Stars as Laboratories for Fundamental Physics - MPP Theory Group

Chapter 5
Two-Photon Coupling of
Low-Mass Bosons
Well-known particles such as neutral pions, or hypothetical ones such
as axions or gravitons, each have a two-photon interaction vertex. It
allows for radiative decays of the form π ◦ → 2γ as well as for the
Primakoff conversion π ◦ ↔ γ in the presence of external electric or
magnetic fields. The Primakoff conversion of photons into gravitons in
putative cosmic magnetic fields could cause temperature fluctuations
of the cosmic microwave background radiation. In stars, the Primakoff
conversion of photons leads to the production of gravitons and axions,
although the graviton luminosity is always negligible. The Primakoff
conversion of axions into photons serves as the basis for a detection
scheme for galactic axions, and was used in several laboratory axion
search experiments. The modified Maxwell equations in the presence
of very low-mass pseudoscalars would substantially modify pulsar electrodynamics.
These physical phenomena are explored and current laboratory
and astrophysical limits on the axion-photon coupling are reviewed.
5.1 Electromagnetic Coupling of Pseudoscalars
The idea that symmetries of the fundamental interactions can be broken
by the “vacuum” or ground state of the fields plays an important
role in modern particle physics. The breakdown of a global symmetry
by the vacuum expectation value of a Higgs-like field leads to the
existence of massless particles, the “Nambu-Goldstone bosons” of the
broken symmetry. One well-known example are the pions which are
165