Measurements of the Dalitz plot distribution (3.17) of 
data
allow one in principle to pin down the form factors
(up to a sign) in the range
.
Measuring the 
branching ratio and
then using
(3.20) gives a relationship between 
and
which is valid
in the approximation (3.8). Furthermore, muon polarization experiments
measure the weighted average of the ratio 
over
the t
range of the experiment [3,25]. On the other hand, the electron modes
are sensitive to 
only, because the other contributions
are suppressed by the factor 
, see eqs.
(3.17), (3.18).
Isospin breaking effects in 
and 
play
a central role in the determination of the Kobayashi-Maskawa matrix element
from 
data, see [26] for a detailed discussion of this
point. In the following we concentrate on the measurement of the slopes
.