Method for extracting the quark mixing parameter cos alpha via B-+/- -> pi(+/-) e(+)e(-)

We show that it is possible to extract the weak mixing angle ct via a measu rement of the rate for B+/- --> pi(+/-)e(+)e(-). The sensitivity to cos alp ha results from the interference between the long and short distance contri butions. The short distance contribution is given in terms of semileptonic form factors. The long distance contribution can be calculated using Ward i dentities and a short distance operator product expansion if the invariant mass of the lepton pair, q(2), is larger than Lambda(QCD)(2). For q(2) grea ter than or equal to 2 GeV2 the branching fraction is approximately 1 X 10( -8)\V-td/0.008\(2). The shape of d Gamma/dq(2) is very sensitive to the val ue of cos alpha at small values of q(2) and varies by 50% when -1 < cos alp ha < 1 at q(2) = 2 GeV2.