R Measurement at BESIII.

The R value is measured with a total of 14 data points with the corresponding center-of-mass energy going from 2.2324 to 3.6710 GeV. The statistical uncertainty of the measured R is less than 0.6%. Two different simulation models, the LUARLW and a new Hybrid generator, are used and give consistent detection and initial-state radiation corrections. An accuracy of better than 2.6% below 3.1 GeV and 3.0% above is achieved for the R value. The precise measurement will be an important constraint for precision tests of the Standard Model, such as calculations of the anomalous magnetic moment of the moun and the running of the QED coupling. [ABSTRACT FROM AUTHOR]