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Pion Form Factor at SND . ( new edition )

Budker Institute of Nuclear Physics, Siberian Branch of the Russian Academy of Sciences. Pion Form Factor at SND . ( new edition ). International Workshop « e+e- Collisions from f to Y » February 27 – March 2, 2006, BINP, Novosibirsk, Russia. M.N. Achasov.

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Pion Form Factor at SND . ( new edition )

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  1. Budker Institute of Nuclear Physics, Siberian Branch of the Russian Academy of Sciences Pion Form Factor at SND. (new edition) International Workshop «e+e- Collisions from f to Y»February 27 – March 2, 2006, BINP, Novosibirsk, Russia M.N. Achasov

  2. SND measurement of the e+e→p +p -process cross section. The e+e→p+p- cross section.SND measurement. SND detector The cross section of the process e+e→p+p- was measured in the SND experiment at VEPP-2M in the energy region √s<1 GeV. This measurements was based on about 4,5 × 106 e+e→p+p- events. The systematic uncertainty of the cross section determination is 1,3 %. (JETP, Vol. 101, №6, 2005, p.p. 1053-1070)

  3. Comparison of the e+e→p+p- cross section measured bySND, CMD-2, KLOE Systematic errors:1,3%- SND, 0,9% - KLOE,0,6% - CMD-2 (Phys.Lett. B575 (2004)). Theyellow area shows the systematic error of the SND measurements. The average deviation betweenSNDandCMD-2: 1,4 ± 1,5 % Systematic errors: 1,3% - SND, 0,8% - CMD-2 (new data). Theyellow area shows the systematic error of the SND measurements. The average deviation betweenSNDandCMD-2:2,5 ± 1,5 %

  4. Calculation of thee+e→e+eg, p+p-g, m+m- gprocesses cross sections play quite an important role in the pion form factor measurement.

  5. Recently it was found, that e+e→p+p-g and m+m- g MC events generators, used in SND analysis, were not quite correct. So we performed reanalysis of SND data usingMCGPJandBABAYAGA generators.

  6. Corrections to thee+e→p+p- process cross section Correction due to inaccuracy of e+e→p+p-g events generator Correction due to inaccuracy of e+e→m+m-g events generator Correction due to inaccuracies of both e+e→p+p-g and e+e→m+m-g events generators

  7. Comparison of the SNDandCMD-2 results onthe e+e→p+p- cross section. The ratio of thee+e→p+p- cross section obtained in CMD-2 (new data) andSND experiments. Theyellow area shows the systematic error.The average deviation betweenSNDandCMD-2:- 0,3 ± 1,6 % The ratio of the e+e→p+p- cross section obtained in CMD-2 (Phys. Lett.B.,575,2004) experiment to the curveof the fit to SND data. Theyellow area shows the systematic error.The average deviation betweenSNDandCMD-2: -1,4 ± 1,5 %

  8. Comparison of the SNDandCMD-2 results onthe e+e→p+p- cross section. The ratio of the e+e→p+p- cross section obtained in CMD-2 (new data) andSND experiments. The yellow area shows the systematic error.

  9. Comparison of the isovector part of the e+e→p+p cross section measured by SND with cross section calculated from the t-→p-p0nt decay spectral function. The ratio of the e+e→p+pcross section calculated from the t-→p-p0nt decay spectral function measured by ALEPH and CLOE-II to the isovector part of the e+e→p+p- cross section measured in SND experiments. Theyellow area shows the systematic error.

  10. Conclusion • Thee+e→p+p-cross section measured with SND detector was corrected. Its value decreased by 2 systematic errors. • A good agreement between SND and CMD-2 data becomes a very good. • On the other hand the agreement with t data becomes worse.

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