Search for a Stochastic Gravitational-wave Background with Torsion-bar Antennas

We performed a simultaneous observational run with prototypes of Torsion-bar Antenna (TOBA) and searched for a stochastic gravitational waves (GW) background. TOBA is a new type of GW detector which measures differential rotation of two test-mass bars caused by tidal force from GWs. It fundamentally has a good sensitivity at lower frequencies, such as 0.1 - 1.0 Hz. The prototype has a 20-cm test mass bar which is levitated by the pinning effect of a superconductor. The data was taken from 1:00 am to 10:00 am on March 11th 2011, at Tokyo and Kyoto in Japan. As a result, we did not detect a stochastic GW background with false alarm rate of 5 %, and set an upper limit on a stochastic GW background. Our 95 % confidence upper limit is Omega(gw)h(0)(2), < 1.2 x 10(19) at 0.06 - 0.9 Hz, where Omega(gw) is the GW energy density per logarithmic frequency interval in units of the critical density and h(0) is the Hubble constant per 100 km/sec/Mpc. We had established the simultaneous observation and the analysis pipeline with two TOBAs, and set an upper limit at a wider frequency band.