1. (Single Arm) Pair Production Cross Section at PrimEx Status Report
Aram Teymurazyan
University of Kentucky
February 24, 2007
PrimEx Collaboration Meeting

2. Outline Event pre-selection Cuts Theoretical calculations
Experimental results
Summary

3. Pair Production in PrimEx
Measured Quantities
Energy of the incident photon
Energies of each lepton
Positions of each lepton

4. Event Sample Selection
Trigger – HyCal_Total_Sum.Tagger_MOR
Production phase only
Miscellaneous checks:
At least 1 reconstructed photon
1 or more clusters in HyCal
EVENTID bank present
At least 1 photon associated with trigger

5. Pair Production Events

6. Applied Cuts 1) 2) 3) 4) 5)
Energy - Momentum conservation is not used in current analysis

7. Pair Production Events

8. Experimental Results

9. Experimental Reults

10. Experimental Results

11. Experimental Results

12. Summary
Measurement of pair production absolute cross section is a check of the absolute photon flux normalization.
Analysis is ongoing.
Agreement with theory at the level of ~3.8 %
Work in progress to reduce the systematic errors to 1 – 2 % level

13. Applied Cuts – Efficiencies

15. State of the Art Calculation of e+e- Cross–Section Considering PrimEx Resolutions
Calculation by Alexandr Korchin:
Bethe-Heitler mechanism of pair production on the nucleus with screening effects due to atomic electrons and Coulomb distortion
Pair production on atomic electrons considering excitation of all atomic states and correlation effects due to presence of other electrons and the nucleus
Radiative corrections (of order ) i) virtual-photon loops and ii) real-photon process like  + →e+ + e- +  + 
Nuclear incoherent contribution  + p →e+ + e- + p
Nuclear coherent contribution (VCS)  + →* + →e+ + e- + 

16. State of the Art Calculation of e+e- Cross–Section Considering PrimEx Resolutions
Calculation by Alexandr Korchin
12C
NIST values for σtotal: GeV and GeV

17. p0 gains
Snake gains

18. p0 gains
Snake gains

19. p0 gains
Snake gains

20. p0 gains
Snake gains

21. Systematic Error Estimation
Experimental Results
Systematic Error Estimation
Photon Flux
2-3%
Target Thickness
0.06%
Background Estimation
0.5%
Calibrations
Total
~3.7%