Nonspinning searches for spinning black hole-neutron star binaries in ground-based detector data: Amplitude and mismatch predictions in the constant precession cone approximation

Current searches for compact binary mergers by ground-based gravitational-wave detectors assume for simplicity the two bodies are not spinning. If the binary contains compact objects with significant spin, then this can reduce the sensitivity of these searches, particularly for black hole-neutron star binaries. In this paper we investigate the effect of neglecting precession on the sensitivity of searches for spinning binaries using nonspinning waveform models. We demonstrate that in the sensitive band of Advanced LIGO, the angle between the binary's orbital angular momentum and its total angular momentum is approximately constant. Under this constant precession cone approximation, we show that the gravitational-wave phasing is modulated in two ways: a secular increase of the gravitational-wave phase due to precession and an oscillation around this secular increase. We show that this secular evolution occurs in precisely three ways, corresponding to physically different apparent evolutions of the binary's precession about the line of sight. We estimate the best possible fitting factor between any nonprecessing template model and a single precessing signal, in the limit of a constant precession cone. Our closed-form estimate of the fitting factor depends only the geometry of the in-band precession cone; it does not depend explicitly on binary parameters, detector response, or details of either signal model. The precessing black hole-neutron star waveforms least accurately matched by nonspinning waveforms correspond to viewing geometries where the precession cone sweeps the orbital plane repeatedly across the line of sight, in an unfavorable polarization alignment.