The extended Water-based Liquid Scintillator Program for ANNIE: Nylon Vessel and Advanced Energy Reconstruction (2025-2027)

• Applicant: Professor Dr. Michael Wurm
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 552099472

Water-based liquid scintillator (WbLS) has been proposed as an advanced detection medium for low-energy astrophysical neutrinos and long-baseline neutrino beam experiments. In WbLS detectors, ionizing particles produce signals through both the scintillation and the Čerenkov effect. The reconstruction of this hybrid signal components greatly enhances particle ID and background discrimination power. Signal separation is further improved by the use of ultra-fast light sensors like LAPPDs. Here, we propose the first full-scale application of WbLS in the reconstruction of GeV neutrino events, utilizing the ANNIE (Accelerator Neutrino Nucleus Interaction Experiment) detector that is located in the Booster Neutrino Beam at Fermilab. This demonstration will be highly relevant for the future use of this novel technique in long-baseline oscillation experiments, currently proposed for the fourth Far Detector Module of the DUNE experiment. The ANNIE experiment uses a 25-ton scale gadolinium loaded water Cherenkov detector to study the cross sections and neutron production of GeV neutrino interactions. ANNIE also serves as a test platform for five ultrafast LAPPDs that provide sub-nanosecond time resolution, greatly enhancing vertex reconstruction and Cherenkov/scintillation separation capabilities. Based on prior funding by the DFG, in 2023 we successfully installed an acrylic vessel holding 365 kg of WbLS in the ANNIE water tank. We achieved a first detection of neutrinos in WbLS and successfully demonstrated pattern-based separation of the superimposed Cherenkov and scintillation signals. It became clear though that a full assessment of the event reconstruction capabilities, especially the reconstruction of recoil protons and neutrons, will require a substantially larger WbLS volume. The goal of the present proposal is to perform the next step in the demonstration of hybrid Cherenkov/scintillation detection by realizing a greatly enlarged WbLS volume in ANNIE. The new vessel will fill the entire central volume of the ANNIE water tank, holding 8 tons of WbLS. Moreover, the tank will be equipped with five additional next-generation LAPPDs to enhance the detection of the isotropic scintillation signal. In this improved setup, both sub-Cherenkov recoil protons and neutrons will be detected by their scintillation signals, allowing for an improved and less biased neutrino energy reconstruction. As part of this coordinated effort with further German universities, Mainz will design and construct the new nylon vessel holding the WbLS, identify the signal of recoil protons and develop the energy reconstruction algorithm that includes neutron ranging, LAPPD high-precision timing and topological event information.

• DFG Programme: Research Grants