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LIGO-G D 1 Toward Enabling Co-Located Interferometric Detectors to Provide Upper Limits on the Stochastic Gravitational Wave Background Nick Fotopoulos, MIT On Behalf of the LIGO Scientific Collaboration UT Brownsville

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LIGO-G D 2 Co-Located Interferometers: Overlap Reduction Function H1-H2 promises significantly enhanced sensitivity over H1-L1, especially at higher frequencies BUT Co-located detectors are subject to environmental correlation

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LIGO-G D 3 H1-H2 Coherence (S4) N = /N ≈ 9.1·10 -6 Very coherent in the instrument’s “sweet spot”. Coherence squared f (Hz) S1-S4 measurements of eff not consistent with zero

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LIGO-G D 4 S1 Results Interferometer Pair eff h 2 eff h 2 / H1-H H1-L H2-L S1

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LIGO-G D 5 Squaring Coherence Sensitivity Theorem: For all Z {PEM channels}, coh(H1,H2)≥coh(H1,Z)coh(H2,Z) Corollary: coh(H1,H2)≥ coh(H1,Z)coh(H2,Z) max Z 1/N 1/N 2

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LIGO-G D 6 Tracking Environmental Coupling in H1-H2 S4 had 107 PEM channels in RDS_R_L1 S5 will have roughly the same

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LIGO-G D 7 Maximum of PEM Coherence Products: Frequency Veto 1/N 2 Threshold Maximum of PEM coherence products follows H1-H2 measured coherence very closely (within error) With this (semi-arbitrary) threshold, 56% bins lost in [50,350]Hz and 48% bins lost in [50,500]Hz, 30% in [50,1024]Hz Vetoed regions (H1-H2 1/N ~ 10 -5, PEM-IFO 1/N ~ due to resolution choices)

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LIGO-G D 8 Success! We are one step closer to setting upper limits with the H1-H2 pair! coh(H1,H2) post-veto histogram exp(-N 2 ) This (unreviewed) pipeline results in noticeably reduced significance for the point estimate We have physical basis for veto

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LIGO-G D 9 Detector Characterization Have determined environmental coupling out to 1kHz. Can identify strongest sources at each frequency!

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LIGO-G D 10 A Few Words on instr eff = instr + GW We must estimate or bound instr Attempts to take this into account in S3 resulted in an GW upper limit a few times worse than the H1-L1 upper limit As we have flagged and eliminated the major sources of instrumental correlation, instr is greatly reduced l Other sources: Incomplete PEM coverage, non-linear environmental couplings…

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LIGO-G D 11 H1-H2 and the Future The new technique: Take maximum across coherence products coh(H1,Z)*coh(H2,Z). The new capability: H1-H2 can provide upper limits at high frequencies, which are inaccessible to H1-L1. S4 was playground and proof of concept; will not publish upper limit from H1-H2. S5 will have “blind” frequency vetoes and the resulting point estimate is planned for publication. With some confidence in H1-H2, we can begin looking for astrophysical sources of stochastic radiation, which is expected to peak at frequencies >200Hz.

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