Request for funds for a high accuracy beam energy measurement system for BESIII

The precise determination of the beam energy at BESIII is extremely important for many physics analyses, in particular, for the precise measurement of the τ mass. Fortunately, a method has been developed by physicists from the Budker Institute of Nuclear Physics (BINP) using back scattered Compton photons produced by CO2 laser beams on both the electron and positron beams in BESIII. This method has been demonstrated at BESSY-I, BESSY-II, VEPP-4M, and VEPP-3 storage rings at Novosibirsk. The expected precision at BESIII is at the level of Δε = 40 keV, or Δε/ε ≈ 2 · 10−5. A collaboration has been formed between BINP, IHEP, and the University of Hawaii to install and commission such a system in BESIII. Hawaii will be responsible for the CO2 laser and a power meter. All groups are responsible for contributing to the installation and commissioning. The total cost of the Hawaii proposal is $119 K.