Contributor info: Dr Manfred Daum

M. Daum, P.-R. Kettle

SciPost Phys. Proc. 5, 012 (2021) · published 6 September 2021 | Toggle abstract · pdf

The most precise value for the $\pi^0$ mass was obtained from the measurement of the mass difference $m_{\pi^-}-m_{\pi^0} = 4.593\,64(48)$\,MeV/c$^2$ in the charge exchange reaction $\pi^-$p $\rightarrow \pi^0$n at PSI. With the most precise charged pion mass value, $m_{\pi^+} = 139.570\,21(14)$\,MeV/c$^2$ and the validity of the CPT theorem ($m_{\pi^-} = m_{\pi^+}$), a value $m_{\pi^0} = 134.976\,57(50)$\,MeV/c$^2$ is obtained. The measurements also revealed, for the first time, evidence of an unexpectedly large contribution from Coulomb de-excitation states during the pionic atom cascade.

M. Daum , P.-R. Kettle

SciPost Phys. Proc. 5, 011 (2021) · published 6 September 2021 | Toggle abstract · pdf

At the Paul Scherrer Institute, the most precise pion mass value was obtained from a precision determination of the muon momentum, $p_{\mu^+}$, in pion decay at rest, $\pi^+ \rightarrow \mu^+ + \nu_{\mu}$. We find $p_{\mu^+} = (29.792\,00 \pm 0.000\,11$ MeV/c and $m_{\pi^+}$ = (139.570\,21 $\pm$ 0.000\,14) MeV/c$^2$. This value is more precise, however, in agreement with the recent compilation of the Particle Data Group for $m_{\pi^-}$. The agreement of $m_{\pi^+}$ with the most precise and recent single measurement, $m_{\pi^-}$ = (139.570\,77 $\pm$ 0.000\,18) MeV/c$^2$ is only fair (2.43 $\sigma$). From our $p_{\mu^+}$ value and using the above mentioned $m_{\pi^-}$ mass and the mass of the muon, $m_{\mu^+} = (105.658\,374\,5 \pm 0.000\,002\,4)$ MeV/c$^2$ we obtain an upper limit for the muon neutrino mass with 90 \%confidence, $m_{\mu_{\nu}} \leq 230$ keV/c$^2$. One also obtains a new quantitative measure of CPT invariance in the pion sector: $(m_{\pi^+} - m_{\pi^-})/m_{\pi}(\mbox{av}) = (-2.9 \pm 2.0)\cdot 10^{-6}$, an improvement by two orders of magnitude.

M. Daum, D. Gotta

SciPost Phys. Proc. 5, 010 (2021) · published 6 September 2021 | Toggle abstract · pdf

The most precise values of the mass of the negatively charged pion have been determined from several measurements of X-ray wavelengths for transitions in pionic atoms at PSI. The Particle Data Group gives the average $m_{\pi^-}= (139.570 61 \pm 0.000 24) MeV/c^2$.