Stars as Laboratories for Fundamental Physics - MPP Theory Group

Chapter 3
Particles Interacting with
Electrons and Baryons
The stellar energy-loss argument is applied to weakly interacting particles
which couple to electrons and baryons. The emission from a normal
stellar plasma can proceed by a variety of reactions, for example
the Compton process γe − → e − χ where χ stands for a single particle
(axion, paraphoton, etc.) or a neutrino pair νν. Other examples of
practical interest are electron bremsstrahlung e − (Z, A) → (Z, A)e − χ
or e − e − → e − e − χ, electron free-bound transitions, and pair annihilation
e − e + → γχ or e − e + → νν. The corresponding energy-loss rates
of stellar plasmas are studied for temperatures and densities which are
of interest for stellar-evolution calculations. For particles coupled to
baryons some of the same processes apply in a normal plasma if one substitutes
a proton or a helium nucleus for the electron. Reactions specific
to a nuclear medium are deferred to Chapter 4. In addition to draining
stars of energy, scalar or vector bosons would mediate long-range forces.
Leptonic long-range forces would be screened by the cosmic neutrino
background. Thermal graviton emission from stars is mentioned.
3.1 Introduction
If weakly interacting particles couple directly to electrons, they can
be produced thermally in stellar plasmas without nuclear processes.
For neutrinos, a direct electron coupling was first contemplated after
the universal V −A theory for their interactions had been proposed in
1958. It was realized immediately that such a coupling would allow
for thermal pair production by bremsstrahlung e − (Z, A) → (Z, A)e − νν
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