THEIA will be a novel, “hybrid” optical neutrino detector, with a rich physics program. THEIA will make use of a number of novel technologies to achieve successful hybrid event detection. Water-based liquid scintillator (WbLS), or slow scintillators can be used to enhance the Cherenkov signal by either reducing or delaying the scintillation component. The use of angular as well as timing information, with sufficiently fast photon detectors, can offer discrimination between Cherenkov and scintillation light for high-energy events even in a standard scintillator like LAB-PPO. Separation can also be enhanced using very fast photon detectors, such as LAPPDs (Large Area Picosecond Photo-Detectors), or spectral sorting using dichroic filters.

Fig 1. The THEIA detector. Upper panel: THEIA-25 sited in the planned fourth DUNE cavern; Lower left panel: an interior view of THEIA-25; Lower center panel: Exterior view of THEIA-100; Lower right panel: an interior view of THEIA-100

THEIA could reside in a cavern the size and shape of those planned for the Deep Underground Neutrino Experiment (DUNE), called Theia-25, and a larger 100-kton version (Theia-100) that could achieve an even broader and more sensitive scientific program (Fig. 1). THEIA is expected to allow for new scientific discovery across a broad spectrum of physics, including long baseline neutrino measurements of oscillations and CP violation searches, solar neutrino measurements of unprecedented precision and scope, and the potential to extend the reach of neutrinoless double beta decay searches to the mass scales implied by a Normal Ordering of neutrino masses. In addition, THEIA represents a major step forward for advancement in other fields – ranging from detection of the Diffuse Supernova Background flux to a measurement of geo-neutrinos with unprecedented statistics.