The mass of the 4\pi^+$

The most precise value for the pion mass was determined from a precision measurement at PSI of the muon momentum in pion decay at rest, \pi^+ \rightarrow \mu^+ + \nu_{\mu}π+→μ++νμ. The result is m_{\pi^+} = 139.570\,21(14)mπ+=139.57021(14)~MeV/c^22. This value is more precise, however, in agreement with the recent compilation of the Particle Data Group for m_{\pi^-}mπ−. The agreement of m_{\pi^+}mπ+ with the recent measurement. This yields a new quantitative measure of CPT invariance in the pion sector: (m_{\pi^+} - m_{\pi^-})/m_{\pi}(\mbox{av}) = (-2.9 \pm 2.0)\cdot 10^{-6}(mπ+−mπ−)/mπ(av)=(−2.9±2.0)⋅10−6, an improvement by two orders of magnitude.