Beam test results of 3D fine-grained scintillator detector prototype for a T2K ND280 neutrino active target

An upgrade of the long baseline neutrino experiment T2K ND280 near detector is under development with the goal to reduce systematic uncertainties in the prediction of number of events at the Super-Kamiokande far detector. The upgrade program includes the design and construction of a new neutrino target, a novel highly granular fully active scintillator detector with 3D WLS fiber readout. The $\sim$2 tonnes 192 × 192 × 56 cm${}^3$ detector will be assembled from 2 × 10${}^6$ plastic scintillator cubes of 1 × 1 × 1 cm${}^3$ size. Each cube is read out by three orthogonal Kuraray Y11 WLS fibers threaded through the detector. A detector prototype made of 125 cubes was assembled and tested in a charged particle beam at CERN in October 2017. This paper presents results from studies of light yield, time resolution and optical cross-talk between the cubes.