Sensitivity of nEXO to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mmultiscripts> <mml:mrow> <mml:mi>Xe</mml:mi> </mml:mrow> <mml:mprescripts/> <mml:none/> <mml:mrow> <mml:mn>136</mml:mn> </mml:mrow> </mml:mmultiscripts> </mml:mrow> </mml:math> 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, ${\ensuremath{\nu}}_{e}+^{136}\mathrm{Xe}\ensuremath{\rightarrow}\phantom{\rule{0ex}{0ex}}{^{136}\mathrm{Cs}}^{*}+{e}^{\ensuremath{-}}$, as well as analogous interactions predicted by models of fermionic dark matter. Due to the recently observed low-lying isomeric states of $^{136}\mathrm{Cs}$, these interactions will create a time-delayed coincident signal observable in the scintillation channel. Here we develop a detailed Monte Carlo simulation 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 ${10}^{\ensuremath{-}9}$, 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 neutrinos from the carbon-nitrogen-oxygen cycle with a statistical uncertainty of 25%, thus contributing a novel and competitive measurement toward addressing the solar metallicity problem. Additionally, nEXO could measure the mean energy of the $^{7}\mathrm{Be}$ neutrinos with a precision of $\ensuremath{\sigma}\ensuremath{\le}1.5\text{ }\text{ }\mathrm{keV}$ and could determine the survival probability of $^{7}\mathrm{Be}$ and pep solar ${\ensuremath{\nu}}_{e}$ with precision comparable to the state of the art. These quantities are sensitive to the Sun's core temperature and to nonstandard neutrino interactions, respectively. Furthermore, the strong background suppression would allow nEXO to search for charged-current interactions of fermionic dark matter in the mass range ${m}_{\ensuremath{\chi}}=0.668--7\text{ }\text{ }\mathrm{MeV}$ with a sensitivity up to three orders of magnitude better than current limits.