Searches for solar Kaluza–Klein axions with gas TPCs

In theories with n extra dimensions beyond the normal 3 + 1, axions may be able to propagate in these extra dimensions and hence acquire Kaluza–Klein excitations. We investigate the possibility of using a low pressure gas time projection chamber (TPC) to search for the two-photon decays of Kaluza–Klein axions produced in the solar interior and subsequently trapped into Earth-crossing orbits. A simulation of the background in a generic 1 m3 TPC due to coincident photon events from radioactive decay in the detector and surrounding laboratory walls was performed. The results of these simulations are discussed with regard to the sensitivity of the TPC to the axion–photon coupling constant ga γγ and local axion number density n0. We find that sensitivities to solar Kaluza–Klein models consistent with limits on exotic solar energy loss are achievable.