Monolithic optical detectors have a long history of success in neutrino physics, from water Cherenkov detectors such as Super-Kamiokande and the Sudbury Neutrino Observatory, to liquid scintillator detectors such as Borexino, KamLAND, and Daya Bay. While those two worlds have existed in parallel for decades, the next generation of detectors is about to exploit the best of both worlds by detecting Cherenkov and scintillation signals simultaneously. In the past, Prof. Orebi Gann and her team at the Berkeley Lab have made critical advances towards this goal by researching novel photon detectors and liquid scintillators such as the water-based liquid scintillator developed at Brookhaven National Lab. Using the CHESS  setup at the Lab, the team was able to achieve a separation of Cherenkov and scintillation light in both pure liquid scintillator (LS) and water-based liquid scintillator (WbLS) utilizing ultra-fast light detectors.
The EOS experiment seeks to perform the first-ever data-driven demonstration of event reconstruction using both Cherenkov and scintillation signals. The Berkeley team’s ground breaking effort is now reflected in the allocation of $10M in funding from the National Nuclear Security Administration (NNSA) to build the EOS experiment. EOS is named for the Titan Goddess of Dawn, and would be a precursor to the multi-kiloton THEIA detector . EOS will be constructed in Berkeley over the next three years. The collaboration includes 14 institutes with a strong contribution from the United States, but also involves international partners from Europe. Building upon the success of CHESS, EOS will go one step further to demonstrate the simultaneous use of Cherenkov and scintillation signals for precision event reconstruction at the few-ton scale. Its data will allow the team to validate and refine event simulations and the design of future instruments. EOS will permit the development of production and handling strategies for WbLS in large-scale detectors. Several variants of WbLS and other novel LS techniques will be investigated with EOS. After the initial phase in Berkeley, a re-deployment of the apparatus can be done at a reactor, accelerator beamline, or at an underground site for additional studies of neutrino events.
The EOS demonstrator results will constitute valuable input for the design of future kiloton and larger detectors like THEIA . These technologies have the potential to impact a broad program of fundamental science, including searches for neutrino-less double beta decay, detection of solar and cosmic neutrinos, and measurements of fundamental symmetry violation if deployed in a neutrino beam. Moreover, the technologies studied with EOS can facilitate the use of neutrinos for remote reactor monitoring.
Stefan Schoppmann and Gabriel Orebi Gann contributed to this news note. Dr. Schoppmann led the studies to explore the possibility of deploying EOS at a reactor in the future. Prof. Orebi Gann is the PI for the EOS project.
 CHESS collaboration – Phys. Rev. C 95, 055801 (2017) and Eur. Phys. J. C 80, 867 (2020)
 THEIA collaboration – Eur. Phys. J. C 80, 416 (2020)