  We show that it is possible to extract the weak mixing angle alpha via a
measurement of the rate for B^+(-) -> \pi^+(-) e^+e^-. The sensitivity to
cos(alpha) results from the interference between the long and short distance
contributions. The short distance contribution can be computed, using heavy
quark symmetry, in terms of semi-leptonic form factors. More importantly, we
show that, using Ward identities and a short distance operator product
expansion, the long distance contribution can be calculated without recourse to
light cone wave functions when the invariant mass of the lepton pair, q^2, is
much larger than LQCDs. We find that for q^2 > 2 GeV^2 the branching fraction
is approximately 1 * 10^{-8}|V_{td}/0.008|^2. The shape of the differential
rate is very sensitive to the value of cos(alpha) at small values of q^2 with
dGamma /dq^2 varying up to 50% in the interval
  -1< cos(alpha)< 1 at q^2= 2 GeV^2. The size of the variation depends upon the
ratio V_{ub}/V_{td}.
