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The most precise value for the
\pi^0
π
0
mass was obtained from the measurement of the mass difference
m_{\pi^-}-m_{\pi^0} = 4.593\,64(48)
m
π
−
−
m
π
0
=
4.593
64
(
48
)
,MeV/c
^2
2
in the charge exchange reaction
\pi^-
π
−
p
\rightarrow \pi^0
→
π
0
n
at PSI. With the most precise charged pion mass value,
m_{\pi^+} = 139.570\,21(14)
m
π
+
=
139.570
21
(
14
)
,MeV/c
^2
2
and the validity of the CPT theorem (
m_{\pi^-} = m_{\pi^+}
m
π
−
=
m
π
+
),
a value
m_{\pi^0} = 134.976\,57(50)
m
π
0
=
134.976
57
(
50
)
,MeV/c
^2
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.