Stars as Laboratories for Fundamental Physics - MPP Theory Group

494 Chapter 13
were observed, constraining various mechanisms that could have removed
neutrinos from the beam such as decays. Equally important, the
nonobservation of a γ-ray burst in coincidence with the neutrino burst
constrains radiative decays of neutrinos and other particles (Sect. 12.4).
More intricate arguments involve signal dispersion, either between
photons and neutrinos, between ν e ’s and ν e ’s, or the intrinsic dispersion
of the ν e burst, constraining various effects that could cause signal
dispersion such as a nonzero neutrino mass or charge.
Most importantly, the inferred cooling time scale of a few seconds
of the newborn neutron star precludes an efficient operation of a nonstandard
cooling agent and thus yields constraints on the emission of
new particles from the SN core, notably of right-handed (r.h.) neutrinos
or axions. This line of reasoning is analogous to the “energy-loss
argument” which for normal stars has been advanced in Chapter 2.
13.2 Basic Characteristics of the Neutrino Burst
13.2.1 Fluence
The neutrinos from a SN are expected to consist of two major components:
the prompt ν e burst, and quasi-thermal emission of about equal
total amounts of energy in (anti)neutrinos of all flavors. The water
Cherenkov detectors would register the ν e burst by virtue of the reaction
ν e + e − → e − + ν e where the scattered electron is strongly forward
peaked, while the cooling signal is registered by ν e + p → n + e + with
an essentially isotropic e + signal. Even though both detectors observed
a forward peaked overall signal, it cannot be associated with ν e -e collisions
(Sect. 11.3.5). Most or all of the events are interpreted as ν e ’s.
The observation of a ν e fluence (time-integrated flux) roughly in
agreement with what is expected from a stellar collapse precludes that
these particles have decayed on their way from the SN to us, yielding
a constraint on their lifetime of (Frieman, Haber, and Freese 1988)
τ νe /m νe ∼ > 6×10 5 s/eV. (13.1)
However, this simple result must be interpreted with care because massive
neutrinos are expected to mix. The heavy ν e admixtures could
decay and may violate this bound.
The ν e ’s were not removed by excessive scattering on cosmic background
neutrinos, majorons, dark-matter particles etc., leading to constraints
on “secret interactions” (Kolb and Turner 1987). Take the