[1] Univ Kentucky, Dept Phys & Astron, Lexington, KY 40506 USA
[2] George Washington Univ, Dept Phys, Washington, DC 20052 USA
[3] JeffersonLab, Newport News, VA 23606 USA
[4] Zhejiang Univ, Zhejiang Inst Modern Phys, Hangzhou 310027, Peoples R China