Novel Constraints on Spin-Dependent Light Dark Matter Scattering

Abstract

We explore the sensitivity of the SNO experiment to light dark matter particles $χ$ 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 ${\rm D}(n,χ\barχ)^3{\rm He}$, and calculate the expected rate within the simplest models of $χ$-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, ${\rm D}(χ,χ)np$, 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 $χ$-nucleon spin-dependent cross sections, showing that cross sections above $σ_{χp} \sim 10^{-33}\,{\rm cm}^{2}$ are generally excluded if $m_χ\leq1.5$\,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.

Figures (5)

• Figure 1: A not-to-scale depiction of emission/detection scheme of MeV-scale DM. $\chi\bar{\chi}$ pairs are emitted in a reactor core, and we concentrate on those produced in heavy water reactors. $\chi\bar{\chi}$ pairs are also emitted in the sun. Detectors purposely installed in proximity $O(30\,\rm m)$ to reactors (short baseline) may be used to detect $\chi\bar{\chi}$ pairs produced in reactors, while neutrino telescopes, such as SNO at long baseline $O(200\,\rm km)$ to reactors may be used to detect $\chi\bar{\chi}$ pairs produced in reactors and the sun. Representative reactions for the pair-creation of DM and subsequent detection are shown in blue.
• Figure 2: Energy distribution $\frac{dF}{dE_{\chi}}$ for two representative values of DM mass, $m_{\chi} = 1\,\rm MeV$ (blue) and $m_\chi = 1.7\,\rm MeV$ (brown). The vertical dashed line of the corresponding color indicates the placement of the energy threshold for a subsequent deuteron disintegration, from $E_\chi> E_{\rm thr}=m_\chi +2.2\,\rm MeV$ until the endpoint at $E_\chi = Q-m_\chi$.
• Figure 3: Mass dependence of branching ratios to $\chi\bar{\chi}$ pairs. Due to the normalization on the non-relativistic $\sigma_{\chi p}$ cross section, ${\rm Br}_{\chi\bar{\chi}}\to \infty$ as $m_\chi\to 0$, and this artificial enhancement has been "detrended" on this plot via an $m_\chi^2/{\rm MeV}^2$ multiplier.
• Figure 4: Flux-averaged deuteron disintegration cross sections $\langle \sigma_{\chi\rm D\!\!\!\!/} \rangle$ normalized on $\sigma_{\chi p} ({\rm MeV}/m_\chi)^2$. These curves are similar in shape to those of Fig. \ref{['fig:Br_rs']}, with a smaller $m_\chi$ at endpoints due to the energy threshold of $R6$.
• Figure 5: Excluded regions on $\{m_{\chi},\sigma_{\chi p}\}$ parameter space, which result from the CANDU produced $\chi$ counting rate at SNO (SNO) and the solar produced $\chi$ counting rate at SNO (Solar). Possible sensitivity reach from Silicon (Si) and Germanium (Ge) nuclear recoil experiments placed near a CANDU reactor are also shown.