Detecting CNO solar neutrinos in next-generation xenon dark matter experiments

We study the prospects for measuring the low-energy components of the solar neutrino flux in future direct dark matter detection experiments. We show that for a depletion of ${}^{136}\mathrm{Xe}$ by a factor of 1000 relative to its natural abundance, and an extension to electron recoil energies of $\sim \mathrm{MeV}$, future xenon experiments with exposure $\sim 1000$ ton-yr can detect the carbon-nitrogen-oxygen (CNO) component of the solar neutrino flux at approximately three-sigma significance. A CNO detection will provide important insight into the metallicity of the solar interior. Precise measurement of low-energy solar neutrinos, including as $\mathit{pp}$, ${}^7\mathrm{Be}$, and $pep$ components, will further improve constraints on the “neutrino luminosity” of the Sun, thereby providing constraints on alternative sources of energy production. We find that a measurement of $L_{\nu }/L_{\odot }$ of order 1% is possible with the above exposure, improving on current bounds from a global analysis of solar neutrino data by a factor of about 7.

Figure 1

Electronic recoil spectrum from solar neutrinos in xenon experiments for elastic $\nu +e^{-}$ scattering. The labels denote the $pp$, ${}^7\mathrm{Be}$, CNO, and $pep$ fluxes. The blue curves show the relevant CNO components (${}^{15}\mathrm{O}$ and ${}^{13}\mathrm{N}$).

Figure 2

The background electronic recoil spectrum from sources relevant to xenon experiments. The solid blue (dashed) line is the $2\nu \beta \beta$ decay of ${}^{136}\mathrm{Xe}$, at natural abundances (depleted to 1% of its natural abundance). The purple line is the background due to ${}^{85}\mathrm{Kr}$ at a concentration of 0.1 ppt. The red line is the spectrum from ${}^{216}\mathrm{Pb}$ due to ${}^{222}\mathrm{Rn}$ emanation, with an activity of $0.1\mu \mathrm{Bq}/\mathrm{kg}$.

Figure 3

The significance of a CNO neutrino detection above the background of $pp$, $pep$, ${}^7\mathrm{Be}$, solar neutrinos, as well as the intrinsic electron recoil background spectrum. The different lines correspond to the level of depletion of ${}^{136}\mathrm{Xe}$ relative to the natural abundance. The top panel assumes uncertainties of 1% for the intrinsic backgrounds, while the bottom panel assumes 0.1% and a factor of 10 lower concentration of Kr and Rn.

Figure 4

Conditional posterior distributions of the flux normalizations for a 200 ton-yr exposure. The different colored contours show different assumptions for the Gaussian priors on the Rn, Kr, and $2\nu \beta \beta$ background components: 10% (red), 1% (blue), and no background uncertainty (green). For all cases, we take the normalization of the $2\nu \beta \beta$ background to be depleted by ${10}^{-3}$ relative to the natural abundance. The contours denote one- and two-sigma credible regions. The red crosses and vertical lines denote the simulated values, i.e., $f_{\alpha }=1$.

Figure 5

Projected fractional uncertainty on the neutrino luminosity of the Sun from multiton xenon dark matter experiments. The top row shows the projected constraints for a 200 ton-yr exposure, and the bottom row shows the projected constraints for a 2000 ton-yr exposure. The left column shows the projected constraints on the total neutrino flux, while the right column shows just the contribution of the CNO flux to the solar luminosity. The shaded regions denote the 90% credible regions. For all cases, the normalization of the $2\nu \beta \beta$ background is depleted by ${10}^{-3}$ relative to the natural abundance.