The ANNIE Water-based Liquid Scintillator Program 2022-2024

水基液体闪烁体（WbLS）是一种用于低能天体物理中微子和长基线中微子束实验的先进探测介质。在WbLS探测器中，电离粒子通过闪烁和切伦科夫效应产生光子信号。两个信号分量的重建大大增强了粒子识别（PID）和背景辨别能力。通过使用超快光传感器进一步改善了信号分离。在这里，我们提出了第一个物理应用的新型闪烁材料在ANNIE（加速器中微子核相互作用实验）实验位于助推中微子束在费米实验室。目前，ANNIE实验使用吨级钆水靶来研究GeV中微子相互作用的截面和中子产生的分布。该提案旨在开发和实现ANNIE的升级，引入WbLS作为目标媒体。在第一阶段，这将通过将一艘装有320公斤WbLS的丙烯酸船部署到ANNIE的水目标中来实现。对于在该容器内相互作用的中微子，WbLS增强的信号重建能力将能够检测强子反冲中的亚切伦科夫粒子，并基于检测到的切伦科夫和闪烁光子的比率增加新的PID能力。ANNIE将很快配备一系列新颖的LAPPD光传感器，提供亚纳秒的计时精度。它们对WbLS中事件重建的影响将是特别感兴趣的，本提案的目标不仅是进一步发展混合事件重建，而且还将其用于对收集到的中微子数据进行物理分析的测试。中性流（NC）中微子相互作用构成了长基线振荡实验的重要背景。此外，这些NC相互作用是特别感兴趣的大体积的水和闪烁体探测器的大气中微子事件的理解。在那里，这类NC事件构成了寻找弥散超新星中微子背景的最持久的背景来源，并限制了间接暗物质搜索的灵敏度。该分析程序将通过引入激光系统来补充，该激光系统用于校准WbLS闪烁响应，并为未来的ANNIE第三阶段的整个探测器设置做好准备，该阶段具有完整的WbLS目标。

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 photon signals by scintillation and the Cherenkov effect. The reconstruction of both signal components greatly enhances particle identification (PID) and background discrimination power. Signal separation is further improved by the use of ultra-fast light sensors. Here, we propose the first physics application of the novel scintillation material in the ANNIE (Accelerator Neutrino Nucleus Interaction Experiment) experiment located in the Booster Neutrino Beam at Fermilab. Currently, the ANNIE experiment uses a ton-scale gadolinium loaded water target to study GeV neutrino interactions for their cross-sections and distributions of neutron production. This proposal aims to develop and realize an upgrade of ANNIE introducing WbLS as target medium. At the first stage, this will be realized by the deployment of an acrylic vessel containing 320 kg of WbLS into ANNIE’s water target. For neutrinos interacting inside this vessel, the enhanced signal reconstruction capabilities of WbLS will enable the detection of sub-Cherenkov particles in hadronic recoils and add novel PID capabilities based on the ratio of Cherenkov and scintillation photons detected. ANNIE will be equipped soon with an array of novel LAPPD light sensors that offer sub-nanosecond timing precision. Their impact on event reconstruction in WbLS will be of special interest.The goal of the present proposal is not only to further develop hybrid event reconstruction but also to put it to the test in physics analyses of the collected neutrino data. Neutral Current (NC) neutrino interactions pose a crucial background for long-baseline oscillation experiments. Moreover, these NC interactions are of special interest for the understanding of atmospheric neutrino events in large-volume water and scintillator detectors. There, this class of NC events constitutes the most persistent source of background in the search for the Diffuse Supernova Neutrino Background and limits the sensitivity in indirect Dark Matter searches. This analysis program will be complemented by the introduction of a laser system for a calibration of the WbLS scintillation response and preparations of the entire detector setup for a future ANNIE Phase III with a complete WbLS target.