AB Lahanas1
, NE Mavromatos2
and DV Nanopoulos3,4,5 
1University of Athens, Physics Department, Nuclear and Particle Physics Section, GR157 71, Athens, Greece.
2King's College London, University of London, Department of Physics, Strand WC2R 2LS, London, UK.
3George P. and Cynthia W. Mitchell Institute for Fundamental Physics, Texas A&M University, College Station, TX 77843, USA.
4Astroparticle Physics Group, Houston Advanced Research Center (HARC), Mitchell Campus, Woodlands, TX 77381, USA.
5Academy of Athens, Division of Natural Sciences, 28 Panepistimiou Avenue, Athens 10679, Greece.