Upper limits to surface-force disturbances on LISA proof masses and the possibility of observing galactic binaries

We have measured surface-force noise on a hollow replica of a LISA proof mass surrounded by its capacitive motion sensor. Forces are detected through the torque exerted on the proof mass by means of a torsion pendulum in the 0.1–30 mHz range. The sensor and electronics have the same design as for the flight hardware, including 4 mm gaps around the proof mass. The measured upper limit for forces would allow detection of a number of galactic binaries signals with signal-to-noise ratio up to \approx 40 for 1 yr integration. We also discuss how LISA Pathfinder will substantially improve this limit, approaching the LISA performance.