LIGO Document P1000112-v27

Searching for stochastic gravitational waves using data from the two co-located LIGO Hanford detectors

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Searches for a stochastic gravitational-wave background (SGWB) using terrestrial detectors typically involve cross-correlating data from pairs of detectors. The sensitivity of such cross-correlation analyses depends, among other things, on the separation between the two detectors: the smaller the separation, the better the sensitivity. Hence, a co-located detector pair is more sensitive to a gravitational-wave background than a non-co-located detector pair. However, co-located detectors are also expected to suffer from correlated noise from instrumental and environmental effects that could contaminate the measurement of the background. Hence, methods to identify and mitigate the effects of correlated noise are necessary to achieve the potential increase in sensitivity of co-located detectors. Here we report on the first SGWB analysis using two Hanford LIGO detectors that addresses the complications arising from correlated environmental noise. We apply correlated noise identification and mitigation techniques to data taken by the two LIGO Hanford detectors, H1 and H2, during LIGO's fifth science run. At low frequencies, 40 - 460 Hz, correlated environmental noise is sufficiently severe that it is not possible to confidently measure or bound the stochastic GW signal. However, at high frequencies, 460-1000, these techniques are shown to be sufficient to set a 95% confidence level (C.L.) upper limit on the gravitational-wave energy (GW) density of \Omega(f) 7.7 x 10^{-4} (f/900 Hz)^3, which improves on previous upper limit by a factor of ~ 180. In doing so, we demonstrate techniques useful for future searches using advanced detectors where correlated noise (e.g., from global magnetic fields) may affect even widely separated detectors.
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