Determination of the thermal noise limit in test of weak equivalence principle with a rotating torsion pendulum**Project supported by the National Natural Science Foundation of China (Grant Nos. 11575160 and 11275075) and the Natural Science Foundation of Key Projects of Hubei Province, China (Grant No. 2013CFA045).

Abstract

Thermal noise is one of the most fundamental limits to the sensitivity in weak equivalence principle test with a rotating torsion pendulum. Velocity damping and internal damping are two of many contributions at the thermal noise, and which one mainly limits the torsion pendulum in low frequency is difficult to be verified by experiment. Based on the conventional method of fast Fourier transform, we propose a developed method to determine the thermal noise limit and then obtain the precise power spectrum density of the pendulum motion signal. The experiment result verifies that the thermal noise is mainly contributed by the internal damping in the fiber in the low frequency torsion pendulum experiment with a high vacuum. Quantitative data analysis shows that the basic noise level in the experiment is about one to two times of the theoretical value of internal damping thermal noise.