A Solar Axion Search Using a Decommissioned LHC Test Magnet

<!--HTML-->Previous solar axion searches have been carried out in Brookhaven (1990) and in Tokyo (2000- ), tracking the Sun with a dipole magnet. QCD inspired axions should be produced after the Big Bang, being thus candidates for the <strong>dark matter</strong>. The Sun is a very useful source of weakly interacting particles for fundamental research. Axions can be produced also in the Sun's core through the scattering of thermal photons in the Coulomb field of electric charges (Primakoff effect). In a transverse magnetic field the Primakoff effect can work in reverse, coherently converting the solar axions or other axion-like particles (ALPS) back into X-ray photons in the keV range. The conversion efficiency increases with $(B⋅L)^2$. In the CAST experiment an LHC prototype dipole magnet (B = 9 T and L = 10 m) with straight beam pipes provides a conversion efficiency exceeding that of the two earlier solar axion telescopes by almost a factor of 100. This magnet is mounted on a moving platform and coupled to both gas filled and solid-state low-background X-ray detectors on either end allowing it to observe the Sun for nearly 1.5 hours at both sunrise and sunset. The rest of the day is devoted to background measurements and, because of the Earth motion, to observations of a large portion of the sky. The 43 mm aperture of the LHC magnet beam pipe requires correspondingly large X-ray detectors, implying a large level of noise. To overcome this problem, CAST uses state-of-the-art analog TPC detectors of the Micromegas type, and in addition, for the first time, an X-ray mirror system from the German space program; the converted X-rays are focused to a few mm spot, improving the signal-to-noise ratio significantly over the original CAST proposal and the earlier solar axion telescopes. Recently, CAST has successfully commissioned and took data with a second X-raysoptics from LLNL, using a technology developed for the NuSTAR space mission. CAST has thus reached its optimum axion detection sensitivity, which is about ten times higher than that of the previous experiments, entering for the first time, the axion searches beyond the limit dictated by astrophysical considerations. CAST data have been used to provide new limits on <strong>Hidden Sector</strong> particles (“paraphotons”). A fifth line installed in CAST, a BaRBE detector being sensitive to a few eV photons, allowed to reach new limits on paraphotons. Revisiting in 2013-2014 the rest mass range below 0.02 eV, CAST could improve its performance for axion-like particles from the Sun, thanks to better performing detectors and the installed second XRT. In addition, with the scheduled lower energy detector threshold, CAST was also able to search for the first time for solar Chameleons (or other as yet not predicted exotica) in an energy region previously inaccessible. Chameleons are candidates for the <strong>dark energy</strong> in the Universe, eventually the biggest mystery in physics.