Stars as Laboratories for Fundamental Physics - MPP Theory Group

540 Chapter 14
emitted from an “axion sphere” rather than the entire volume of the
protoneutron star.
These bounds were derived assuming equal couplings to protons and
neutrons. However, a glance at Fig. 14.4 reveals that KSVZ axions essentially
do not couple to neutrons while C p ≈ −0.36. For DFSZ axions
the couplings vary with cos 2 β, although for cos 2 β ≈ 0.5 about the same
values as for KSVZ axions apply which are thus taken as generic. Assuming
a proton fraction of about 0.3 for the relevant regions of the SN
core I estimate an effective nucleon coupling of C N ≈ 0.3 1/2 0.36 ≈ 0.2.
Therefore,
0.01 eV ∼ < m a ∼ < 10 eV,
0.6×10 6 GeV ∼ < f a ∼ < 0.6×10 9 GeV (14.37)
are formally adopted as the SN 1987A excluded axion parameters.
Axions on the “trapping side” of the SN argument can still be excluded
because they would have caused additional events in the IMB
and Kamiokande water Cherenkov detectors. The excluded range of
Eq. (13.2) translates into the approximate mass exclusion range of
20 eV−20 keV.
These limits on the axion mass and decay constant are summarized
in Fig. 14.5. The slanted end of the SN 1987A exclusion bar is a
reminder of the potentially large uncertainty of this limit. Except for
very special choices of model-dependent parameters axions with a mass
above 0.01 eV are excluded.
The high-mass end of the stellar exclusion bars in Fig. 14.5 has
not been worked out in detail because of their overlap with laboratory
limits. The globular cluster limits apply without modification up to a
mass of, say, 30 keV because the temperature in the cores of HB stars
and red giants are about 10 keV. However, axions with masses in this
range interact much more strongly than those at the low-mass end of the
exclusion bar so that the Boltzmann suppression of the emission rate
for a large mass is partly balanced by the increased coupling strength.
14.5 Cosmological Limits
The stellar-evolution bounds on axions have received much attention
because they push the Peccei-Quinn scale to such large values that
axions appear to play a significant cosmological role if they exist at
all. Therefore, to place the stellar constraints into context it may be
useful to close this chapter with a brief summary of the cosmological