  New developments in liquid scintillators, high-efficiency, fast photon
detectors, and chromatic photon sorting have opened up the possibility for
building a large-scale detector that can discriminate between Cherenkov and
scintillation signals. Such a detector could exploit these two distinct signals
to observe particle direction and species using Cherenkov light while also
having the excellent energy resolution and low threshold of a scintillator
detector. Situated in a deep underground laboratory, and utilizing new
techniques in computing and reconstruction techniques, such a detector could
achieve unprecedented levels of background rejection, thus enabling a rich
physics program that would span topics in nuclear, high-energy, and
astrophysics, and across a dynamic range from hundreds of keV to many GeV. The
scientific program would include observations of low- and high-energy solar
neutrinos, determination of neutrino mass ordering and measurement of the
neutrino CP violating phase, observations of diffuse supernova neutrinos and
neutrinos from a supernova burst, sensitive searches for nucleon decay and,
ultimately, a search for NeutrinoLess Double Beta Decay (NLDBD) with
sensitivity reaching the normal ordering regime of neutrino mass phase space.
This paper describes Theia, a detector design that incorporates these new
technologies in a practical and affordable way to accomplish the science goals
described above. We consider two scenarios, one in which Theia would reside in
a cavern the size and shape of the caverns intended to be excavated for the
Deep Underground Neutrino Experiment (DUNE) which we call Theia 25, and a
larger 100 ktonne version (Theia 100) that could achieve an even broader and
more sensitive scientific program.
