1. Energy determination at BEPC-II  Satellite Meeting «On the Need for a super-tau-charm factory» September 26 – 27, 2008, BINP, Novosibirsk, Russia M.N. Achasov (on behalf of BINP,IHEP and Univ. of Hawaii collaboration) Budker Institute of Nuclear Physics, Siberian Branch of the Russian Academy of Sciences

2. Introduction Why in e+e- experiments the accurate beam energy calibration is important in the «c-  » energy region ? The  -lepton mass determination m  =1776,76±0,15 MeV/c 2  lepton is fundamental particle, its mass is important parameter of the Standard Model. The masses of  and D mesons are also of interest.

3. Accuracy of the  mass measurement in BEPC-II – BESIII experiment Statistic uncertainty <20 keV (one week of data taking) Systematic uncertainty (without accuracy of beam energy determination) about 20 keV; include the following sources – luminosity, detection efficiency, branching ratios, background subtraction, energy spread, theoretical uncertainty ) Total error is about 30 keV. [Y.K.Wang, X.H.Mo, C.Z.Yuan, J.P.Liu, Optimization of the data taking strategy for a high precision  mass measurement, Nucl. Instr. and Meth. A583, 2007, pp. 479-484; X.H. Mo, Study of high precision  mass measurement at BESIII, In Proc. Of TAU’06, Nucl. Phys. Proc. Suppl., 169, 2007, pp. 132-139] The most important source of uncertainty is the precision of the absolute beam energy

4. Approaches to the beam energy determination Calibration of the energy scale from scan of the J  and  ′ resonances. Expected accuracy is about 100 keV. [BES Collaboration, Measurement of the mass of  lepton, Phys. Rev. D 53, 1996, pp. 20-31 ] Beam energy measurement using Compton backscattering of monochromatic laser radiation on the e ± beams. This approach was approved in collider experiment at VEPP-4M [V.E.Blinov, A.V.Bogomyakov, N.Yu.Muchnoi, S.A..Nikitin, I.B.Nikolaev, A.G.Shamov, V.N.Zhilich, Review of beam energy measurements at VEPP-4M collider, in Proc. Of INSTR’08]

5. General idea of the Compton backscattering approach The maximum energy of backscattered photon  max is strictly related with the electron energy  so if one measured  max then the electron energy can be obtained: where  0 is the laser photon energy. The  max which is measured by High Purity Germanium (HPGe) detector with ultimately high resolution (~10 -3 ) allows to have statistical accuracy in the beam energy measurement about 10 -5. The systematic accuracy is mostly defined by absolute calibration of the detector. The accurate calibration could be done in the photon energy range up to 10 MeV by using the  active radionuclides.

6. Accuracy of the method The method was tested using the resonant depolarization technique in experiments at VEPP- 4M ( accuracy is about 1 keV.) The comparison of the energy measurements results shows that the Compton backscattering method accuracy is about 40 keV The  mass expected accuracy is about 50 keV

7. BEPC-II electron-positron collider

8. Layout of the beam energy calibration system

9. Deployment place

10. Components of the system Source of photons - CO 2 laser with radiation wavelength =10,6  m (Univ. of Hawaii, USA) Optical system which provide the laser beam transportation to the beams (BINP, Russia). Coaxial HPGe detector (50 mm diameter, 50 mm height) with energy resolution 2,5 keV for 6 MeV photons (BINP, Russia). Laser to vacuum insertion system (BINP, Russia) Sources of  for HPGe calibration (IHEP, China)

11. Optical system Laser will be here

12. Design drawing of vacuum box New Vacuum Chamber

13. Laser to vacuum insertion system Short vacuum chamber with copper mirror Long vacuum chamber

14. Conclusion The BEPC-II energy calibration system is in progress now. We hope to finish its installation and testing in 2009. Then the preliminary physical results can be obtained to the end of 2010.