| Detector setup of the NINJA physics run. The ECC was installed in the most upstream of the neutrino beam and detected charged particle tracks from the neutrino interaction vertices. A muon was detected in Baby MIND in the most downstream. Baby MIND consists of detector modules (orange) and magnet modules (gray). An emulsion shifter and a scintillation tracker were installed between them to connect muon tracks. The $z$ direction is defined along the beam and the location of the scintillation tracker is set as $z = \SI{0}{\cm}$. |
| Distribution of position of the tracks accumulated over a period of one month in the NINJA previous run. The tracks inside a red rectangle is used in the evaluation of the position resolution. |
| Distribution of angle of the tracks accumulated over a period of one month in the NINJA previous run. The track density is highest around the point $(\tan\theta_x, \tan\theta_y) = (0, -0.6)$. |
| Setup of the detectors in the J-PARC NM building. The NINJA detectors were surrounded by the WAGASCI detectors in the T2K experiment. The center positions of the detectors were not identical to maximize the muon angle acceptance of the WAGASCI detectors. |
| Conceptual design of the scintillation tracker for the NINJA physics run. Each scintillator bar (orange) is aligned with a gap whose width is 1/3 of the scintillator bar. The scintillator bars are attached to and supported by the structure (blue). The black line between the second and third layers is a shading black sheet inserted to avoid optical cross-talk between the scintillators. |
| The scintillator bar used in the tracker. A wavelength-shifting fiber (light green) is set on a groove. |
| The scintillation tracker for the NINJA physics run. Scintillation light from each plastic scintillator bar is read out by the wavelength-shifting fibers and the MPPCs. |
| Event rate of the tracker. The event rate is stable for the whole period. |
| Analysis flow of the track matching and the position and angle reconstructions of the muon tracks. |
| Schematic view of the tentative position reconstruction in the scintillation tracker. The red scintillators are the hit scintillators. For the left cluster, black solid lines determine the minimum and maximum position candidates and position is reconstructed as their average. Dotted lines are examples not satisfying the hit pattern. The perpendicular line cannot make the pattern in the right cluster, and the averaged position of all hit scintillators are used instead. Each star represents the reconstructed position of the cluster. |
| Distributions of the position differences between the tentative positions in the scintillation tracker and the extrapolated positions from Baby MIND. The left and right figures show the horizontal and vertical distributions, respectively. The selection criteria are shown with red lines. |
| Distributions of the position differences between the tentative positions in the scintillation tracker and the extrapolated positions from Baby MIND. The left and right figures show the horizontal and vertical distributions, respectively. The selection criteria are shown with red lines. |
| Distribution of two-dimensional angles reconstructed from the scintillation tracker and Baby MIND. The layers of the scintillation tracker are almost perpendicular to the neutrino beam and the distribution is consistent with it. |
| Schematic view of the position reconstruction in the tracker. The red scintillators are the hit scintillators in one cluster. Black solid lines determine the minimum and maximum position candidates and dotted lines are examples not satisfying the hit pattern. |
| Position distribution reconstructed from the scintillation tracker and Baby MIND. The distribution is not uniform since the scintillation tracker and Baby MIND are not centered and the angle acceptances for the muon tracks are different for different positions. A red rectangle shows an area used in the evaluation of the efficiency described in Section~\ref{ssec:performance:matcheff}. |
| Product of the detection efficiency of the scintillation tracker and the matching efficiency between the scintillation tracker and Baby MIND. Expected distribution of Baby MIND reconstructed angle of muon tracks from neutrino-water interactions in the ECCs is shown by a gray histogram. |
| Position difference distributions of the scintillation tracker and the emulsion shifter. The left shows the vertical and the right shows the horizontal distributions. |
| Position difference distributions of the scintillation tracker and the emulsion shifter. The left shows the vertical and the right shows the horizontal distributions. |
| Angle dependence of the position resolution of the scintillation tracker. |
| Angle difference distribution of the scintillation tracker and the emulsion shifter. The left and right figures show the horizontal and vertical distributions. |
| Angle difference distribution of the scintillation tracker and the emulsion shifter. The left and right figures show the horizontal and vertical distributions. |
| Angle dependence of the angle resolution of the scintillation tracker. |