In mid-October, a neutrino detector that was designed, built and tested at CERN was loaded onto four trucks to begin a month-long journey to Japan. Once safely installed at the J-PARC laboratory in Tokai, the “Baby MIND” detector will record muon neutrinos generated by beams from J-PARC and play an important role in understanding neutrino oscillations at the T2K experiment.

Weighing 75 tonnes, Baby MIND (Magnetised Iron Neutrino Detector) is bigger than its name suggests. It was initiated in 2015 as part of the CERN Neutrino Platform (CERN Courier July/August 2016 p21) and was originally conceived as a prototype for a 100 kt detector for a neutrino factory, specifically for muon-track reconstruction and charge-identification efficiency studies on a beamline at CERN (a task defined within the earlier AIDA project). Early in the design process, however, it was realised that Baby MIND was just the right size to be installed alongside the WAGASCI experiment located next to the near detectors for the T2K experiment, 280 m downstream from the proton target at J-PARC.

T2K studies the oscillation of muon (anti)neutrinos, especially their transformation into electron (anti)neutrinos, on their 295 km-long journey from J-PARC on the east coast of Japan to Kamioka on the other side of the island. The experiment discovered electron-neutrino appearance in a muon-neutrino beam in 2013 and earlier this year reported a two-sigma hint of CP violation by neutrinos, which will be explored further during the next eight years. Another major current target is to remove the ambiguity affecting the measurement of the neutrino mixing angle θ23.

Baby MIND will help in this regard by precisely tracking and identifying muons produced when muon neutrinos from the T2K beamline interact with the WAGASCI detector. This will allow the ratio of cross-sections in water and plastic scintillator (the active material in WAGASCI) to be determined, helping researchers understand  energy reconstruction biases that affect target nuclei-dependent neutrino fluxes and cross-sections. “Besides the water-to-scintillator ratio, the interest of the experiment is to measure a slightly higher-energy beam and compare the energy distribution (simply reconstructed from the muon angle and momentum, that Baby MIND measures) for the various off-axis positions relevant to the T2K and NOVA beams,” says Baby MIND spokesperson Alain Blondel.

Since its approval in December 2015, the Baby MIND collaboration – comprising CERN, the Institute for Nuclear Research of the Russian Academy of Sciences, and the universities of Geneva, Glasgow, Kyoto, Sofia, Tokyo, Uppsala, Valencia and Yokohama – has designed, prototyped, constructed and tested the Baby MIND apparatus, which includes custom designed magnet modules, electronics, scintillator sensors and support mechanics.

Significant departure

The magnet modules were the responsibility of CERN, and mark a significant departure from traditional magnetised-iron neutrino detectors, which have large coils threaded through the entire iron mass. Each of the 33 two-tonne Baby MIND iron plates is magnetised by its own aluminium coil, a feature imposed by access constraints in the shaft at J-PARC and resulting in a highly optimised magnetic field in the tracking volume. Between them, plastic scintillator slabs embedded with wavelength-shifting fibres transmit light produced by the interactions of ionising particles to silicon photomultipliers.

The fully assembled Baby MIND detector was qualified with cosmic rays prior to tests on a beamline at the experimental zone of CERN’s Proton Synchrotron in the East Area during the summer of this year, and analyses showed the detector to be working as expected. First physics data from Baby MIND are expected in 2018. “That new systems for the Baby MIND were designed, assembled and tested on a beamline in a relatively short period of time (around two years) is a great example of people coming together and optimising the detector by using the latest design tools and benefiting from the pool of experience and infrastructures available at CERN,” says Baby MIND technical co-ordinator Etam Noah.