The PDG uses data from two experiments [4,7], both of which have
been
sensitive mainly to the SD
term in (1.15). In [7], 56
events
with 
MeV, 
MeV and 
have been identified, whereas the later experiment [4] has
collected 51 events with 
MeV, 
MeV and
. In these kinematical regions, background from
is absent because 
MeV. The combined result of both experiments is
[4]
For SD
, the bound
has been obtained from a sample of electrons with energies 220 MeV 
MeV [4]. Using (1.21,1.22), the
result (1.28) leads to
The bound (1.29) on the other hand implies [4]
from where one concludes [4] that 
is outside the range
to 
,
As we already mentioned, the interference terms INT
in 
are small and can hardly ever be measured.
As a result of this,
the amplitudes A,V and the ratio 
determined from 
are
subject to the ambiguities (1.25), (1.27).
Here, the interference terms INT
are nonnegligible in appropriate regions
of phase space (see Figs. (1.1,1.2)). Therefore, this
decay allows one in principle to
pin down V and A. The PDG uses data from two experiments
[5,8]. In [5],
the momentum spectrum of the muon was measured in the region (1.20).
In total 
SD
events
have been found with 216 MeV/c 
230 MeV/c and 
MeV, which leads to
In order to identify
the effect of the SD
terms, the region 120 MeV/c
150 MeV/c was searched. Here, the background from 
decays
was very serious. The authors found 142 
candidates and
conclude that
The result (1.33) is consistent with
(1.30), and the bound (1.34) is worse than the
result (1.31) obtained from 
. The
branching ratios which follow [5] from
(1.33,1.34) are displayed in table
1.2, where we also show the 
results [7,4].
The entry SD
+INT
for 
is based on additional
constraints from 
[5].
Table 1.2: Measured
branching ratios 
. The 
data are from
[7,4], the 
data from
[5,8]. The last column corresponds [5]
to the cut
(1.20).
