Searching hidden-sector photons inside a superconducting box

We propose an experiment to search for extra 'hidden-sector' U(1) gauge bosons with small gauge kinetic mixing 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 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 2μeV ≤ 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. (authors)