Stars as Laboratories for Fundamental Physics - MPP Theory Group

What Have We Learned from SN 1987A 495
scattering on cosmic background neutrinos as an example. The presentday
density of primordial neutrinos is about 100 cm −3 in each neutrino
and antineutrino flavor. With a distance to the Large Magellanic
Cloud of about 50 kpc = 1.5×10 23 cm one has a column density between
SN 1987A and Earth of about 10 25 cm −2 so that the ν e -ν cross
section must be less than about 10 −25 cm 2 . If the cosmic background
neutrinos are massless they have a temperature of about 1.8 K and
so ⟨E ν ⟩ ≈ 3T ν ≈ 5×10 −4 eV. Because the measured SN neutrinos
have
√
a characteristic energy of 30 MeV the center of mass energy is
s ≈ 200 eV.
The cross-section bound is not particularly impressive compared
with a standard weak cross section of order G 2 Fs ≈ 10 −51 cm 2 . However,
νν cross sections have never been directly measured and so the
SN 1987A limit provides nontrivial information. As an example, neutrinos
could scatter by majoron exchange, or they could scatter directly
on a background of primordial majorons. Kolb and Turner then
found a certain constraint on the neutrino-majoron Yukawa coupling
(Sect. 15.7.2). As another example, the proposition that the solar neutrino
flux could be substantially depleted by scatterings on cosmic background
particles (Slad’ 1983) is excluded.
Other particles besides neutrinos may have been emitted from the
SN and could have caused detectable events. Engel, Seckel, and Hayes
(1990) have discussed the case of axions; they can be absorbed in water
by oxygen nuclei, a 16 O → 16 O ∗ , which subsequently produce γ rays by
decays of the sort 16 O ∗ → 16 O γ, 16 O ∗ → 15 O n γ, and 16 O ∗ → 15 N p γ.
The γ rays would cause electromagnetic cascades and so they are detectable
about as efficiently as e ± . The axion emission was estimated by
identifying their unit optical depth for a given interaction strength in a
simplified model of the SN temperature and density profile. More than
10 extra events would be expected at Kamiokande for an axion-nucleon
Yukawa coupling in the range
1×10 −6 ∼ < g aN ∼ < 1×10 −3 (13.2)
which is thus excluded. In the middle of this interval, up to 300 additional
events would have been expected. However, axions with couplings
in this interval are also excluded by other methods (Sect. 14.4).
13.2.2 Energy Distribution
The energy distribution of the events at the IMB and Kamiokande
detectors broadly confirms the expected quasi-thermal emission with