Search for sub-electronvolt solar axions using coherent conversion of axions into photons in magnetic field and gas helium

Abstract A search for solar axions has been performed using an axion helioscope which is equipped with a 2.3-m long 4 T superconducting magnet, PIN-photodiode X-ray detectors, and a telescope mount mechanism to track the sun. In addition, a gas container to hold dispersion-matching gas has been developed and a mass region up to m a =0.27 eV was newly explored. From the absence of any evidence, analysis sets a limit on axion–photon coupling constant to be g aγγ 6.8–10.9 ×10 −10 GeV −1 for the axion mass of 0.05 a eV at 95% confidence level, which is more stringent than the limit inferred from the solar-age consideration. This result gives currently the most stringent observational limit on the existence of solar axions in this mass region.