A new evaluation of the pion weak form factors

Abstract

We have used the PIBETA large acceptance detector for a precise measurement of the ${\pi }^{+}\to e^{+}\nu \gamma$ radiative decay at rest, with 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.

Comments: Talk presented at the 2nd International Conference on Nuclear and Particle Physics with CEBAF, held 26-31 May 2003 in Dubrovnik, Croatia. 7 pages, 3 figures. Proceedings will be published in the journal Fizika B