New Precise Measurement of the Pion Weak Form Factors in pi+ ---> e+ nu gamma Decay

We have measured the pi{sup +}->e{sup +}{nu}{gamma} branching ratio over a wide region of phase space, based on a total of 65 460 events acquired using the PIBETA detector. Minimum-{chi}{sup 2} fits to the measured (E{sub e}{sup +}, E{sub {gamma}}) energy distributions result in the weak form factor value of F{sub A}=0.0119(1) with a fixed value of F{sub V}=0.0259. An unconstrained fit yields F{sub V}=0.0258(17) and F{sub A}=0.0117(17). In addition, we have measured a=0.10(6) for the dependence of F{sub V} on q{sup 2}, the e{sup +}{nu} pair invariant mass squared, parametrized as F{sub V}(q{sup 2})=F{sub V}(0)(1+aq{sup 2}). The branching ratio for the kinematic region E{sub {gamma}}>10 MeV and {theta}{sub e}{sup +}{sub {gamma}}>40 deg. is measured to be B{sup expt}=73.86(54)x10{sup -8}. Earlier deviations we reported in the high-E{sub {gamma}}-low-E{sub e}{sup +} kinematic region are resolved without a tensor term. We also derive new values for the pion polarizability {alpha}{sub E}=2.78(10)x10{sup -4} fm{sup 3} and neutral pion lifetime {tau}{sub {pi}}{sub 0}=(8.5+-1.1)x10{sup -17} s.