Searching Axions through Coupling with Spin: The QUAX Experiment

The axion is one of the main candidates of dark matter and was originally introduced to solve the strong CP problem of quantum chromodynamics. Here we present a proposal to search for QCD axions with mass in the 200 mu eV range, assuming that they make a dominant component of dark matter. Since the axion can couple to the spin of a fermion, its presence can be seen as the existence of an equivalent rf field with frequency and amplitude fixed by the axion mass and coupling, respectively. This equivalent magnetic field would produce spin flips in a magnetic sample placed inside a static magnetic field, which determines the resonant interaction at the Larmor frequency. Spin flips would subsequently emit radio frequency photons that can be detected by a suitable quantum counter in an ultra-cryogenic environment. An updated report of the experimental results will be presented, together with a preliminary measurement and a projection of future improvements.