1. Jefferson Laboratory, 12000 Jefferson Ave., Newport News, VA 23606, USA
2. Department of Physics and Astronomy, University of South Carolina, 712 Main St., Columbia, SC 29208, USA
3. School of Physics, Nanjing University, Nanjing 210093, China
4. Institute for Nonperturbative Physics, Nanjing University, Nanjing 210093, China