Solar atmospheric neutrinos: A new neutrino floor for dark matter searches

As is well known, dark matter direct detection experiments will ultimately be limited by a “neutrino floor,” due to the scattering of nuclei by MeV neutrinos from, e.g., nuclear fusion in the Sun. Here we point out the existence of a new neutrino floor that will similarly limit indirect detection with the Sun, due to high-energy neutrinos from cosmic-ray interactions with the solar atmosphere. We have two key findings. First, solar atmospheric neutrinos $\lesssim 1\mathrm{TeV}$ cause a sensitivity floor for standard weakly interacting massive particles (WIMP) scenarios, for which higher-energy neutrinos are absorbed in the Sun. This floor will be reached once the present sensitivity is improved by just 1 order of magnitude. Second, for neutrinos $\gtrsim 1\mathrm{TeV}$, which can be isolated by muon energy loss rate, solar atmospheric neutrinos should soon be detectable in IceCube. Discovery will help probe the complicated effects of solar magnetic fields on cosmic rays. These events will be backgrounds to WIMP scenarios with long-lived mediators, for which higher-energy neutrinos can escape from the Sun.

Figure 1

The $\mathrm{SA}\nu$ flux spectrum. Below 300 GeV, we use the SSG1991 models [32] (upper: Naive; lower: Nominal); above 300 GeV, we use the IT1996 model [34]. All are shown within the angular cone of the Sun (${\theta }_{\text{Sun}}$). We also show the $\mathrm{EA}\nu$ flux spectrum within ${\theta }_{\text{Sun}}$ and within the neutrino-muon separation angle (${\theta }_{\nu \mu }$).

Figure 2

The total muon spectrum ($\mathrm{starting}+\text{entering}$) of the $\mathrm{SA}\nu$ flux (Nominal model) for IceCube with ten years of live time. Also shown is the muon spectrum from the $\mathrm{EA}\nu$ background within the neutrino-muon opening angle. The $\mathrm{SA}\nu$ signal exceeds the background at $E_{\mu }\gtrsim 2\mathrm{TeV}$.

Figure 3

Left: The total muon spectrum of the $\mathrm{SA}\nu$ in IceCube with 1 year of live time, compared with several DM spectra in the $\tau \overline{\tau }$ channel, obtained with WimpSim [67]. For high DM masses, the spectra become indistinguishable due to neutrino absorption in the Sun. For presentation, the DM annihilation rate for different masses is taken to have a fixed value of ${10}^{19}{\mathrm{s}}^{-1}$ (see text for details). Right: The same, but for Super-K, and the annihilation rate is ${10}^{20}{\mathrm{s}}^{-1}$.

Figure 4

Left: The indirect detection neutrino sensitivity floors for SD DM cross section for Super-K and IceCube are shown in the bottom. We show the case for the $\tau \overline{\tau }$ channel, and the solid (dashed) line corresponds to the Naive (Nominal) case. For comparison, we show, in black dotted line, the floor obtained using the IceCube effective area [30] with the Naive model. We also show current indirect detection limits from Super-K [29] and IceCube [30]; the white space between them and the floors shows the remaining parameter space that can be probed by solar DM searches. For perspective, we also show the direct detection limits from PICO [75]. Right: Same as the left, but for the $b\overline{b}$ channel.