Picoradiant tiltmeter and direct ground tilt measurements at the Sos Enattos site

We report the tilt sensitivity reached by the ARCHIMEDES tiltmeter in the 2–20 Hz frequency region, where seismic noise is expected to give an important limitation to the sensitivity in the next future Gravitational Waves detection, particularly through Newtonian noise. The tilt noise level $$\tilde{\theta }(f)$$ is about $$10^{-12} \mathrm{rad}/\sqrt{\mathrm{Hz}}$$ in most of the band, reaching the minimum of $$\tilde{\theta } = 7\cdot 10^{-13} \mathrm{rad}/\sqrt{\mathrm{Hz}}$$ around 9 Hz. The tiltmeter is a beam balance with a 0.5 m suspended arm and interferometric optical readout, working in closed loop. The results have been obtained by a direct measurement of the ground tilt at the Sos Enattos site (Sardinia, Italy). This sensitivity is a requirement to use the tiltmeter as part of an effective Newtonian noise reduction system for present Gravitational Waves detectors, and also confirms that Sos Enattos is among the quietest sites in the world, suitable to host the third-generation Gravitational Waves detector Einstein Telescope.