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$$\in$ [0, 1]) between Th and U, and the projected uncertainties on the crust contributions of 8.3% (Th) and 7.0% (U).