Picoradiant tiltmeter and direct ground tiltmeasurements 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 (theta) over tilde (f) is about 10(-12)rad/root Hz in most of the band, reaching the minimum of (theta) over tilde =7 center dot 10(-13)rad/root 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.