A new evaluation of the pion weak form factors

We have used the PIBETA large-acceptance detector for a precise measurement of the pi+->e+nigama radiative decay at rest, with a broad phase space coverage. Using the CVC value for the pion vector form factor, F_V=0.0259(5), we have obtained a new value of the pion axial-vector form factor gamma=F_A/F_V=0.429(14). However, significant deviations from the SM predictions are evident in our data. The discrepancy can be accounted for by introducing a pion tensor form factor of F_T=-0.0016(3). The question of the true nature of the observed deviations remains open pending further theoretical and experimental study.