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

IntroductionWhy in e+e- experiments the accurate beam energy calibration is important in the c-t energy region ? The t-lepton mass determination mt=1776,760,15 MeV/c2 t-lepton is fundamental particle, its mass is important parameter of the Standard Model.The masses of y and D mesons are also of interest.

Accuracy of the t mass measurement in BEPC-II BESIII experimentStatistic uncertainty

Approaches to the beam energy determinationCalibration of the energy scale from scan of the J/y and y resonances. Expected accuracy is about 100 keV.[BES Collaboration, Measurement of the mass of t 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 INSTR08]

General idea of the Compton backscattering approachThe maximum energy of backscattered photon wmax is strictly related with the electron energy e, so if one measured wmax then the electron energy can be obtained:

where w0 is the laser photon energy. The wmax 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 g-active radionuclides.

Accuracy of the methodThe 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 keVThe t mass expected accuracy is about 50 keV

BEPC-II electron-positron collider

Layout of the beam energy calibration system

Deployment place

Components of the systemSource of photons - CO2 laser with radiation wavelength l=10,6mm (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 g for HPGe calibration (IHEP, China)

Optical system Laser will be here

Design drawing of vacuum boxNew Vacuum Chamber

Laser to vacuum insertion systemShort vacuum chamber with copper mirrorLong vacuum chamber

ConclusionThe 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.