Stars as Laboratories for Fundamental Physics - MPP Theory Group

Particles Interacting with Electrons and Baryons 91
proposed as an explanation of CP conservation in strong interactions.
It leads to the prediction of axions which will be discussed in some detail
in Chapter 14. The most restrictive limits on their coupling strength
arise from the stellar energy-loss argument, and it cannot be excluded
that in fact they play an important role in the evolution of some stars
(Sect. 2.2.5). No wonder that axions have played a primary role in
studies concerning the impact of new weakly interacting particles on
stellar evolution.
The main focus of this chapter is an application of the stellar energyloss
argument to weakly interacting bosons which couple to electrons by
a variety of interaction structures. The relevant processes are entirely
analogous to those which emit neutrino pairs except for the plasma process
which requires a two-body final state. Therefore, I will presently
study photo and bremsstrahlung production of weakly interacting particles,
including standard neutrinos.
One may consider the same processes with protons substituted for
electrons. For neutrino emission this variation is of no interest because
the rate is much smaller. It is significant for low-mass bosons which
couple only to baryons.
The energy-loss argument will be systematically applied, yielding
restrictive limits on the possible Yukawa and gauge couplings of novel
bosons to electrons and baryons. For very low-mass scalar or vector
bosons these limits are discussed in the context of those arising from
the absence of novel long-range interactions.
3.2 Compton Process
3.2.1 Vector Bosons
The simplest process for the emission of weakly interacting particles
from the hot and dense interior of a star is the Compton process where
a photon from the heat bath interacts with an electron and is thus converted
into a neutrino pair, an axion, or some other boson (Fig. 3.1).
These processes are analogous to the usual Compton scattering of photons.
Therefore, I begin with this well-known case which is based on the
standard electron-photon interaction L int = ie ψ e γ µ ψ e A µ with the electron
charge e, the electron Dirac field ψ e , and the photon field A. With
the fine structure constant α = e 2 /4π ≈ 1/137, the electron mass m e ,
and σ 0 ≡ πα 2 /m 2 e the total Compton cross section is (e.g. Itzykson and