Stars as Laboratories for Fundamental Physics - MPP Theory Group

462 Chapter 12
12.3.3 New Particles
Besides neutrinos, stars can also produce other weakly interacting particles
by both plasma and nuclear processes. With a temperature of
about 1.3 keV in the solar center the former reactions would produce a
relatively soft spectrum and so I focus on nuclear reactions where MeV
energies are available. If the new particle is a boson and if it couples
to nucleons, it will substitute for a photon with certain relative rates r
in reactions with final-state γ-rays. A short glance at the nuclear reaction
chains shown in Fig. 10.2 reveals that a particularly useful case is
p + d → 3 He + γ with E γ = 5.5 MeV. This reaction occurs about 1.87
times for every 4 He nucleus produced by fusion in the Sun and so it
must occur about 1.7×10 38 s −1 . A certain fraction of the particles produced
will be reabsorbed or decay within the Sun. If their probability
for escaping is p the Sun emits r p 1.7×10 38 s −1 of the new objects.
If the new particle is a scalar boson a it will have a decay channel
a → 2γ. Because this decay is isotropic in a’s rest frame the spectrum
of decay photons is box-shaped (Fig. 12.1) with an upper endpoint of
5.5 MeV. If a fraction q of the particles decays between the Sun and
Earth the local γ flux is r p q 2.2×10 10 cm −2 s −1 MeV −1 . The upper limit
photon flux shown in Fig. 12.4 at 5.5 MeV is 0.8×10 −3 cm −2 s −1 MeV −1
(Peterson et al. 1966). From there, Raffelt and Stodolsky (1982) found
the general upper bound r p q < 4×10 −14 . They also calculated r, p,
and q for the specific case of “standard axions” and were able to derive
a strong limit on the properties of this hypothetical particle. Together
with many laboratory constraints (Particle Data Group 1994) standard
axions are now entirely excluded, the main motivation to consider “invisible
axions” instead (Chapter 14).
12.4 Supernova 1987A
12.4.1 Decay Photons from Low-Mass Neutrinos
The most significant constraints on radiative particle decays from stellar
sources can be derived on the basis of the neutrino burst from supernova
(SN) 1987A. The neutrino observations and their interpretation were
discussed in Chapter 11. For the present purpose it is enough to know
that the neutrinos arrived in a short burst lasting a few seconds with
a total emitted energy per flavor of about 1×10 53 erg = 6.2×10 58 MeV.
For ν e the measured spectral distribution is consistent with a thermal