Pion Form Factor at SND. (new edition)

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Pion Form Factor at SND. (new edition)
Budker Institute of Nuclear Physics,
Siberian Branch of the Russian
Academy of Sciences
M.N. Achasov

• International Workshop ee- Collisions from
  f to Y
  February 27 March 2, 2006,
• BINP, Novosibirsk, Russia

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SND measurement of the ee? ? p p - process
cross section.
The ee? ? pp- cross section. SND
measurement.
SND detector
The cross section of the process ee? ? pp- was
measured in the SND experiment at VEPP-2M in the
energy region vslt1 GeV. This measurements was
based on about 4,5 106 ee? ? pp- events. The
systematic uncertainty of the cross section
determination is 1,3 .
(JETP, Vol. 101,
?6, 2005, p.p. 1053-1070)
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Comparison of the ee? ? pp- cross section
measured by SND, CMD-2, KLOE
Systematic errors 1,3 -
SND, 0,9 - KLOE, 0,6 - CMD-2 (Phys.Lett. B575
(2004)). The yellow area shows the
systematic error of the SND measurements.
The average deviation
between SND and CMD-2 1,4 1,5
Systematic errors 1,3 - SND,
0,8 - CMD-2 (new data). The yellow area
shows the systematic error of the SND
measurements. The average
deviation between SND and CMD-2 2,5 1,5
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Calculation of the ee? ? ee? g, pp-g, mm- g
processes cross sections play quite an important
role in the pion form factor measurement.
process role
ee? ? ee? g normalization
ee? ? mm- g Background subtraction (vs lt 500 MeV)
ee? ? pp-g Radiative correction and detection efficiency determination
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• Recently it was found, that
  ee? ? pp-g and mm- g
  MC events generators, used in SND analysis,
  were not quite correct. So we performed
  reanalysis of SND data using MCGPJ and BABAYAGA
  generators.

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Corrections to the ee? ? pp- process cross
section
Correction due to inaccuracy of ee? ? pp-g
events generator
Correction due to inaccuracy of ee? ? mm-g
events generator
Correction due to inaccuracies of both ee? ?
pp-g and ee? ? mm-g events generators
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Comparison of the SND and CMD-2 results onthe
ee? ? pp- cross section.
The ratio of the ee? ? pp- cross section
obtained in CMD-2 (new data) and SND
experiments. The yellow area shows the systematic
error. The average deviation between SND and
CMD-2 - 0,3 1,6
The ratio of the ee? ? pp- cross section
obtained in CMD-2 (Phys. Lett.B.,575,2004)
experiment to the curve of the fit to SND data.
The yellow area shows the systematic error. The
average deviation between SND and CMD-2
-1,4 1,5
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Comparison of the SND and CMD-2 results onthe
ee? ? pp- cross section.
The ratio of the ee? ? pp- cross section
obtained in CMD-2 (new data) and SND experiments.
The yellow area shows the systematic error.
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Comparison of the isovector part of the ee?
? pp cross section measured by SND with cross
section calculated from the t-?p-p0nt decay
spectral function.

• The ratio of the ee? ? pp cross section
  calculated from the t-?p-p0nt decay
  spectral function measured by ALEPH and CLOE-II
  to the isovector part of the ee? ? pp- cross
  section measured in SND experiments. The yellow
  area shows the systematic error.

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Conclusion

• The ee? ? pp- 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.