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

We study the sensitivity of nEXO to solar neutrino charged-current interactions, \(\nu_e\)+\(^{136}\)Xe→\(^{136}\)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 read- out. 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 σ ≤ \(1.5\) 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.

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