Abstract
We propose an experiment to search for extra "hidden-sector" U(1) gauge bosons with small gauge kinetic mixing χ with the ordinary photon, predicted by many extensions of the Standard Model. The setup consists of a highly sensitive magnetometer inside a superconducting shielding which are in turn placed inside a strong (but sub-critical) magnetic field. Photon – hidden-sector photon – photon oscillations would allow the magnetic field to "leak" into the shielded volume and register on the magnetometer. In our setup the magnetometer directly measures the field strength (∝χ2), improving the sensitivity over "light-shining-through-walls experiments which measure the regeneration probability (∝χ4). In the mass range 2 μeV≲mγ'≲200 meV the projected sensitivity is in the χ∼5×10−9 to χ∼10−6 range. This surpasses current astrophysical and laboratory limits by several orders of magnitude —ample room to discover new physics.