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
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*∫Hdl 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)
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:
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
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 !
Resonance depolarization at VEPP-4M Touschek effect: Intra-beam cross-section depends on polarization. Scattered particles hit the scintillation counters
Resonance depolarization at VEPP-4M 200 kHz per 2 mA in a bunch =(fpol-funpol)/fpol f =IBS counting rate
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
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
Partial depolarization of the beam at VEPP-4M (spin gymnastic…)
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
Compton backscattering VEPP-4M, 2006-09, 900-2000 MeV
Compton backscattering at VEPP-4M The precision of energy measurement is 30-50 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
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
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=1.8-4.0 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=1.8-4.0 Gev (important for g-2, and s determination) High precision test of CPT theorem by comparison of spin precession frequencies of electrons and positrons
KEDR detector at VEPP-4M In operation since 2002
High precision mass measurement with KEDR detector at VEPP-4M An example of J/ mass measurement
High precision mass measurement with KEDR detector at VEPP-4M MJ/ψ= 3096.917 ± 0.010 ± 0.007 MeV Mψ’ = 3686.117 ± 0.012 ± 0.015 MeV Mψ’’= 3773.5 ± 0.9 ± 0.6 MeV The precision was improved 3-6 times compare to PDG value
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
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
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=1.8-4.0 Gev High precision test of CPT theorem by comparison of spin precession frequencies of electrons and positrons