Test of Violation of Einstein's Equivalence Principle By Galactic Dark Matter

We report here the results of operation of a torsion balance with a period of ∼ 1.27 x 10⁴ s. The analysis of data collected over a period of ∼ 115 days shows that the difference in acceleration towards the Galactic Center of test bodies made of aluminum and quartz was (0.8 ± 1.3) x 10^-15 ms^-2. This sets a bound on the violation of the equivalence principle by forces exerted by Galactic dark matter, expressed as an Eötvös parameter η_DM = (1.7 ± 2.8) x 10^-5, and improves upon the earlier bounds significantly. We also present preliminary results on the violation of the equivalence principle by forces exerted by the sun.