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 $11.9\times10^{ 32}$ free target protons. The expected number of detected geo- and reactor antineutrinos is $218\,^{ +28}_{ -20}$ and $170\,^{ +24}_{ -20}$, respectively. The precision of the fitting procedure has been evaluated to be 6.72% and 8.55% for geo- and reactor antineutrinos, respectively. We also demonstrate the sensitivity towards fitting individual Th and U contributions, with best fit values of $N_\text{ Th}=39\,^{ +18}_{ -15}$ and $N_\text{ U}=180\,^{ +26}_{ -22}$. We obtain $(\text{ Th}/\text{ U})=4.3\pm2.6$ after one year of data taking, and within ten years, the relative precision of the (Th/U) mass ratio will be reduced to 15%. Finally, from the fit results of individual Th and U contributions, we evaluate the mantle signal to be $S_\text{ mantle} = 9.0\,\pm [4.2,4.5]$NIU. This was obtained assuming a full-range positive correlation ($\rho_c\in[0, 1]$) between Th and U, and the projected uncertainties on the crust contributions of 8.3% (Th) and 7.0% (U). When considering systematic uncertainties on the signal and background shape and fluxes, the mantle signal becomes $S_\text{ mantle} = 9.3\,\pm [5.2,5.4]$NIU.
Categories: hep-ex physics.geo-ph physics.ins-det