We  entertain  the  idea  that a
suitable background of cold (very low momentum)
pseudoscalar    particles   or   condensate,may
trigger a background that effectively generates
Lorentz-invariance  violation. This aether-like
background  induces a Chern-Simons modification
of  QED.  Physics  is  different  in  different
frames   and,   in   the   rest  frame  of  the
pseudoscalar  background, high momentum photons
can  decay  into  pairs. The threshold for such
decay depends quadratically on the rest mass of
the  particles. This mechanism could explain in
a  natural  way  why  antiprotons are absent in
recent  cosmic  ray measurements (PAMELA, CAST,
FERMI-LAT). A similar signal could be used as a
probe of pseudoscalar condensation in heavy ion
collisions at RHIC (PHENIX anomaly).