FIZIKA B 13 (2004)  1, 127 - 134

A NEW EVALUATION OF THE PION WEAK FORM FACTORS

DINKO POČANIĆ
(for the PIBETA collaboration)

Physics Department, University of Virginia, Charlottesville, VA 22904-4714, USA
E-mail address: pocanic@virginia.edu)

Received 27 October 2003;        Accepted 26 April 2004
Online 16 July 2004

We have used the PIBETA large-acceptance detector for a precise measurement of the p⁺®e⁺ng  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 g=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.

PACS numbers: 14.40.Aq, 13.30.Cz, 11.30.Rd
UDC 539.126.3

Keywords: pion weak form factor, rare decays, radiative decay

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