UC Berkeley

Abstract: We present the sensitivity of the Theia experiment to low-energy geo- and reactor antineutrinos. For this study, we consider one of the possible proposed designs, a 17.8-ktonne fiducial volume Theia-25 detector filled with water-based liquid scintillator placed at Sanford Underground Research Facility (SURF). We demonstrate Theia’s sensitivity to measure the geo- and reactor antineutrinos via inverse-beta decay interactions after one year of data taking with $$11.9\times {10}^{32}$$ 11.9 × 10 32 free target protons. Considering all uncertainties on input throughout the whole analysis chain, the expected number of geo- and reactor antineutrinos is $$220{}_{-24}^{+30}$$ 220 - 24 + 30 (stats+syst) and $$168{}_{-24}^{+26}$$ 168 - 24 + 26 (stat+sys), respectively, after one year of data taking. The corresponding expected fit precision of a sole experiment is evaluated at 8.7% and 10.1%, respectively. We also demonstrate the sensitivity towards fitting individual Th and U contributions, with best fit values of $$N_{\text{Th}}={40}_{-22}^{+26}$$ N Th = 40 - 22 + 26 (stat+sys) and $$N_{\text{U}}={180}_{-24}^{+30}$$ N U = 180 - 24 + 30 (stat+sys). Finally, from the fit results of individual Th and U contributions, we evaluate the mantle signal to be $$S_{\text{mantle}}=9.3\pm [5.2,5.4]$$ S mantle = 9.3 ± [ 5.2 , 5.4 ]  NIU (stat+sys). This was obtained assuming a full-range positive correlation ($${\rho }_c\in [0,1]$$ ρ c ∈ [ 0 , 1 ] ) between Th and U, and the projected uncertainties on the crust contributions of 8.3% (Th) and 7.0% (U).