The sign of the dipole-dipole potential by axion exchange
  We calculate a dipole-dipole potential between fermions mediated by a light
pseudoscalar, axion, paying a particular attention to the overall sign. While
the sign of the potential is physical and important for experiments to discover
or constrain the axion coupling to fermions, there is often a sign error in the
literature. The purpose of this short note is to clarify the sign issue of the
axion-mediated dipole-dipole potential. As a by-product, we find a sign change
of the dipole-dipole potenital due to the different spin of the mediating
particle.
