Stars as Laboratories for Fundamental Physics - MPP Theory Group

90 Chapter 3
(Pontecorvo 1959; Gandel’man and Pinaev 1959) or by photoproduction
γe − → e − νν (Ritus 1961; Chiu and Stabler 1961). The set of
processes important for normal stars was completed by Adams, Ruderman,
and Woo (1963) who discovered the plasma process γ → νν.
Neutrino emission by these reactions is now a standard aspect of stellarevolution
theory.
If other weakly interacting particles were to exist which couple directly
to electrons they could essentially play the same role and thus
add to the energy loss of stars. The main speculation to be followed
up in this chapter is the possible existence of weakly interacting bosons
that would couple to electrons. Among standard particles the only lowmass
bosons are photons and probably gravitons. The former dominate
the radiative energy transfer in stars, the latter are so weakly interacting
that their thermal emission is negligible (Sect. 3.7). Why worry
about others
Such a motivation arises from several sources. Low-mass bosons
could mediate long-range forces between electrically neutral bodies for
which gravity is the only standard interaction. It is an interesting
end in itself to set the best possible bounds on possible other forces
which might arise from the exchange of novel scalar or vector bosons.
Their existence seemed indicated for some time in the context of the
“fifth-force” episode alluded to in Sect. 3.6.3 below. It is also possible
that baryon or lepton number play the role of physical charges similar
to the electric charge, and that a new gauge interaction is associated
with them. The baryonic or leptonic photons arising from this hypothesis
are intriguing candidates for weakly interacting low-mass bosons
(Sect. 3.6.4). It will turn out, however, that typically massless bosons
which mediate long-range forces are best constrained by experiments
which test the equivalence principle of general relativity. Put another
way, to a high degree of accuracy gravity is found to be the only longrange
interaction between neutral bodies.
Long-range leptonic forces can be screened by the cosmic neutrino
background. In this case the stellar energy-loss argument remains of
importance to limit their possible strength (Sect. 3.6.4).
The remaining category of interesting new bosons are those which
couple to the spin of fermions and thus do not mediate a long-range
force between unpolarized bodies. In the simplest case their CP-conserving
coupling would be of a pseudoscalar nature. Such particles arise
naturally as Nambu-Goldstone bosons in scenarios where a global chiral
U(1) symmetry is spontaneously broken at some large energy scale. The
most widely discussed example is the Peccei-Quinn symmetry that was