Sensitivity of nEXO to $^{136}$Xe Charged-Current Interactions: Background-free Searches for Solar Neutrinos and Fermionic Dark Matter

TL;DR

This paper demonstrates that the nEXO detector can achieve background-free measurements of solar neutrinos and fermionic dark matter interactions by utilizing time-delayed signals from $^{136}$Cs isomeric states, significantly improving detection sensitivity.

Contribution

The study introduces a detailed Monte Carlo simulation of scintillation signals in nEXO, enabling background discrimination and precise measurements of solar neutrino fluxes and dark matter interactions.

Findings

Achieves background discrimination of order 10^{-9}

Projects 25% uncertainty in CNO solar neutrino flux measurement

Enhances sensitivity to fermionic dark matter by up to three orders of magnitude

Abstract

We study the sensitivity of nEXO to solar neutrino charged-current interactions, $\nu_e + ^{136}$Xe$\rightarrow ^{136}$Cs$^* + e^-$, as well as analogous interactions predicted by models of fermionic dark matter. Due to the recently observed low-lying isomeric states of $^{136}$Cs, these interactions will create a time-delayed coincident signal observable in the scintillation channel. Here we develop a detailed Monte Carlo of scintillation emission, propagation, and detection in the nEXO detector to model these signals under different assumptions about the timing resolution of the photosensor readout. We show this correlated signal can be used to achieve background discrimination on the order of $10^{-9}$, enabling nEXO to make background-free measurements of solar neutrinos above the reaction threshold of 0.668 MeV. We project that nEXO could measure the flux of CNO solar neutrinos with a statistical uncertainty of 25%, thus contributing a novel and competitive measurement towards addressing the solar metallicity problem. Additionally, nEXO could measure the mean energy of the $^7$Be neutrinos with a precision of $\sigma \leq 1.5$ keV and could determine the survival probability of $^{7}$Be and $pep$ solar $\nu_e$ with precision comparable to state-of-the-art. These quantities are sensitive to the Sun's core temperature and to non-standard neutrino interactions, respectively. Furthermore, the strong background suppression would allow nEXO to search for for charged-current interactions of fermionic dark matter in the mass range $m_\chi$ = $0.668$-$7$ MeV with a sensitivity up to three orders of magnitude better than current limits.