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