Accelerator and reactor complementarity in coherent neutrino-nucleus scattering

We study the complementarity between accelerator and reactor coherent elastic neutrino-nucleus elastic scattering ($\mathrm{CE}\nu \mathrm{NS}$) experiments for constraining new physics in the form of nonstandard neutrino interactions (NSI). First, considering just data from the recent observation by the Coherent experiment, we explore interpretive degeneracies that emerge when activating either two or four unknown NSI parameters. Next, we demonstrate that simultaneous treatment of reactor and accelerator experiments, each employing at least two distinct target materials, can break a degeneracy between up and down flavor-diagonal NSI terms that survives analysis of neutrino oscillation experiments. Considering four flavor-diagonal ($ee/\mu \mu$) up- and down-type NSI parameters, we find that all terms can be measured with high local precision (to a width as small as $\sim 5\%$ in Fermi units) by next-generation experiments, although discrete reflection ambiguities persist.

Figure 1

The degeneracy-lifting effect of detector complementarity is exhibited for Si and Ge targets, where NSI contributions interfere coherently within a single amplitude (left), or incoherently as a sum of squares, where flavor contributions are either distinguishable (middle), or indistinguishable (right).

Figure 2

Posterior probabilities of NSI parameters using the current Coherent data, allowing for two nonzero NSI parameters. The contours show the 68% and 95% credible regions, and the red cross indicates the standard model value.

Figure 3

Posterior probabilities of effective NSI parameters using the current Coherent data, allowing for four flavor diagonal parameters to be nonzero. The left panel takes flat priors on the $\epsilon$’s in the range $[-1.5:1.5]$, and the right panel uses flat priors in the range $[-2.5:2.5]$. The contours show the 68% and 95% credible regions, and the red crosses indicate the standard model value. In particular, notice that these two-dimensional projections are space-filling with respect to definition of the prior, and thus do not represent any experimental constraints.

Figure 4

Comparison between an unbinned reconstruction (blue) and a binned reconstruction (red) of simulated future reactor and accelerator data. For the unbinned case we take a single energy bin, while for the binned case we take ten energy bins. The contours show the 90% credible regions and the red crosses indicate the simulated standard model value.

Figure 5

Projected posterior probabilities of effective NSI with future accelerator and reactor data, allowing for four flavor diagonal parameters to be nonzero. The contours show the 68% and 95% credible regions, and the red cross indicates the simulated standard model value.

Figure 6

Projected posterior probabilities of the four NSI parameters with future accelerator and reactor data. Here we have marginalized over the uncertain experimental backgrounds and fluxes from the respective neutrino sources. The contours show the 68% and 95% credible regions, and the red cross indicates the simulated standard model value.

Figure 7

Projected posterior probabilities of the four NSI parameters with future accelerator and reactor data. Here we assume zero experimental background for the accelerator detectors, all other uncertainties are marginalized over. The contours show the 68% and 95% credible regions, and the red cross indicates the simulated standard model value.

Figure 8

Current posterior distribution on the four NSI parameters with Coherent data. Here flux and background uncertainties are marginalized over. The contours show the 68% and 95% credible regions.