ANNIE

ANNIE is a gadolinium-doped water Cherenkov detector performing measurements in the Booster Neutrino Beam (BNB) at Fermilab in Batavia (Illinois) in the United States. The primary goal is the determination of the neutron multiplicity generated by muon neutrino interactions in water as a function of the momentum transfer: The results will impact the reconstruction of neutrino interactions in long-baseline oscillation experiments but also the search for proton decay and the Diffuse Supernova Neutrino Background in future large-scale detector.

Beyond its physics program, ANNIE acts as a testbed for novel detector technologies:

• The use of gadolinium-doped water as neutrino target in ANNIE greatly enhances the neutron detection efficiency compared to a normal Water Cherenkov detectors and is vital for a precise measurement of the final neutron number.
• Furthermore, the experiment utilizes state-of-the-art technology to enhance its vertex and track reconstruction capabilities: Large Area Picosend Photodetectors (LAPPDs) allow the reconstruction of picosecond-timing of single photons whilst simultaneously allowing for sub-cm position resolution within the 20cm x 20cm detection area. In early 2024, the experiment is taking data with 3 fully functional LAPPDs mounted in the ANNIE Water Tank.
• In 2023, ANNIE performed the first detection of neutrinos in a novel target medium, Water-based Liquid Scintillator (WbLS). Neutrinos interacting in WbLS not only produce Cherenkov but as well a weak scintillation signal, enhancing the sensitivity to low energy particles below the Cherenkov threshold. A first publication describes the results of a test run with 365 kg of WbLS deployed in a transparent acrylic vessel inside the ANNIE water tank.

Left: Inside view of the ANNIE water tank; middle: schematic view of the experimental setup; right: the SANDI acrylic vessel for Water-based Liquid Scintillator first deployed in spring 2023.

Based on the successful first deployment of WbLS in ANNIE, the collaboration is now moving towards the installation of a larger WbLS volume in the ANNIE water tank. The goal is the demonstration of the full event reconstruction capability in an extended WbLS volume in combination with an array of ultra-fast LAPPDs. For this, the Mainz group is developing a transparent Nylon Vessel that will separate the inside WbLS from the outside water in contact with the detector materials. This vessel will fill the entire active volume of the ANNIE water tank and contain 8 tons of WbLS. Moreover, we are developing novel reconstruction techniques to identify the scintillation light of recoil protons in neutrino interactions, exploring the potential to improve the energy resolution for the incident neutrinos. Both will be important ingredients in the preparation of a future large scale neutrino experiment based on the new technology.

Left: hybrid signal in water-based scintillator -- recoil protons below the Cherenkov threshold give a visible contribution to the overall light signal, also neutron capture signals on Gd are enhanced; right: schematic layout of the planned Nylon Vessel holding 8 tons of WbLS, to be designed and constructed in Mainz.