[1] Princeton Univ, Dept Astrophys Sci, Princeton, NJ 08544 USA
[2] Stanford Univ, Dept Phys, Kavli Inst Particle Astrophys & Cosmol, Stanford, CA 94305 USA
[3] Stanford Univ, Stanford Linear Accelerator Ctr, Stanford, CA 94305 USA
[4] Univ Chicago, Enrico Fermi Inst, Chicago, IL 60637 USA
[5] Kavli Inst Cosmol Phys, Dept Astron & Astrophys, Chicago, IL 60637 USA