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The highs and lows of the QCD axion

The QCD axion has been proposed more than 30 years ago to explain the smallness of the neutron's dipole moment. It is an excellent Dark Matter candidate and the search for it has been ongoing ever since its conception. In my talk I will describe two experiments that will explore complementary parts of the axion's parameter space. When the axion's Compton wavelength is tens of microns up to several centimeters, it can be detected through the monopole-dipole and dipole-dipole interaction it mediates in matter. In our ARIADNE proposal, the tiny effect of this interaction can be measured using polarized He-3. When the axion's Compton wavelength is larger than a kilometer, it matches the size of astrophysical black holes and its presence can be diagnosed through the superradiance effect that causes BHs to spin down. During this process, a cloud of axions forms a gravitational atom around the BH nucleus. Monochromatic gravitational waves produced by atomic transitions in this cloud turn BHs into astrophysical beacons that are well within the reach of Advanced LIGO experiment — after discovering gravitational waves, Advanced LIGO may also diagnose the existence of a new particle.