Signal Formation Mechanisms in a Double Micro-Mesh Gaseous Detector Using the T2K Gas Mixture

As part of a major upgrade to the near detector of the T2K experiment, two gaseous Time Projection Chambers (TPCs), positioned above and below the active target, were constructed, installed, and brought into operation. Each High-Angle TPC features a large active volume enclosed by field cages with rectangular cross-sections and lightweight composite walls. Readout is performed by two planes, each equipped with eight Encapsulated Resistive Anode Micromegas (ERAM) detectors. A deep understanding of the detector, particularly the signal formation process, is crucial to fully exploit its performance. This work investigates the signal induction mechanism in Micromegas-based gaseous detectors operating with the T2K gas mixture. Specifically, it aims to develop a detailed understanding of a novel Thick-GEM-like detector incorporating two micro-mesh layers. This sophisticated configuration enables the separation of the electron and ion components of the avalanche. The study combines simulations carried out with Garfield++ and experimental data obtained from a dedicated test setup, and both approaches confirm that, with an optimized detector configuration, the signal induced by electrons can be effectively isolated while the ion-induced component is significantly suppressed. This result is a valuable first step to have a detailed knowledge of the single components of the signals. Further studies on the detector would lead to a deeper understanding of the ion component of the signal, which still has several unclear aspects such as its ion composition and the mobility of each ion type in the gas mixture, and applications to detectors having a resistive anode layer.