1. High precision measurement of the beam energy at VEPP-4M collider by Yury Tikhonov KEDR&VEPP-4M collaboration Budker INP, Novosibirsk
Outlook
Introduction
Resonance depolarization technique
Compton backscattering
Applications
Conclusion
11 Pisa meeting on advanced detectors, La Biodola, Isola Elba, Italy, May 24-30

2. Introduction
The energy in a storage ring is determined by the integral of the transverse magnetic field (H) and the orbit length (L):
E~L*∫Hdl
The precision of this method is limited to dE/E~10ˉ³ and it is not sufficient for particle physics (particle mass and total cross section measurements, spectroscopy etc)
A few method was invented for the absolute energy measurement in a storage ring:
-resonance depolarization method, dE/E~10ˉ⁷ (Budker INP, 1975)
-dependence of the SR intensity on beam energy, dE/E~10ˉ⁵ (Budker INP, 1978)
-Compton backscattering, dE/E~10ˉ⁴ (BESSY, 1997)

3. Resonance depolarization method
Electrons and positrons in a storage ring become polarized due to Sokolov-Ternov effect (1964): probability of SR with spin flip depends on spin direction to magnetic field
p ~ R/
The spin precession frequency depends on the beam energy:

4. Resonance depolarization method BEAM ENERGY SPREAD AND SPIN SPECTRA at VEPP-4M
In homogeneous magnetic field a width of the spin spectra is ~10ˉ⁹ !
In real storage ring it is ~10ˉ⁷ due to betatron oscillations and nonlinearity of magnetic field and noise in magnet system

5. Resonance depolarization method
The spin precession frequency can be measured by resonance depolarization by external electromagnetic field
So, methods for polarization measurements are needed. A few method have been invented and developed:
-intra-beam scattering (Touschek effect)
-angle asymmetry of the backscattered polarized laser photons on a beam
-angle asymmetry of the backscattered polarized SR photons on a beam
-spin dependence of SR intensity
(Budker INP, SLAC, DESY, Cornel. CERN……)
The resonance depolarization method has been used in many experiments for high precision measurements of particle masses and total cross section measurements (VEPP-2M, VEPP-4, VEPP-4M, DORIS-2, CESR, LEP….):
Kˉ ,K°,K⁺, ,,,J/, (nS),(nS),  precision 10ˉ⁴ ÷ 10ˉ⁵ (
Recent development at VEPP-4M: precision 10ˉ⁶ !

6. Resonance depolarization at VEPP-4M
Touschek effect:
Intra-beam cross-section
depends on polarization.
Scattered particles hit
the scintillation counters

7. Resonance depolarization at VEPP-4M
200 kHz
per 2 mA
in a bunch
=(fpol-funpol)/fpol
f =IBS counting rate

8. Energy measurement at VEPP-4M by RD with an accuracy of 5
Energy measurement at VEPP-4M by RD with an accuracy of 5*10-7 using the IBS polarimeter
1-funpol/ fpol

9. e- e+ Scan speed=55 eV/sec Ep - Ee =(1.32±0.14) keV
Comparison of the depolarization frequencies of electrons and positrons VEPP-4M (electron-positron energy gap measurement)
Scan speed=55 eV/sec e- e+
Ep - Ee =(1.32±0.14) keV

10. Partial depolarization of the beam at VEPP-4M (spin gymnastic…)

11. The example of the long-term beam energy behaviour measured by RD
The example of the long-term beam energy behaviour measured by RD. Error bars show the mean deviation from the fit

12. Compton backscattering
VEPP-4M, , MeV

13. Compton backscattering at VEPP-4M
The precision of energy measurement
is KeV ( 1keV for RD!)
BUT!
No polarized beams needed
Energy measurements during
experiments (in case of any jumps RD
used)
Energy spread can be measured with
precision of 5%
A few isotopes used for detector calibration and stability check
The final calibration by resonance depolarization

14. Resonance depolarization and Compton backscattering at VEPP-4M
The VEPP-4M beam energy is know with precision better 20 keV at
any time and it is sufficient for all applications

15. High precision measurement of the beam energy at VEPP-4M: applications
The motivations for development of high precision measurements of beam energy:
Improvement of the mass measurement of J/ and  - families and D-mesons (important for spectroscopy)
Improvement of the -lepton mass measurement (test of lepton universality)
Precision measurement of the e+e- hadrons cross section in the region 2E= Gev (important for g-2,  and s determination)
High precision test of CPT theorem by comparison of spin precession frequencies of electrons and positrons
The motivations for development of high precision measurements of beam energy:
Improvement of the mass measurement of J/ and  - families and D-mesons (important for spectroscopy)
Improvement of the -lepton mass measurement (test of lepton universality)
Precision measurement of the e+e- hadrons cross section in the region 2E= Gev (important for g-2,  and s determination)
High precision test of CPT theorem by comparison of spin precession frequencies of electrons and positrons

16. KEDR detector at VEPP-4M
In operation since 2002

17. High precision mass measurement with KEDR detector at VEPP-4M
An example of J/ mass measurement

18. High precision mass measurement with KEDR detector at VEPP-4M
MJ/ψ= ± ± MeV
Mψ’ = ± ± MeV
Mψ’’= ± 0.9 ± 0.6 MeV
The precision was improved 3-6 times compare to PDG value

19. High precision mass measurement with KEDR detector at VEPP-4M
-lepton mass measurement
ee→ττ
τ-lepton mass
Best results now: precision is 1.5 better then PDG value

20. High precision test of CPT
Spin precession frequency  is equal to:
So, comparison of the precession frequency of electron
and positron is sensitive to m , e and g-2 !
Now the differences of the electron and positron mass,
charge and g-2 are below 6*10ˉ , 8*10ˉ and 4*10ˉ
respectively
Our preliminary study shows that we can compare
precession frequency for electrons and positron
with precision of 10ˉ⁹

21. Conclusion
The new development of the methods for high precision measurement of beams energy allows to realize a wide physics program with KEDR at VEPP-4M:
Improvement of the mass measurement of J/ and  - families and D-mesons
Improvement of the -lepton mass measurement
Precision measurement of the e+e- hadrons cross section in the region 2E= Gev
High precision test of CPT theorem by comparison of spin precession frequencies of electrons and positrons