Abstract
We report the first measurement of the absolute branching fraction for . This measurement is based on of annihilation data produced at , which is just above the threshold. The data were collected with the BESIII detector at the BEPCII storage rings. The branching fraction is determined to be , representing a significant improvement in precision over the current indirect determination. As the branching fraction for is the benchmark for those of other semileptonic channels, our result provides a unique test of different theoretical models, which is the most stringent to date.
- Received 9 October 2015
DOI:https://doi.org/10.1103/PhysRevLett.115.221805
© 2015 American Physical Society