LUX Releases Spin-Dependent Limits

While yesterday’s big news was clearly the gravitational wave result, LUX also put its first spin-dependent WIMP interaction limits on the arXiv. In direct detection experiments, the spin-independent limit is typically stronger because the amplitudes add together in coherent nuclear scattering, leading to a dependence that scales like a polynomial factor of the atomic number. Spin-dependent interactions, typically from axial vector interactions, give rise to terms related to the individual nucleons’ spins. Nucleons tend to arrange themselves so that the spins mostly cancel, so the spin-dependent terms tend to be smaller than the spin-independent terms by a factor of approximately A2 if you assume that the fundamental couplings are the same. Because the cross section ends up with some angular momentum factors in terms, you need to know the isotopic abundances very well to get a reliable spin-dependent measurement. In this result, LUX gets the best result of any direct detection experiment for WIMP-neutron spin-dependent scattering and is about an order of magnitude behind in the WIMP-proton channel (xenon is not the best nucleus to use for spin-dependent proton interactions).