1. Department of Physics and Astrophysics, University of California, Los Angeles, CA 90095, USA
2. Key Laboratory of Quark & Lepton Physics (MOE) and Institute of Particle Physics, Central China Normal University, Wuhan 430079, China
3. Department of Physics, Purdue University, West Lafayette, IN 47907, USA
4. Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA