Stars as Laboratories for Fundamental Physics - MPP Theory Group

Radiative Particle Decays 483
12.4.9 Axions
Another hypothetical particle that could have been emitted abundantly
from SN 1987A is the axion. The impact of the axionic energy loss is
discussed in Sect. 13.5. If axions are more strongly interacting than a
certain limit, implying that their mass is larger than a few eV, they
are emitted from the surface of the SN core with a luminosity similar
to that of neutrinos. Kolb and Turner (1989) found that the axion
fluence from SN 1987A would have been F a ≈ 6×10 10 cm −2 m −12/11
eV
with m eV ≡ m a /eV at a temperature of T a ≈ 15 MeV m −4/11
eV . From
the GRS fluence limits, Kolb and Turner found that m a must be less
than a few 10 eV, a bound which is less restrictive than, for example,
the limit from globular cluster stars (Sect. 5.2.5).
12.4.10 Supernova Energetics
To derive the various GRS limits one had to assume that the radiative
decays occurred outside of the progenitor’s envelope and so neutrinos
falling into the shaded area in Fig. 12.16 were not accessible to these
arguments. However, in this case the stellar envelope itself serves as a
“detector” as discussed by Falk and Schramm (1978) many years ago;
see also Takahara and Sato (1986). Supernova observations in general,
and those of SN 1987A in particular, indicate that of the approximately
3×10 53 erg of released gravitational binding energy only a small fraction
on the order of one percent becomes directly visible in the form of the
optical explosion as well as the kinetic energy of the ejecta. In contrast,
even if only one of the neutrino species decayed radiatively within the
progenitor, about 30% of the binding energy would light up!
If the lifetime were so short that the parent neutrinos would never
get far from the SN core one would not have to worry. Therefore, the
critical range of decay times is between the core dimensions of about
30 km = 10 −4 s and the envelope radius of about 100 s. If the neutrinos
had nonradiative decay modes, and if their total laboratory lifetime fell
into this range, one could only conclude that B < γ ∼ 10 −2 .
If they decayed only into radiation, and taking E ν to be 10 MeV,
the quantity τ γ /m ν cannot lie between about 10 −11 and 10 −5 s/eV (for
heavy neutrinos τ γ includes the e + e − channel). For an effective transition
moment µ eff , an interval between about 10 2 and 10 5 µ B m −2
eV is
excluded, moderately interesting only for large masses. Then, however,
the cosmological limits strongly suggest the presence of nonradiative
decay channels.