Extracting ever more precise information from long-baseline neutrino oscillation experiments calls for a better understanding of neutrino production and neutrino interactions with their target detectors, through experimental programmes dedicated to neutrino cross-section measurements and validation of neutrino interaction models. The challenge is in the extrapolation from interaction rates at the near detectors to predictions for observed events at the far detector as a function of neutrino oscillation parameters. To address this challenge, the general approach is to reduce the associated systematic errors from flux, detector and neutrino interaction model uncertainties.

The T2K WAGASCI experiment aims at measuring neutrino cross-sections in water (the target material of the Super-Kamiokande far detector) and hydrocarbon (CH) around 1 GeV. It is part of an extensive program of cross-section measurements at the near detector hall by the T2K collaboration that includes the existing ND280 near detector, and an upgrade to the ND280 due online in 2022. 

Figure 1. The WAGASCI experiment detector layout: WG=WAGASCI, PM=Proton Module, NJ=Ninja, SM=Side muon range detector, BM=Baby MIND.

The Baby MIND detector was designed, assembled and tested at CERN as part of the Neutrino Platform NP05 project (link here). It will track muons produced when neutrinos interact with detector materials through the dominant Charged Current Quasi Elastic (CCQE) neutrino-nucleon interactions, measuring their momentum and identifying their charge with high efficiency in the momentum range 0.4 to 5 GeV/c. The charge ID capabilities are especially important to reject background events, which arise from the beam itself. The muon neutrinos of interest are produced from the decay of positive pions. Each such decay also produces a positive muon. It is the decay of these secondary muons that generates the unwanted backgrounds, electron neutrinos and muon anti-neutrinos. By changing the polarity of the magnetic horns that focus the pions before decay, it is possible to obtain a muon anti-neutrino beam from negative pions. The fraction of unwanted background events from secondary muon neutrinos can be as large as 30% in this beam mode.

Figure 2. Baby MIND at CERN (top) during beam tests at the PS experimental hall in 2017 and at J-PARC (bottom) at the end of the 2-week installation phase in 2018.

The main CERN hardware contribution is the design and construction of the warm magnet system, a novel system proposed by the EP-ADO group. The 33 ARMCO steel magnet modules each with their own air-cooled aluminium coils, mark a departure from the more traditional layout with a coil around a large monolithic steel block. The design offers the advantage of very uniform magnetic field lines across the detector volume that are fully contained within the magnet steel resulting in moderate 10 kW power consumption for 1.5 T. Compactness and modularity are further advantages as the 2 tonne magnet modules can be handled independently for transport and installation. 

Baby MIND was installed at J-PARC over a 2-week period in 2018. The magnet and scintillator module assemblies were lowered into the experiment pit from the surface of the Neutrino Monitor (NM) building through a restricted shaft access opening one at a time. They were then transported at the pit floor level from the shaft area to their intended positions in support structures hosting 3 or 4 modules. Handling operations were delicate, having to comply at all times with very stringent earthquake safety directives at J-PARC, which is hosted within a Japanese Atomic Energy Agency (JAEA) campus. Commissioning of Baby MIND at J-PARC was then carried out to check the detector was fully functional after transport and re-assembly, and confirmed good synchronisation of the readout electronics with the T2K beam.

Figure 3. A magnet module being lowered into the pit at J-PARC.

Figure 4. View from the surface floor of the Neutrino Monitoring (NM) building at J-PARC showing the lowering of a Baby MIND magnet and scintillator module assembly down the shaft to the pit (B2) floor, past the ND280 detector which is enclosed in the CERN-donated UA1 magnet.​

The WAGASCI experiment will operate two main neutrino target types, the fully active plastic scintillator Proton Module, and the WAGASCI detector modules. The latter consist of a 3D grid-like lattice of rectangular cuboids with active plastic scintillator walls and passive water-filled inner volumes. A very high water to scintillator ratio (4:1) is obtained in this way. This high ratio, combined with the 4 Pi tracking capabilities and low detection threshold for protons and pions open up a range of possibilities for cross-section measurements in water.

Figure 5. Event display obtained during the Baby MIND commissioning run in 2018: neutrino interaction upstream of Baby MIND with the resulting muon incident on Baby MIND from the left.

WAGASCI is located at an off-axis angle of 1.5 degrees, different to the 2.5 degrees off-axis angle of the T2K ND280 and Super-K detectors which are 280 m and 295 km from the neutrino source respectively. Placing the detector "off-axis" with respect to the primary proton beam axis narrows the neutrino beam spectrum and lowers its peak. By subtracting fluxes at ND280 and WAGASCI, even narrower "pseudo-monochromatic" beams can be obtained, from 0.2 to 0.9 GeV and from 0.6 to 2 GeV. This method will be studied through the measurement of flux-integrated cross-sections at these two fluxes.

Figure 6. Energy spectra obtained by using different off-axis angle fluxes. The top two plots show the energy distribution of the fluxes (left) and neutrino interactions (right) for ND280 (2.5 degrees off-axis) and WAGASCI (1.5 degrees off-axis). The bottom plots show the spectra obtained by subtraction of ND280 and WAGASCI fluxes.

The WAGASCI concept has already produced two of the most precise measurements of neutrino interaction cross-sections in water at 0.9 GeV and 1.5 GeV mean neutrino energies. With the complete WAGASCI setup that now includes Baby MIND with its ability to tell the charge of the outgoing muon, and the side MRDs that extend the range of angles over which muons can be observed, we can look forward to a wide program of cross-section measurements on H₂0, CH and Fe including absolute cross-sections, cross-section ratios, inclusive and exclusive double-differential cross-sections.