New Precise Measurement of the Pion Weak Form Factors inπ+→e+νγDecay

Abstract

We have measured the π+→e+νγ branching ratio over a wide region of phase space, based on a total of 65 460 events acquired using the PIBETA detector. Minimum-χ2 fits to the measured (Ee+, Eγ) energy distributions result in the weak form factor value of FA=0.0119(1) with a fixed value of FV=0.0259. An unconstrained fit yields FV=0.0258(17) and FA=0.0117(17). In addition, we have measured a=0.10(6) for the dependence of FV on q2, the e+ν pair invariant mass squared, parametrized as FV(q2)=FV(0)(1+aq2). The branching ratio for the kinematic region Eγ>10 MeV and θe+γ>40° is measured to be Bexpt=73.86(54)×10−8. Earlier deviations we reported in the high-Eγ–low-Ee+ kinematic region are resolved without a tensor term. We also derive new values for the pion polarizability αE=2.78(10)×10−4 fm3 and neutral pion lifetime τπ0=(8.5±1.1)×10−17 s.Received 15 April 2008DOI:https://doi.org/10.1103/PhysRevLett.103.051802©2009 American Physical Society