The T2K ND280 Upgrade

Abstract

In view of the J-PARC program of upgrades of the beam intensity, the T2K collaboration is preparing towards an increase of the exposure aimed at reaching sensitivity for leptonic CP violation at 3$\sigma$ level for a significant fraction of the possible $\delta_{CP}$ values. To reach this goal, an upgrade of the T2K near detector ND280 will be installed at J-PARC in 2022, with the aim of reducing the combined statistical and systematic uncertainties to better than 4\%. We have developed an innovative concept for this neutrino detection system, comprising the Super-Fine-Grained-Detector (SuperFGD), two High Angle TPC (HA-TPC) and six TOF planes. The SuperFGD, a highly segmented scintillator detector, acting as a fully active target for the neutrino interactions, is a novel device with dimensions of approximately $~1.9\times1.9\times0.6~\text{m}^3$ and a total mass of about 2 ton. It consists of about 2 millions small scintillator cubes each $1~\text{cm}^3$. Each cube is optically isolated. The signal readout from each cube is provided by wavelength shifting fibers inserted through the cubes and connected to micro-pixel avalanche photodiodes MPPCs. The total number of channels will be $\sim$60,000. We have demonstrated that, by providing three 2D projections, this detector delivers excellent PID, timing, and tracking performance, including a $4\pi$ angular acceptance, especially important for short proton and pion tracks. The HA-TPC will be used for 3D track reconstruction, momentum measurement and particle identification. These TPCs, with overall dimensions of $2\times2\times0.8~\text{m}^3$, will be equipped with 32 resistive Micromegas. The thin field cage (3 cm thickness, 4\% rad. length) will be realized with laminated panels of Aramid and honeycomb covered with a kapton foil with copper strips. The $34\times42~\text{cm}^2$ resistive bulk Micromegas will use a 500 kOhm/square DLC foil to spread the charge over the pad plane, each pad being appr. $1~\text{cm}^2$. The front-end cards, based on the AFTER chip, will be mounted on the back of the Micromegas and parallel to its plane. The time-of-flight (TOF) detector will allow to reject events generated in the passive areas of the detector and improve particle identification. The TOF will consist of 6 planes with about $5~\text{m}^2$ surface area surrounding the SuperFGD and the TPCs. Each plane will be assembled with 2.2 m long cast plastic scintillator bars with light collected by arrays of large-area MPPCs from two ends. The time resolution at the bar centre is 150 ps. A report on the design of these detectors, their performance, the results of the test beam and the plan for the construction is provided.