The BESIII Collaboration has measured the R ratio in low center-of-mass (c.m.) energy region, published on Feb. 10, 2022 online by Physics Review Letters [Phys. Rev. Lett.128, 062004 (2022)].

The R value, defined as the ratio of cross section at 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 in 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 the electroweak precision physics program at future colliders.

In this measurement, a total of 14 data points are used with the corresponding c.m. energy going from 2.2324 to 3.6710 GeV. Taking the advantage of the large data samples, the statistical uncertainty of the measured R ratio is less than 0.6%. A plenty of efforts have been made to develop and investigate two different simulation models, the existing inclusive LUARLW generator and a new Hybrid one integrated a few exclusive generators, with which the inclusive hadronic event production from electron-positron annihilations are reproduced very well. The two simulation models give consistent detection efficiencies and initial-state-radiation corrections, with a maximum total difference less than 2.3%. An accuracy of better than 2.6% below 3.1 GeV and 3.0% above is achieved in the R ratios, much improved from previous results at the level of 3~6%.

URL: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.062004