Stars as Laboratories for Fundamental Physics - MPP Theory Group

Chapter 14
Axions
The idea of axions is introduced and their phenomenological properties
are reviewed. The constraints on pseudoscalars that have been derived
throughout this book are systematically applied to axions.
14.1 The Strong CP-Problem
All fermions, with the possible exception of neutrinos, have magnetic
dipole moments—see Tab. 14.1 for several important examples. The
minimal electromagnetic coupling of charged spin- 1 fermions (charge q,
2
mass m) automatically yields a “Dirac moment” q/2m. Higher-order
amplitudes lead to an additional “anomalous” contribution, which is
the only one for neutral particles. For example, the magnetic moments
of massive neutrinos calculated in the standard model were given in
Eq. (7.16). The QED prediction of the electron anomalous magnetic
moment is perhaps the most stunning quantitative success of theoretical
physics.
On the other hand, no particle electric dipole moment has ever
been detected—see Tab. 14.1 for some upper limits. At first this is
quite satisfying because an electric dipole moment would allow one to
distinguish between matter and antimatter in an absolute sense. 88 The
electroweak and strong gauge interactions are CP-conserving, so the
observed broad symmetry between particles and antiparticles appears
natural.
88 In the nonrelativistic limit the electric dipole operator d must be proportional
to the spin operator s which is an axial vector. Because the electric field is a polar
vector the energy d·E reverses sign under P and thus under CP whence its absolute
sign yields an absolute distinction between fermions and antifermions.
524