A new torsion pendulum for testing the limits of free-fall for LISA test masses

On-ground verification of the precision with which a test mass can be in perfect free-fall, without any stray forces, is among the most challenging aspects of preparing for LISA and LISA Pathfinder. This study aims at improving the sensitivity in torsion pendulum measurements of the stray forces arising in the interaction between a test mass and the capacitive position sensor that surrounds it. Measurements are performed with pendulum suspensions based on both tungsten and higher quality factor uncoated fused silica torsion fibers. The results achieved with the fused silica pendulum establish more stringent upper limits on the excess force noise attributable to the sensor—at a level that roughly coincides with the LISA Pathfinder flight goal around 1 mHz. Additionally, these measurements demonstrate a force sensitivity improvement over what can be achieved with thermal noise-limited tungsten over a wide range of frequencies, with significant further improvements still possible.