Exploring nu signals in dark matter detectors

Abstract

We investigate standard and non-standard solar neutrino signals in direct dark matter detection experiments. It is well known that even without new physics, scattering of solar neutrinos on nuclei or electrons is an irreducible background for direct dark matter searches, once these experiments each the ton scale. Here, we entertain the possibility that neutrino interactions are enhanced by new physics, such as new light force carriers (for instance a "dark photon") or neutrino magnetic moments. We consider models with only the three standard neutrino flavors, as well as scenarios with extra sterile neutrinos. We find that low-energy neutrino--electron and neutrino--nucleus scattering rates can be enhanced by several orders of magnitude, potentially enough to explain the event excesses observed in CoGeNT and CRESST. We also investigate temporal modulation in these neutrino signals, which can arise from geometric effects, oscillation physics, non-standard neutrino energy loss, and direction-dependent detection efficiencies. We emphasize that, in addition to providing potential explanations for existing signals, models featuring new physics in the neutrino sector can also be very relevant to future dark matter searches, where, on the one hand, they can be probed and constrained, but on the other hand, their signatures could also be confused with dark matter signals.

Figures (8)

• Figure 1: Expected event rates in dark matter detectors from (a) solar neutrino--electron scattering and (b) solar neutrino--nucleus scattering in germanium. In (a), we use units of events per keVee per year per $N_A$ electrons (where $N_A$ is the Avogadro number) to be able to compare rates in different materials. Thick black lines correspond to the total event rate, while thin lines break the rate up into contributions from different neutrino production processes. We also show the observed electron recoil spectra in XENON-100 Aprile:2011vb (see text for details) and Borexino Borexino:2011rx, from the low-threshold analysis of CDMS data Ahmed:2010wy, and the event spectra from CoGeNT Aalseth:2011wp and DAMA Bernabei:2008yi. Since CoGeNT and DAMA cannot distinguish nuclear recoils from electron recoils, we interpret their data as electron recoil in the left panel and as nuclear recoils in the right panels.
• Figure 2: Expected event spectra in a dark matter detector from new physics in the scattering of solar neutrinos on electrons. The different colored curves correspond to (A) a model where the neutrino has a magnetic dipole moment of $\mu_\nu=0.32\times 10^{-10} \mu_B$ and (B, C, D) models where the scattering is enhanced by the exchange of a new light gauge boson $A'$ with couplings $g_e$ to electrons and $g_\nu$ to neutrinos. The latter case is for instance realized in the model from section \ref{['sec:kinetic-mixing']}, where Standard Model particles couple to the $A'$ through its kinetic mixing with the photon, but there is also a sterile neutrino $\nu_s$ directly charged under $U(1)'$. To keep the discussion general, we assume the $\nu_e \to \nu_s$ transition probability to be energy-independent, and we have absorbed the corresponding flux suppression into a redefinition of $g_\nu^2$. The black curve shows the Standard Model rate from figure \ref{['fig:SMspectrum']}, and the red curves and data points show the observed electron recoil rates in XENON-100 Aprile:2011vb (see section \ref{['sec:rates']} for details), Borexino Borexino:2011rx, CoGeNT Aalseth:2011wp, and DAMA Bernabei:2008yi. (Note that CoGeNT and DAMA cannot distinguish nuclear recoils from electron recoils, so their data can be interpreted as either.)
• Figure 3: Expected event spectra in a dark matter detector from $A'$-enhanced scattering of heavy sterile neutrinos on electrons (thick colored lines, see figure legend for the parameters used). We have assumed the $A'$ mass to be almost negligible, and we have chosen the cross section such that the CoGeNT excess can be explained. Black lines show the count rate in the Standard Model, and red curves show the observed event rates in XENON-100 Aprile:2011vb (see section \ref{['sec:rates']} for details), Borexino Borexino:2011rx, CoGeNT Aalseth:2011wp and DAMA Bernabei:2008yi. We have accounted for the kinematic suppression of heavy neutrino production for the ${{}^{7}\rm Be}$ neutrinos (solid colored lines), but not of the pp, ${}^{13}$$\rm N$, ${}^{15}$$\rm O$, ${}^{17}$$\rm F$, ${}^{8}$$\rm B$ and hep neutrinos (dashed colored line), which are produced as parts of 3-body final states.
• Figure 4: Expected event spectra in dark matter detectors from solar neutrino--nucleus scattering in (a) germanium, (b) CaWO$_4$, (c) NaI(Tl), and (d) xenon. Note that for NaI(Tl), we use units of keVee rather than keVnr for $E_r$ because due to the different quenching factors for Na (0.3) and I (0.09) Bernabei:1996vj, the nuclear recoil energy cannot be uniquely reconstructed. Colored curves correspond to (A) a scenario with a neutrino magnetic moment $\mu_\nu = 0.32 \times 10^{-10} \mu_B$, (B) a model with active neutrino--nucleus scattering through a light $A'$ boson (for instance the $U(1)_{B-L}$ model from section \ref{['sec:B-L']}), and (C), (D) a model in which 2% of the solar neutrino flux oscillate into a Standard Model singlet $\nu_s$, which couples to atomic nuclei for instance via a light $U(1)_B$ gauge boson (section \ref{['sec:gauged-B']}) Pospelov:2011ha. The relevant model parameters, in particular the mass of the $A'$ and its coupling to nucleons ($g_p = g_n \equiv g_N$) are listed in the legend. Where applicable, we have assumed active--sterile mixing with $\sin^2\theta_{24} = 0.02$ and $\Delta m^2_{42} = 10^{-10}$ eV$^2$. The black curves show the Standard Model rate, and the red curves and data points show the observed spectra of nuclear recoil candidates in CoGeNT Aalseth:2011wp, in the low-threshold data set from CDMS Ahmed:2010wy, in CRESST Angloher:2011uu, and in DAMA Bernabei:2008yi. The approximate Xenon-100 exclusion region is obtained by converting Xenon-100's observed rate of signal candidates (3 events/100.9 days/48 kg between 8.4 keVnr and 44.6 keVnr Aprile:2011hi) into the units of our plots.
• Figure 5: Relative annual modulation fraction $(R_{\rm Jun} - R_{\rm Dec}) / (R_{\rm Jun} + R_{\rm Dec})$ of the rate of $A'$-mediated sterile neutrino--electron scattering as a function of the recoil energy $E_r$ and the mass squared difference between active and sterile neutrinos. For simplicity, we use a two-flavor vacuum oscillation framework here. One can clearly distinguish five different regimes, in which the rate is dominated by pp neutrinos, ${}^{7}$$\rm Be$ neutrinos, pep neutrinos, CNO neutrinos, and ${}^{8}$$\rm B$ neutrinos, respectively.