Novel Constraints on Spin-Dependent Light Dark Matter Scattering

We explore the sensitivity of the SNO experiment to light dark matter particles $\chi$ with spin-dependent interactions with nucleons. We show that the pair-production of MeV scale dark matter is possible in heavy water (CANDU) reactors via ${\rm D}(n,\chi\bar\chi)^3{\rm He}$, and calculate the expected rate within the simplest models of $\chi$-nucleon interactions. %Heavy water nuclear reactors serve as an excellent production method for spin-dependent dark matter. Owing to a sizable $Q$-value for this reaction, a large fraction of DM particles produced this way are above the threshold for deuteron disintegration, ${\rm D}(\chi,\chi)np$, which adds to the SNO neutral current signal. Evaluating the CANDU-to-SNO scheme for the production and detection of DM, we derive novel constraints for the $\chi$-nucleon spin-dependent cross sections, showing that cross sections above $\sigma_{\chi p} \sim 10^{-33}\,{\rm cm}^{2}$ are generally excluded if $m_\chi \leq1.5$\,MeV. An isospin-mirror reaction will occur in the Sun, and for the kinematically allowed region it excludes a portion of parameter space with cross sections on the order $10^{-37}\,{\rm cm}^{2}$. We also evaluate the potential sensitivity of small ``near" detectors placed in close proximity to a CANDU reactor to search for a coherent nuclear recoil, finding subdominant sensitivity.