Stars as Laboratories for Fundamental Physics - MPP Theory Group

Particles Interacting with Electrons and Baryons 109
The same case was treated numerically by Raffelt and Weiss (1995)
who implemented the compound energy-loss rate of Fig. 3.6 with varying
values of α ′ in several red-giant evolutionary sequences. The results
of this work have been discussed in Sect. 2.5.2 where it was used
as one justification for the simple 10 erg g −1 s −1 energy-loss constraint
that was derived there. This detailed numerical study yielded the same
limit Eq. (3.44) on a pseudoscalar Yukawa coupling to electrons.
3.6 Astrophysical Bounds on Yukawa and Gauge
Couplings
3.6.1 Pseudoscalars (Axions)
The main concern of this chapter was novel bosons which interact with
electrons by a dimensionless Yukawa coupling. It will become clear
shortly that these results can be easily translated into limits on baryonic
couplings as well. It may be useful to pull together the main
results found so far, and discuss them in the context of other sources
of information on the same quantities.
The best studied case of boson couplings to electrons is that of
pseudoscalars because the existence of such particles is motivated by
their role as Nambu-Goldstone bosons of a spontaneously broken chiral
symmetry of the fundamental interactions. Within this class, axions
(Chapter 14) have been most widely discussed; they usually serve as a
generic example for low-mass pseudoscalars. For vector bosons which
couple by means of a “magnetic moment” (Eq. 3.3) such as “paraphotons”
the same limits apply apart from an extra factor of 2 in the
emission rate from the two polarization states of these particles.
The simplest constraint on the Yukawa coupling g a of pseudoscalars
(axions) to electrons (α a = ga/4π) 2 arises from the argument that the
age of the Sun precludes any novel energy-loss mechanism to be more
efficient than the surface photon luminosity (Sect. 1.3.2). The relevant
emission processes are the Compton reaction with the energy-loss rate
given in Eq. (3.23), and bremsstrahlung with electrons scattering on
electrons, protons, and helium nuclei; the emission rate was given in
Eq. (3.28). An integration over a typical solar model yields an axion
luminosity (Raffelt 1986a)
L a = α a 6.0×10 21 L ⊙ , (3.45)
with about 25% from the Compton process, 25% from ee bremsstrahlung,
and 50% from bremsstrahlung by electrons scattering on nuclei.