Measurement of the muon momentum in pion decay at rest using a surface muon beam

doi:10.1016/0370-2693(94)91419-2

Physics Letters B

Volume 335, Issue 2, 1 September 1994, Pages 231-236

https://doi.org/10.1016/0370-2693(94)91419-2 Get rights and content

Abstract

We have measured the momentum of muons from the decay π⁺ → μ⁺ν_μ at rest to be p_μ⁺ = 29.79207±0.00012 MeV/c. This result leads to a laboratory upper limit of 0.16 MeV (CL = 0.9) for the muon-neutrino mass. The cosmological upper limit of the neutrino mass, the muon mass and the new p_μ⁺-value yield the pion mass m_π⁺ = 139.57031 ± 0.00016 MeV. The mean kinetic energy of the pions stopped in the isotropic-graphite production target immediately before their decay is found to be $T_{π^{+}} = 0.415 ± 0.027 eV$ .

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Results from neutrino mass and oscillation experiments now set the mass of the muon neutrino to less than 2 eV/c². This fact, together with our former measurement of the muon momentum in pion decay at rest, $p_{μ^{+}} = (29.792 00 \pm 0.000 11)$ MeV/c, allows us to directly determine the charged pion mass with 1 ppm precision which constitutes the most precise value of the charged pion mass to date, $m_{π^{+}} = (139.570 21 \pm 0.000 14) MeV / c^{2}$ . This value is within 1.44 σ of the Particle Data Group's compilation of the charged pion mass value, $m_{π^{\pm}} = (139.570 61 \pm 0.000 24)$ MeV/c². From $p_{μ^{+}}$ we derive the kinetic energy of the muon, $T_{μ^{+}} = (4.119 84 \pm 0.000 03)$ MeV and the mass difference, $m_{π^{+}} - m_{μ^{+}} = (33.911 84 \pm 0.000 14)$ MeV/c². From our new $m_{π^{+}}$ value, assuming CPT invariance ( $m_{π^{-}} = m_{π^{+}}$ ) and our measured mass difference $D_{π} = m_{π^{-}} - m_{π^{0}} = (4.593 64 \pm 0.000 48)$ MeV/c² we obtain a new value for the neutral pion mass, $m_{π^{0}} = (134.976 57 \pm 0.000 50)$ MeV/c². One also obtains a new quantitative measure of CPT invariance in the pion sector: $(m_{π^{+}} - m_{π^{-}}) / m_{π} (av) = (- 2.9 \pm 2.0) \cdot 10^{- 6}$ , an improvement by two orders of magnitude.
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¹: Present address: Seminar für Angewandte Mathematik, Eidgenössische Technische Hochschule, Rämistrasse 101, CH-8092 Zürich, Switzerland.

²: Present address: Institut für Teilchenphysik, Eidgenössische Technische Hochschule, CH-5232 Villigen-PSI, Switzerland.

³: Present address: Paul-Scherrer-Institut Zürich, Badenerstrasse 569, CH-8048 Zürich, Switzerland.

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Measurement of the muon momentum in pion decay at rest using a surface muon beam

Abstract

Phys. Lett. B

Nucl. Instr. and Meth. A

Journal of Nuclear Materials

Nuc. Phys. A

Nuc. Phys. B

Phys. Rev. D

CERN report 73-16