WAGASCI-BabyMIND Neutrino Detector

• WAGASCI-BabyMIND is an integrated hybrid neutrino detector that combines water and hydrocarbon targets with specialized vertex, muon, and angular measurement modules.
• Its off-axis placement near a 0.7 GeV peaked neutrino beam enhances sensitivity to differences in muon neutrino cross sections, crucial for oscillation studies.
• The advanced muon spectrometry, featuring magnetized iron and scintillator tracking, achieves high momentum and charge identification efficiencies to reduce systematic uncertainties.

The WAGASCI-BabyMIND detector is an integrated, hybrid neutrino detection apparatus located in the T2K near detector hall at J-PARC, designed to perform precision measurements of muon neutrino ($\nu_\mu$) charged-current (CC) interactions on both water and hydrocarbon targets, particularly in final states without charged pions. Installed in 2018 and positioned 1.5° off-axis to the T2K beam, WAGASCI-BabyMIND comprises multiple specialized modules for interaction vertex detection, muon spectrometry, and enhanced angular coverage, facilitating stringent constraints on neutrino-nucleus cross sections, thus reducing systematic uncertainties relevant for oscillation analyses and CP violation searches.

1. Detector Architecture and Modular Components

The WAGASCI-BabyMIND system employs several distinct subdetectors, enabling simultaneous measurement of $ν_\mu$ CC events on both water (H$_2$O) and hydrocarbon (CH) targets:

• Vertex Detectors:
  • WAGASCI Modules: These modules integrate a three-dimensional grid of plastic scintillator submerged in water, with a mass ratio of roughly 4:1 (H$_2$O to CH). The fine-grained 3D grid, with $\sim$2.5 cm cubic cells, yields highly efficient spatial localization and track reconstruction with $\epsilon(\theta) \geq 70\%$ for $p_\mu > 50$ MeV/c (Quilain, 2016).
  • Proton Module (PM): Provides a pure CH target, crucial for separation of water and hydrocarbon induced signals.
• Downstream Muon Detectors:
  • BabyMIND: A magnetized iron spectrometer comprising 33 ARMCO steel magnet modules (each individually magnetized to $B = 1.5$ T), interleaved with 18 plastic scintillator modules for high-resolution tracking. The individual magnet modules utilize a sewing-pattern coil configuration for uniform $\vec{B}$ fields (Antonova et al., 2017, Antonova et al., 2017, Antonova et al., 2017).
  • Wall Muon Range Detectors (Wall-MRDs): Extend muon angular coverage, especially for tracks exiting the main spectrometer at high angles, and utilize wavy-pattern wavelength-shifting fibers for improved light yield and timing precision (Abe et al., 9 Sep 2025).
• Electronics & Readout:
  • Hamamatsu Multi Pixel Photon Counters (MPPCs) are used for scintillator readout, with dual (high/low gain) signal chains and custom front-end electronics built around the CITIROC ASIC and 400 MHz internal clock (Antonova et al., 2017, Parsa, 2020).

The combined apparatus enables the identification of muon tracks’ momentum and charge, reconstructs event topology, and supports high statistical event selection and calibration across a broad solid angle.

2. Off-Axis Placement and Neutrino Flux Characterization

WAGASCI-BabyMIND is situated 1.5° off-axis from the neutrino beam center, resulting in a neutrino energy spectrum peaked near 0.7 GeV, as compared to 0.6 GeV at ND280 (2.5° off-axis) (Abe et al., 9 Sep 2025). This off-axis configuration yields:

• A harder neutrino spectrum, with increased contributions from interaction modes such as single-/multi-pion production and multinucleon knock-out processes, facilitating model constraints beyond CCQE dominance.
• Enhanced sensitivity for comparing water and hydrocarbon cross sections in a broader kinematic regime—directly informing the modeling relevant for far detector (Super-Kamiokande/Hyper-Kamiokande) analyses.

This differential flux is visualized in Figure flux of (Abe et al., 9 Sep 2025), displaying the relative peak shift and shaping the target phase space for cross section measurements.

3. Muon Spectrometry: Magnetized Iron and Scintillator Tracking

BabyMIND functions as a precision muon spectrometer:

• Magnetized Iron Modules:

Each magnet module generates a horizontal $B$-field ($1.5$ T), achieved with $140$ A current and $350$ W dissipation per module (Antonova et al., 2017). The bending of muon trajectories is used for momentum and charge determination:

$R = \frac{p}{qB}\,, \quad p = qBR$

for track curvature $R$ and charge $q$.

• Scintillator Tracking:

The 18 active planes (horizontal and vertical orientations) measure consecutive spatial hits. The dual-ended fiber and SiPM readout achieve light yields exceeding $37.5$ photoelectrons (vertical) and $65$ (horizontal) per MIP, averaged across modules, with $<10\%$ asymmetry (Antonova et al., 2017).

• Performance:
  • Reconstruction efficiencies reach $>95\%$ (momentum) and $>90\%$ (charge identification) in the core design kinematic range ($0.2$–$6$ GeV/c for pencil-beam muons) (Antonova et al., 2017).
  • In beam and cosmic ray tests, timing resolutions and detection efficiencies met design goals (Antonova et al., 2017).
  • The design with individually-magnetized segments and variable pitch between iron plates improves resolution at low momenta by minimizing multiple scattering effects (Antonova et al., 2017).

