Search technique to observe precessing compact binary mergers in the advanced detector era

Gravitational-wave signals from compact binary coalescences are most effectively identified by matched filter searches. These searches match the data against a pre-generated bank of gravitational-wave templates. Currently, all modelled gravitational-wave searches use templates that restrict the component spins to be aligned (or anti-aligned) with the orbital angular momentum. This means that they are less sensitive to gravitational-wave signals from precessing binaries, implying that a significant fraction of signals may remain undetected. In this work, we introduce a matched filter search that is sensitive to signals generated from precessing binaries. We take advantage of the fact that a gravitational-wave signal from a precessing binary can be decomposed into a power series of five harmonics. This allows us to create a generic-spin template bank that is only $\sim 3$ times larger than existing aligned-spin banks. Our new search shows a $\sim 100\%$ increase in sensitive volume for neutron star black hole binaries with total mass larger than $17.5\, M_{\odot}$ and in-plane spins $>0.67$, and improves sensitivity by $\sim60\%$ on average across the full generic spin neutron-star black-hole parameter space. In addition, our generic spin search performs as well as existing aligned-spin searches for neutron star black hole signals with insignificant in-plane spins. We anticipate that this improved technique will identify significantly more gravitational-wave signals, and help shed light on the unknown spin distribution of binaries in the universe.