Probing light mediators at ultralow threshold energies with coherent elastic neutrino-nucleus scattering

Light neutral mediators, with mass $\lesssim 1\mathrm{GeV}$, are common features of extensions to the Standard Model. Current astrophysical and terrestrial experiments have constrained the model parameter space, and planned experiments around the world promise continued improvement in sensitivity. In this paper we study the prospects for probing light neutral mediators using terrestrial stopped pion and reactor sources in combination with ultralow threshold nuclear and electron recoil detectors. We show that the coherent neutrino-nucleus and neutrino-electron scattering channels provide complementary sensitivity to light mediators. With low-threshold detectors, we show that most stringent bounds on models arise from the nuclear scattering process, improving upon previous bounds from electron scattering of solar neutrinos by nearly an order of magnitude for mediator masses $\gtrsim 0.1\mathrm{GeV}$.

Figure 1

Sample recoil rates for the four BSM scenarios with the following mediator types: scalar (top left), pseudoscalar (top right), vector (bottom left), or axial vector (bottom right). The solid and dotted lines are for Ge and Si detectors respectively. The mediator mass is taken to be 1 keV and the coupling chosen such that the interaction is approximately the strength of the SM. The sharp features in the electron rates are due to atomic energy levels.

Figure 2

Left: 90% confidence intervals on the standard model $\mathrm{CE}\nu \mathrm{NS}$ cross section from a selection of experiments. Right: Exposure required to reach a $5\sigma$ fluctuation from the background-only hypothesis.

Figure 3

Discovery limits for neutrino scattering off germanium nuclei (left) and electrons (right), for the different BSM models: scalar, pseudoscalar, vector, and axial vector (from top to bottom).

Figure 4

The same as Fig. 3 but for silicon detectors.

Figure 5

Discovery limits vs threshold for neutrino scattering off germanium nuclei (left) and electrons (right), for the different BSM models: scalar, pseudoscalar, vector, and axial vector (from top to bottom).

Figure 6

Predicted exclusion curves in the coupling-mediator mass parameter space in a $U(1{)}_{\mathrm{B}-\mathrm{L}}$ model for electron (red) and nuclear (blue) recoils observed by germanium detectors (phase I and II are solid and dashed respectively) with a total exposure of 10 000-days, thresholds situated 2 m from a ${\overline{\nu }}_e$ flux sourced by a 1 MW reactor. The green curves show the current (4466 kg day) and future (10 000 kg day) exclusion limits from the COHERENT experiment. The shaded regions are exclusion curves from other experiments and observations, a compilation from [32].