Geo- and reactor antineutrino sensitivity at THEIA

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 × 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 (stats+syst) and 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 Th = 40 - 22 + 26 (stat+sys) and 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 mantle = 9.3 ± [ 5.2 , 5.4 ] NIU (stat+sys). This was obtained assuming a full-range positive correlation ( ρ c ∈ [ 0 , 1 ] ) between Th and U, and the projected uncertainties on the crust contributions of 8.3% (Th) and 7.0% (U).