The BESIII Collaboration has measured the R ratio in the low center-of-mass (c.m.) energy region. The results were published online in Physics Review Letters.

The R value, defined as the ratio of the cross section at the lowest order between the inclusive hadronic process e⁺e_^- → hadrons and the QED process e⁺e^- → μ⁺μ ^-, is a very important quantity in particle physics. Precision measurements of the R ratio below 5 GeV contribute to the standard model (SM) prediction of the muon anomalous magnetic moment. The R ratio also contributes to the determination of the QED running coupling constant evaluated at the Z pole. This observable provides another precision test for the SM and is essential for electroweak precision physics programs at future colliders.

In this measurement, a total of 14 data points were used, with the corresponding c.m. energy ranging from 2.2324 to 3.6710 GeV. Taking advantage of the large data samples, the statistical uncertainty of the measured R ratio is less than 0.6%.

Plenty of effort was made to develop and investigate two different simulation models—the existing inclusive LUARLW generator and a new hybrid one that integrates a few exclusive generators by which inclusive hadronic event production from electron-positron annihilations is reproduced very well. The two simulation models give consistent detection efficiencies and initial-state radiation corrections, with a maximum total difference of less than 2.3%. An accuracy of better than 2.6% below 3.1 GeV and 3.0% above has been achieved in the R ratios, which is much improved from previous results at the level of 3~6%.