TY  - JOUR
PB  - SciPost Foundation
DO  - 10.21468/SciPostPhysProc.5.011
TI  - The mass of the $\pi^+$
PY  - 2021/09/06
UR  - https://scipost.org/SciPostPhysProc.5.011
JF  - SciPost Physics Proceedings
JA  - SciPost Phys. Proc.
VL  - 
IS  - 5
SP  - 011
A1  - Daum, Manfred
AU  - Kettle, Peter-R.
AB  - 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.
ER  -

@Article{10.21468/SciPostPhysProc.5.011,
	title={{The mass of the $\pi^+$}},
	author={M. Daum  and P.-R. Kettle},
	journal={SciPost Phys. Proc.},
	pages={011},
	year={2021},
	publisher={SciPost},
	doi={10.21468/SciPostPhysProc.5.011},
	url={https://scipost.org/10.21468/SciPostPhysProc.5.011},
}