With the joint effort from the staff of Beijing Electron Positron Collider (BEPCII) and the collaborators of the BEijing Spectrometer (BESIII) experiment for 4 months, the BESIII Collaboration has finished the data taking in the low energy region of 2-3 GeV on May 1, 2015.
The purpose of this round of experiment is the R-value measurement and the Quantum Chromodynamics (QCD) relevant studies in the low energy. The center-of-mass energy covers from 3.08 GeV just below the J/psi resonance to the lowest 2.0 GeV that BEPCII can reach in principle but never reached before, which is a big challenge to the accelerator. The BEPCII and BESIII people not only managed to finish colliding data collection at 19 energy points and separated-beam mode at 2 energy points according to the run plan, but also added a new point 2.05 GeV based on the result of quick analysis. The total integrated luminosity is 525.5 pb^-1, which is the largest data sample in the world in the same energy region. The data is important to the precision measurement of R values and the experimental test of QCD theory, and has unique advantage compared to other high energy experiments like Belle in Japan and BaBar in America using initial state radiation technique for similar studies.
The BESIII Collaboration currently has 52 member institutions from 12 countries, about 400 researchers, and 24 of them are from the University of Science and Technology of China (USTC). The USTC team plays a leading role in the project of R measurement and QCD studies, which is the only case in the history of the BES (I, II, III) experiments. The USTC team took a large fraction of shifts during the data collection: in the 17-week running, Profs Guangshun Huang, Wenbiao Yan and Haiping Peng acted as Weekly Coordinator for 6 weeks, and Academician Zhengguo Zhao also made frequent trips to the BESIII control room for experiment monitoring.
R value is the ratio of the total cross section of electron-positron annihilation into hadrons to that for muon pairs, and it is an important input parameter in the Standard Model calculation. Because the perturbative theory is not applicable in the low energy, the experimental measurements and tests of QCD are mandatory. BEPCII/BESIII is the only facility for high energy physics in China, which operates in the energy of 2 – 4.6 GeV. This work is supported in part by Key Project of the National Natural Science Foundation of China and Fundamental Research Project (973) of the Ministry of Science and Technology of China.
(HUANG Guangshun,School of Physical Sciences)
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