4. Physics Analysis: Cross Section Measurement Procedure

The cross section measurement in (Abe et al., 9 Sep 2025) proceeds as follows:

• Dataset and Selection:
  • Fiducial volume cuts inside WAGASCI or PM
  • Track number and PID cuts, employing the “MUon Confidence Level” (MUCL):

  $$\mathrm{MUCL} = P \sum_{i=0}^{n-1} \frac{(-\ln{P})^i}{i!} \,,\quad P = \prod_{i=1}^n \mathrm{CL}_i$$ - $\mathrm{CL}_i$ is calculated per hit for PID discrimination. - Charge determination by log-likelihood tests of track curvature in BabyMIND.

• Signal Extraction:

Binned likelihood fits in muon momentum ($p_\mu$) and angle ($\cos \theta_\mu$) yield the interaction signal, correcting for efficiency, background, and flux.

• Cross Section Calculation:

Integrated and differential cross sections are computed as:

$$\frac{d\sigma}{dx_i} = \frac{\hat{N}^{\mathrm{exp,\,sig}_i}}{\epsilon_i \,\Phi \,N^{\mathrm{FV}_\mathrm{nucleons}} \Delta x_i}$$

where $\hat{N}^{\mathrm{exp,\,sig}_i}$ is the best-fit signal yield, $\epsilon_i$ the detection efficiency, $\Phi$ the integrated flux, $N^{\mathrm{FV}_\mathrm{nucleons}}$ the fiducial volume nucleon count, and $\Delta x_i$ the bin width.

• Main Results:
  • Hydrocarbon (CH): $1.26 \pm 0.18 \times 10^{-39}$ cm$^2$/nucleon
  • Water (H$_2$O): $1.44 \pm 0.21 \times 10^{-39}$ cm$^2$/nucleon
  • Both values incorporate statistical and systematic uncertainties and are presented for events with no charged pions in final state (Abe et al., 9 Sep 2025).

5. Model Comparisons and Differential Cross Sections

• Monte Carlo Generators:

The measured cross sections are evaluated against NEUT v5.3.2 and GENIE v2.8.0 predictions (including alternative NEUT configurations with modified $M_A^{QE}$).

• Compatibility:

Data are compatible with the generator models, supporting their underlying nuclear and cross section implementations. The agreement is quantified using a $\chi^2$ approach:

$$\chi^2 = \sum_{ij} \left[\left( \frac{d\sigma}{dx} \right)^{\mathrm{data}}_i - \left( \frac{d\sigma}{dx} \right)^{\mathrm{model}}_i\right] \mathbf{V}^{-1}_{ij} \left[\left( \frac{d\sigma}{dx} \right)^{\mathrm{data}}_j - \left( \frac{d\sigma}{dx} \right)^{\mathrm{model}}_j\right]$$

where $\mathbf{V}_{ij}$ is the covariance matrix. Reported $\chi^2/$NDF values ($\sim$6–12 for 12 bins) support compatibility within uncertainties (Abe et al., 9 Sep 2025).

• Differential Measurement:

Differential cross sections as functions of $p_\mu$ and $\cos\theta_\mu$ reveal bin-by-bin consistency with model predictions, with minor deviations within error bands. Tables and figures in (Abe et al., 9 Sep 2025) document these results in detail.

6. Operational Performance and Physics Run Outcomes

• Data Quality:

The 2019–2020 physics run yielded a $97\%$ data collection efficiency (Parsa, 2020). The detector exhibited robust calibration and timing performance, with precise event grouping and synchronization.

• Beam Monitoring:

BabyMIND’s charge identification discriminates “wrong-sign” muon events, enabling continuous monitoring of neutrino/antineutrino beam composition, which is vital for systematic error control (Parsa, 2020, Antonova et al., 2017).

• Interaction Topologies:

The measured sample includes various interaction modes: CCQE (37.6%), CC-$1\pi$ (21%), CC-$n\pi$ (15.5%), and NC (26%), as per Monte Carlo estimates on iron (Parsa, 2020).

• Event Selection & Reconstruction:

Examples of CC QE interactions ($\nu_\mu + n \rightarrow \mu^{-} + p$) are resolved, including clear track bending signatures under the BabyMIND $B$-field, and multi-track/shower interactions where applicable.

7. Impact on Neutrino Oscillation Analyses and Future Directions

The hybrid WAGASCI-BabyMIND apparatus directly reduces one of the dominant sources of systematic uncertainty in oscillation studies—mismatched target materials and incomplete angular phase space. These improved measurements:

• Enhance constraints on $ν_\mu$ cross sections for water targets, essential for accurate extrapolation to far detectors employing water Cherenkov technology.
• Improve the reliability of neutrino oscillation and CP violation searches in T2K and Hyper-Kamiokande by minimizing cross section model dependence and wrong-sign contamination.
• Establish a foundation for further precision measurements with expanded datasets and improved detector calibration, setting the stage for future upgrades and multi-material analyses.

This suggests ongoing data taking and method refinement will further bolster model discrimination and systematic control, providing essential inputs for the next generation of long-baseline neutrino experiments.