of the time-of-flight distributions we conclude that the pionic hydrogen atoms have a mean kinetic energy ${\overline{T}}_{\pi p}<12\mathrm{}$ eV (90% confidence limit) at the time of the charge exchange.