Name: Koji ISHIDOSHIRO
 Institution: IPNS, KEK
 Title: Observational Upper Limit on a Gravitational Wave Background at 0.2 Hz with a Torsion-bar Antenna
 Key Words: Gravitational wave background
 Abstract: We present the observational upper limit on a gravitational
wave background (GWB) at 0.2 Hz. 
A gravitational wave background (GWB) emerges from the random superposition of a large number of unresolved and independent gravitational waves (GWs). 
The generation of a GWB could result from the cosmological process, just as the cosmic microwave background (CMB) but occurring much earlier Universe. 
Detection and constraint for such GWB  will reveal how the Universe evolves in its very early epoch. A number of experiments have been performed to search for a GWB. However, the low frequency band 0.01 ¡Ý 1 Hz is still unexplored
For low frequency GW observations, we have developed a novel GW antenna, called a torsion-bar antenna. The antenna operated one-night observation in 2009. 

In this poster, we show our antenna, data analysis, its result and future prospects.