1. APC-Astroparticule et Cosmologie (UMR-CNRS 7164), Université de Paris, F-75006 Paris, France
2. Institute of Theoretical Physics & Research Center of Gravitation, School of Physical Science and Technology, Lanzhou University, No. 222, South Tianshui Road, Lanzhou, Gansu 730000, P. R. China
3. Faculdade de Ciências, Instituto de Astrofísica e Ciências do Espaço, Universidade de Lisboa, Ed. C8, Campo Grande, 1769-016 Lisboa, Portugal
4. Lanzhou Center for Theoretical Physics, School of Physical Science and Technology, Lanzhou University, No. 222, South Tianshui Road, Lanzhou, Gansu 730000, P. R. China