[论文解读] Constraints on directionality effect of nuclear recoils in a liquid argon time projection chamber

20k Collaboration, P. Agnes+265 more|arXiv (Cornell University)|Jul 28, 2023

Dark Matter and Cosmic Phenomena被引用 2

一句话总结

本研究通过测量已知角度下的中子诱发核 recoil，调查液氩时间投影室（TPC）在暗物质探测中的方向敏感性。利用INFN-LNS的ReD实验数据，采用Cataudella电荷复合模型进行似然分析，未发现显著的方向性效应，对电离云长宽比R = 1.037 ± 0.027的上限置信度为90%时R < 1.072。

ABSTRACT

The direct search for dark matter in the form of weakly interacting massive particles (WIMP) is performed by detecting nuclear recoils (NR) produced in a target material from the WIMP elastic scattering. A promising experimental strategy for direct dark matter search employs argon dual-phase time projection chambers (TPC). One of the advantages of the TPC is the capability to detect both the scintillation and charge signals produced by NRs. Furthermore, the existence of a drift electric field in the TPC breaks the rotational symmetry: the angle between the drift field and the momentum of the recoiling nucleus can potentially affect the charge recombination probability in liquid argon and then the relative balance between the two signal channels. This fact could make the detector sensitive to the directionality of the WIMP-induced signal, enabling unmistakable annual and daily modulation signatures for future searches aiming for discovery. The Recoil Directionality (ReD) experiment was designed to probe for such directional sensitivity. The TPC of ReD was irradiated with neutrons at the INFN Laboratori Nazionali del Sud, and data were taken with 72 keV NRs of known recoil directions. The direction-dependent liquid argon charge recombination model by Cataudella et al. was adopted and a likelihood statistical analysis was performed, which gave no indications of significant dependence of the detector response to the recoil direction. The aspect ratio R of the initial ionization cloud is estimated to be 1.037 +/- 0.027 and the upper limit is R &lt; 1.072 with 90% confidence level

研究动机与目标

• 测试液氩TPC中的漂移电场是否在核 recoil 中引起方向依赖的电荷复合效应。
• 评估通过角度调制特征实现方向性暗物质探测的潜力。
• 利用中子辐照实验数据验证Cataudella电荷复合模型。
• 对液氩中初始电离云的长宽比R施加约束。

提出的方法

• 在INFN-LNS使用72 keV中子辐照ReD TPC，产生具有已知反冲方向的核 recoil。
• 测量闪烁光（S1）和电致发光（S2）信号，以确定电荷复合行为。
• 应用似然统计分析，将Cataudella方向性模型预测的S2信号与实验数据进行比较。
• 根据反冲方向θr将数据分为四个角度子集，以检验方向性偏差。
• 使用机器学习技术分析预测误差，评估与无方向性原假设的一致性。
• 从χ²检验中计算得到p值 = 0.23，并推导出R的90%置信上限。

实验结果

研究问题

• RQ1 液氩TPC中的漂移电场是否在核 recoil 中引起方向依赖的电荷复合效应？
• RQ2 探测器对核 recoil 的响应是否随反冲动量与漂移场之间夹角的变化而变化？
• RQ3 Cataudella电荷复合模型是否与已知方向核 recoil 的实验数据一致？
• RQ4 在假设无方向性效应的前提下，电离云长宽比R的上限是多少？

主要发现

• 在ReD数据中未观察到显著的方向性效应，χ²检验的p值为0.23。
• 初始电离云的长宽比R测量值为R = 1.037 ± 0.027。
• R的90%置信水平上限为R < 1.072。
• 数据与无方向敏感性的原假设一致。
• 灵敏度研究证实，若S1存在7%的差异（如SCENE所暗示），本数据集可在3.2σ水平下检测到。
• 结果支持Cataudella电荷复合模型的有效性，并对方向性暗物质搜寻策略施加了约束。