Stars as Laboratories for Fundamental Physics - MPP Theory Group

570 Chapter 16
16.2 Minimally Extended Standard Model
16.2.1 Cosmological Mass Limit for All Flavors
Neutrinos with nonstandard properties would have more radical implications
in astrophysics. A minimal and most plausible extension of the
standard model is the possibility that they have masses and mixings
like the other fermions. Taking this hypothesis in a literal sense means
that neutrinos would need to have Dirac masses like the charged fermions.
Therefore, one needs to postulate the existence of right-handed
neutrinos and Yukawa couplings to the standard Higgs field to generate
masses and mixings. What do we know about neutrinos in the context
of this Minimally Extended Standard Model
Perhaps the most dramatic lesson is that the mass of all sequential
neutrinos (ν e , ν µ , ν τ ) must be less than the cosmological limit of
approximately 30 eV (Sect. 7.1.5). This conclusion is not to be taken
for granted as massive neutrinos with mixings can decay by virtue of
ν → ν ′ γ, and by ν → ν e e + e − if m ν exceeds about 2m e . While the
standard-model radiative decays are too slow to avoid the cosmological
limit, the e + e − channel can be fast on cosmological time scales if the
mixing angle is not too small. This decay channel is an option only
for ν τ which may have a mass of up to 24 MeV while the experimental
mass limits on the other flavors are below 0.16 MeV.
Such a “heavy” ν τ , however, can be excluded by several arguments.
The stellar evolution one based on SN arguments was presented in
Sect. 12.5.2. The bremsstrahlung emission of photons in ν τ → ν e e + e − γ
would produce a γ-ray flux in excess of the SMM limits for SN 1987A
unless sin 2 2θ < e3 ∼ 10 −9 . (For a detailed dependence of this limit on the
assumed mass see Fig. 12.18.) In addition, the integrated positron flux
from all galactic supernovae over the past, say, 100,000 years would
exceed the observed value unless the decays are very fast (near the SN)
or very slow (outside of the galactic disk). Together, these limits leave
no room for a heavy ν τ (Fig. 12.19). In addition, the mass range
0.5 MeV < ∼ m < ν ∼ 35 MeV can be excluded on the basis of big bang
nucleosynthesis arguments (Fig. 7.2) unless the neutrinos are shorter
lived than permitted by the Minimally Extended Standard Model.
Neutrino masses near the cosmological limit would be important for
cosmology as they could contribute some or all of the dark matter of
the universe. The latter option is disfavored by theories of structure
formation. However, a subdominant “hot dark matter” contribution in
the form of, say, 5 eV neutrinos might be cosmologically quite welcome.