The ANNIE Water-based Liquid Scintillator Program 2022-2024

水基液体闪烁体（WbLS）被提出作为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。两种信号分量的重构大大提高了粒子识别（PID）和背景识别能力。超高速光传感器的使用进一步改善了信号分离。在这里，我们提出了这种新型闪烁材料在费米实验室加速器中微子束中的ANNIE（加速器中微子核相互作用实验）实验中的首次物理应用。目前，ANNIE实验使用吨级载钆水靶来研究GeV中微子相互作用的截面和中子产生的分布。本课题旨在开发并实现以WbLS为目标介质的ANNIE升级版。在第一阶段，这将通过将一个含有320公斤WbLS的丙烯酸容器部署到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光子和闪烁光子的比例增加新的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引起人们的特别关注。本提案的目标不仅是进一步发展混合事件重建，而且将其用于收集的中微子数据的物理分析中。中性电流（NC）中微子相互作用是长基线振荡实验的重要背景。此外，这些NC相互作用对于理解大体积水和闪烁体探测器中的大气中微子事件具有特殊的意义。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源，并限制了间接暗物质搜索的灵敏度。该分析计划将通过引入激光系统来补充，用于校准WbLS闪烁响应，并为未来ANNIE第三阶段的完整WbLS目标准备整个探测器设置。

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.