Research On The Test Of Weak Equivalence Principle With A High-Q Torsion Balance

Weak Equivalence Principle(WEP) is the foundation of the General Theory of Relativity, which states that all objects fall with the same acceleration in a homogeneous gravitational field. Experimentally, the basic method testing the WEP is to compare the accelerations of different test bodies. Meanwhile, a high precision measurement is also used to search for a new composition dependent interaction, which constitutes a frontier of particle physics. The highest sensitivity test is carried out by the University of Washington E(o|¨)t-Wash group, using a rotating torsion balance. And the differential acceleration measurement, with a sensitivity of 3×10^-15m/s2, introduces the best limitation of E(o|¨)tv?s parameters,η(Be-Ti)<1.8×10^-13.At present, fundamental limitations of sensitivity in WEP test with a torsion balance are the effect of gravity gradients and thermal noises. We have discussed some preliminary calculations and measurements based on these two factors in this thesis.First, the torque between the pendulum and attractive mass can be expressed by an expansion in spherical multipole moments, described by the couplings of the gravity gradient moments and gradient fields. There are two ways to minimize the couplings. One way is to measure the ambient gravity gradients and compensate it by installing suitably shaped compensators. The other way is to minimize the gradient moments of the pendulum.The thermal torque noise of a torsion balance is known as S_t(f)∝1/Q, where higher Q fibers are less noisy. Fused silica fibers have been known to have very high Qs. Here, the fabrication and the Qs measurements of silica fibers are described. Our results indicate that Qs of fibers is as high as 2×10⁵ when the diameter is lager than 30μm. This allows the thermal noise of silica fibers to achieve a factor of 6 lower than that of the tungsten fiber, which is used in E(o|¨)t-Wash group's WEP test with Q 4000. Finally, we made fibers conducting with thin metal films. The measurements suggest that suitable coating materials and thickness will benefit for retaining large Qs of fibers. Such fibers will improve the sensitivity of torsion pendulum used for WEP test.