1. Measurements of the Electromagnetic Form Factor of the Proton Huihong Li June 9th 2004

2. Outline 1.Introduction 2.Event selection 3.Luminosity 4.Efficiency 5.Result 6.Summary

3. 1. Introduction ▪ The nucleon is not pointlike, but by far there is no perfect theory to describe its complex structure. We can only empirically use electromagnetic form factor to describe the distributions of inner charge and magnetic moment. ▪ Four form four form factors :,,, ▪ The early measurements of proton form factor were in the space-like region, and there are a lot of results which show that empirical dipole formula can work well for proton form factors.

4. ▪ While in the time-like region, the each experiment group's data is very limited. Especially for 6 < s <9, there is no data at present. ▪ Two processes, and, are used to measure proton form factor. In the figure and table, the studies of these two processes from other experiments are shown.

5. 2. Event selection

11. 3. Luminosity ▪ The integrated luminosity at each data point was calculated via an analysis of wide-angle Bhabha events. ▪ The method was as follows: We first select signal Bhabha events using BSC information only. The efficiency of this selection procedure is determined by using a non-BSC selection method to find Bhabha events in the data and comparing efficiencies. The motivation for using a data- based efficiency deterimation was to avoid known discrepancies between the MC and data angular distributions due to ribs in the BSC. ▪ The final calculation was:

12. 4. Efficiency ▪ With M.C data the efficiency can be calculated. Here we use v10402 SIMBES to generate M.C data. In the generator of p2bb( ), some corrections about the initial state radiation, Coulomb effect and final state have been taken into account. ▪ Initial state radiation correction: * In 1985, Kuraev and Fadin presented to use the structure function of the electron and positron to calculate the initial state radiative correction. * This scheme is a universal component of the corrections to processes with an arbitrary final state(such as, ).

14. ▪ Coulomb effect correction: ▪ Final state correction

15. ▪ Efficiency: With the generator taken account of initial radiative correction, Coulomb effect, and final state correction, 10 thousand M.C events for each energy point are generated. The detection efficiency is calculated by submitting M.C events to the same cut criterion used for real data.

16. 5. Result ▪ Form factor

17. ▪ Error analysis

20. 6. Summary ▪ The proton form factors have been presented for 10 center- of-mass energies between 2.00 and 3.07 GeV about 2 batches data: R scan data taking in 1999 which are reconstructed by v103 and other 3 points data in 2004. ▪ There is no one signal at 2.9GeV, so the upper limits of the cross section and form factor are presented at 90% C.L. ▪ The result is consistent to the PQCD prediction, the express in experiment is, in which =0.3GeV is the QCD scale parameter and C is a free parameter.