Abstract
String theory compactifications may generate many light axion-like particles (ALPs) with weak couplings to electromagnetism. In general, a large number of ALPs may exist, with a linear combination having a potentially observable coupling to electromagnetism. The basis in which only one ALP couples to electromagnetism is in general misaligned with the mass basis. This leads to mixing between the `electromagnetic' ALP and a number of `hidden' ALPs that do not interact directly with the photon. The process is analagous to neutrino oscillations. I will discuss the phenomenological consequences of this mixing for astrophysical ALP signals, in particular showing that it may significantly reduce the predicted signal in experiments such as the CERN Axion Solar Telescope.
Highlights
We will consider a string axiverse scenario with N axion fields φi
As the mass of the QCD axion is generated by QCD effects, we will assume that the QCD basis is approximately aligned with the mass basis, and mass mixing between the QCD axion and the axion-like particles (ALPs) is negligible
The axion-like particle that mixes with the photon may not be a mass eigenstate but a linear combination of many mass eigenstate ALPs [20]
Summary
Each φi would typically interact rather weakly with electromagnetism, but the combined effect of many such fields leads to a potentially observable coupling between the axion sector and the photon [20]. We will move to a basis in which only one field, φQCD, couples to gluons. This field is the QCD axion [5]. The mass and coupling to photons of φQCD are set by its interaction with the QCD sector, in particular by mixing with the neutral pion [21]. We will refer to these as axion-like particles or ALPs. Note that we are no longer in the mass basis, and so we expect some mixing between φQCD and the other φi.
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