Fitting the Annual Modulation in DAMA with Neutrons from Muons and Neutrinos

The DAMA/LIBRA experiment searches for evidence of dark matter scattering off nuclei. Data from DAMA show $9.2\sigma$ evidence for an annual modulation, consistent with dark matter having a cross section around $2\times {10}^{-40}{\mathrm{cm}}^2$. However, this is excluded by other direct detection experiments. We propose an alternative source of annual modulation in the form of neutrons, which have been liberated from material surrounding the detector by a combination of ${}^8\mathrm{B}$ solar neutrinos and atmospheric muons. The phase of the muon modulation lags 30 days behind the data; however, we show that adding the modulated neutrino component shifts the phase of the combined signal forward. In addition, we estimate that neutrinos and muons need $\sim 1000{\mathrm{m}}^3$ of scattering material in order to generate enough neutrons to constitute the signal. With current data, our model gives as good a fit as dark matter, and we discuss prospects for future experiments to discriminate between the two.

Figure 1

The DAMA signal is composed of neutrons liberated in the material surrounding the detector by both solar neutrinos (dotted line) and atmospheric muons (dashed line). Both components have fixed phases, with only their amplitudes as free parameters. Individually, neither of these has the correct phase to fit the data; however, in combination, the fit quality is excellent.

Figure 2

Contours (and best-fit point is denoted by star) of the modulation residuals for the muon $A_{\mu }$ and neutrino $A_{\nu }$ induced neutron signal in Eq. (3), for the case where the phases are marginalized over. Shown also are approximate values for the day where the signal peaks for selected values of $A_{\nu }/A_{\mu }$.

Figure 3

Comparison of models for the DAMA data. The model proposed in this Letter is shown as the solid cyan line, composed of neutrons produced by solar neutrinos and atmospheric muons [with fixed phases $({\varphi }_{\nu },{\varphi }_{\mu })=(3,179)$ days]. Adding the solar neutrino contribution to that from muons shifts the phase forward by $\sim 30$ days, markedly improving the fit to the data.

Figure 4

Approximate peak day of the $\text{neutrino}+\text{muon}$ signal at four different labs. Deeper labs have a lower muon flux [42] and so a phase closer to that of the solar neutrinos.