Wikipedia
A Photo-meson comes from the word, meson; "An elementary particle that is composed of a quark and an antiquark, such as a kaon or pion. (Meson composed of rarer quarks are much heavier.)" A photo-meson is defined as "a meson ejected from an atomic nucleus as a result of the incidence of a gamma ray or other high energy photon"
Research currently focused on photo-mesons, is to yield understanding and possible use of energy, as "Photomeson production is the main energy loss for relativistic nucleons in dense radiation fields like the cosmic microwave background and the radiation fields in Gamma Ray Bursts (GRB) and jets of Active Galactic Nuclei (AGN)." Further understanding of photomesons will aid to understanding of nuclear physics & quantum mechanics; cosmic rays, quarks, antiquarks, photons, etc.
Ultrahigh-energy nucleons, which eject from Stars, through the process of creating photo-mesons, creates Cosmic Microwave Background Radiation (CMBR), Active Galactic Nucleus (AGN jet and Gamma Ray Bursts (GRB) photo fields. This process is a transfer / decay of energy, from Ultrahigh-energy nucleons. A secondary product, of transfer / decay or energy appears in the product of neutrinos & gamma-ray fluxes.