750 GeV diphotons from closed string states

a Department of Physics and Astronomy, Lehman College, City University of New York, NY 10468, USA b Department of Physics, Graduate Center, City University of New York, NY 10016, USA c Department of Astrophysics, American Museum of Natural History, NY 10024, USA d Departamento de Física, Universidad Nacional de La Plata, C.C. 67, (1900) La Plata, Argentina e LPTHE, UMR CNRS 7589, Sorbonne Universités, UPMC Paris 6, 75005 Paris, France f Albert Einstein Center, Institute for Theoretical Physics, Bern University, Sidlerstrasse 5, CH-3012 Bern, Switzerland g Department of Physics, Northeastern University, Boston, MA 02115, USA h Department of Physics, National Taiwan Normal University, Taipei, 116, Taiwan i Max-Planck-Institut für Physik, Werner-Heisenberg-Institut, 80805 München, Germany j Arnold Sommerfeld Center for Theoretical Physics, Ludwig-Maximilians-Universität München, 80333 München, Germany