New Definition of the Neutrino Floor for Direct Dark Matter Searches

The neutrino floor is a theoretical lower limit on WIMP-like dark matter models that are discoverable in direct detection experiments. It is commonly interpreted as the point at which dark matter signals become hidden underneath a remarkably similar-looking background from neutrinos. However, it has been known for some time that the neutrino floor is not a hard limit, but can be pushed past with sufficient statistics. As a consequence, some have recently advocated for calling it the “neutrino fog” instead. The downside of current methods of deriving the neutrino floor are that they rely on arbitrary choices of experimental exposure and energy threshold. Here we propose to define the neutrino floor as the boundary of the neutrino fog, and develop a calculation free from these assumptions. The technique is based on the derivative of a hypothetical experimental discovery limit as a function of exposure, and leads to a neutrino floor that is only influenced by the systematic uncertainties on the neutrino flux normalizations. Our floor is broadly similar to those found in the literature, but differs by almost an order of magnitude in the sub-GeV range, and above 20 GeV.

Figure 1

Present exclusion limits on the spin-independent DM-nucleon cross section (assuming equal proton or neutron couplings) [7, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71]. Beneath these limits we show three definitions of the neutrino floor for a xenon target. The previous discovery-limit-based neutrino floor calculation shown by the dashed line is taken from the recent APPEC report [72] (based on the technique of Ref. [32]). The envelope of 90% C.L. exclusion limits seeing one expected neutrino event is shown as a dotted line. The result of our work is the solid orange line. We define this notion of the neutrino floor to be the boundary of the neutrino fog, i.e., the cross section at which any experiment sensitive to a given value of $m_{\chi }$ leaves the standard Poissonian regime and begins to be saturated by the background.

Figure 2

A graphical description of the technique we adopt to map the neutrino fog and plot its boundary. In the main panel we show the spin-independent DM parameter space, coloring the section below the neutrino floor by the value of $n$, defined as the index with which a discovery limit scales with the number of background events, i.e., $\sigma \propto N^{-1/n}$. The neutrino fog is defined to be the regime for which $n>2$, with the neutrino floor being the cross section for a given mass where this transition occurs. The top right panel shows the evolution of $\sigma$ with $N$ at $m_{\chi }=5.5\mathrm{GeV}$ between the two cross sections labeled “a” and “b” on the main panel.

Figure 3

Left: The neutrino floor for various popular direct detection targets. The general trend is for lighter targets to have floors shifted towards higher masses. The full topology of the neutrino fog for these same targets is shown in Fig. S1 of the Supplemental Material [115]. Right: xenon’s neutrino floor, showing the effect of improvements to neutrino flux uncertainties. The solid line shows our baseline calculation, whereas the dot-dashed and dashed lines show the result after reducing the systematic uncertainties on the solar and atmospheric fluxes by a factor of 10.