Search for a stochastic gravitational-wave background using a pair of torsion-bar antennas

We have set a new upper limit on the stochastic gravitational-wave background using two prototype torsion-bar antennas (TOBAs). A TOBA is a low-frequency gravitational-wave detector with bar-shaped test masses rotated by the tidal force of gravitational waves. As a result of simultaneous 7-hour observations with TOBAs in Tokyo and Kyoto in Japan, our upper limit with a confidence level of 95% is Ωgwh₀²<1.9×10¹⁷ at 0.035--0.830 Hz, where h₀ is the Hubble constant in units of 100 km/s/Mpc and Ωgw is the gravitational-wave energy density per logarithmic frequency interval in units of the closure density. We successfully updated the upper limit and extended the explored frequency band.