Regular Article - Theoretical Physics

The branching ratio revisited

¹ Institut für Kernphysik, Institute for Advanced Simulation and Jülich Center for Hadron Physics, Forschungszentrum Jülich, 52425, Jülich, Germany
² Helmholtz-Institut für Strahlen- und Kernphysik, Universität Bonn, 53115, Bonn, Germany
³ Bethe Center for Theoretical Physics, Universität Bonn, 53115, Bonn, Germany
⁴ Division of Nuclear Physics, Department of Physics and Astronomy, Uppsala University, Box 516, 75120, Uppsala, Sweden
⁵ High Energy Physics Department, National Centre for Nuclear Research, ul. Hoza 69, 00-681, Warsaw, Poland
⁶ Kavli Institute for Theoretical Physics, Kohn Hall, University of California, Santa Barbara, CA, 93106-4030, USA

^* e-mail: kubis@hiskp.uni-bonn.de

Received: 30 November 2016
Accepted: 26 January 2017
Published online: 14 February 2017

Abstract

We analyze the most recent data for the pion vector form factor in the timelike region, employing a model-independent approach based on dispersion theory. We confirm earlier observations about the inconsistency of different modern high-precision data sets. Excluding the BaBar data, we find an updated value for the isospin-violating branching ratio . As a side result, we also extract an improved value for the pion vector or charge radius, , where the first uncertainty is statistical as derived from the fit, while the second estimates the possible size of nonuniversal radiative corrections. In addition, we demonstrate that modern high-quality data for the decay will allow for an even improved determination of the transition strength .