Stars as Laboratories for Fundamental Physics - MPP Theory Group

Axions 539
Bounds on the Yukawa coupling to electrons of various novel particles
were derived in Chapter 3; for pseudoscalars a summary was given
in Tab. 3.1. The most restrictive limit was obtained from the delay of
helium ignition in low-mass red giants that would be caused by excessive
axion emission; in terms of the axion-electron Yukawa coupling it
is g ae ∼ < 2.5×10 −13 . With Eq. (14.27) this translates into
m a C e ∼ < 0.003 eV and f a /C e ∼ > 2×10 9 GeV. (14.33)
Axions which interact too strongly to escape freely from the interior
of stars would still contribute to the transfer of energy. For the Sun,
this issue was studied in Sect. 1.3.5. One easily finds that for C e = 1
Fig. 1.2 excludes axion masses below about 50 keV.
In hadronic axion models C e = 0 at tree level and so no interesting
bounds on m a and f a obtain. In the DFSZ model, C e was given in
Eq. (14.28). Taking the number of families to be N f = 3 one finds
m a cos 2 β ∼ < 0.01 eV and f a / cos 2 β ∼ > 0.7×10 9 GeV. (14.34)
These limits depend on the parameter cos 2 β which, in principle, can
be equal to 0.
The axion-photon coupling is best constrained by the lifetime of
horizontal-branch (HB) stars as outlined in Sect. 5.2.5. The limit
Eq. (5.23) translates into
m a ξ ∼ < 0.4 eV and f a /ξ ∼ > 1.5×10 7 GeV, (14.35)
where ξ was defined in Eq. (14.25). In addition, approximately the
mass range 4−14 eV is excluded by the “telescope search” for a line
from the radiative decay of cosmic axions (Fig. 12.23).
The most restrictive limit on the axion-nucleon coupling arises from
the duration of the neutrino signal of SN 1987A. The formally excluded
range for the axion-nucleon Yukawa coupling was specified in
Eq. (13.11); as discussed in Sect. 13.5 it is fraught with uncertainties
because no reliable calculation of the axion emission rate from a nuclear
medium is available at the present time. In terms of the axion mass
and axion decay constant the nominally excluded range is
0.002 eV ∼ < C N m a ∼ < 2 eV,
3×10 6 GeV ∼ < f a /C N ∼ < 3×10 9 GeV. (14.36)
The case of large m a (small f a ) is the trapping regime where axions
contribute to the energy transfer in a SN core, and where they are