New sensitivity to solar WIMP annihilation using low-energy neutrinos

Dark matter particles captured by the Sun through scattering may annihilate and produce neutrinos, which escape. Current searches are for the few high-energy neutrinos produced in the prompt decays of some final states. We show that interactions in the solar medium lead to a large number of pions for nearly all final states. Positive pions and muons decay at rest, producing low-energy neutrinos with known spectra, including ${\overline{\nu }}_e$ through neutrino mixing. We demonstrate that Super-Kamiokande can thereby provide a new probe of the spin-dependent WIMP-proton cross section. Compared to other methods, the sensitivity is competitive and the uncertainties are complementary.

Figure 1

Solar WIMP capture rate as a function of WIMP mass for two values of the SD WIMP-proton scattering cross section. Evaporation (see text) is not included.

Figure 2

Low-energy neutrino yield (summed over flavors) per WIMP annihilation ($\sqrt{s}=2m_{\chi }$) in the solar core, obtained by simulating pion-induced hadronic showers in the solar medium. Reference lines are shown for the cases where all or only 10% of the annihilation energy goes into producing pions. The high-energy neutrino yield for ${\tau }^{+}{\tau }^{-}$, relevant for current searches, is also shown.

Figure 3

Detectable signal in Super-K induced by low-energy ${\overline{\nu }}_e$ from solar WIMP annihilation, along with the measured background (both 4.1 live-time years). The signal shape is independent of WIMP properties, and its normalization scales with ${\sigma }_{\chi p}^{\mathrm{SD}}$ (here chosen to be at the edge of exclusion). For a WIMP mass of 10 GeV, the signal shown corresponds to a cross section of $4.5\times {10}^{-37}{\mathrm{cm}}^2$.

Figure 4

New sensitivity to ${\sigma }_{\chi p}^{\mathrm{SD}}$ using low-energy ${\overline{\nu }}_e$ in Super-K. Possible improvements using neutron tagging with gadolinium (details see text) are included in the projected sensitivity for 4 years of Hyper-Kamiokande [30] data. For $m_{\chi }\lesssim 4\mathrm{GeV}$, evaporation of solar WIMPs will degrade the signal. Published upper limits (90% C.L.) from direct scattering [36, 37, 38] and high-energy neutrino searches are shown [39, 40], along with the possible DAMA/LIBRA signal region [41, 42].