The T2K experiment

Using a beam of muon neutrinos and antineutrinos produced at the J-PARC accelerator, located at Tokai on the east coast of Japan, the T2K experiment is studying how they are transformed into neutrinos and antineutrinos of other flavours respectively. The measurement is carried out by comparing the (anti-)neutrinos observed at detectors close to the production of the beam with those detected at Super-Kamiokande, a gigantic detector made up of 50,000 tonnes of ultra-purified water, now doped with Gadolinium, and located around 300 km away. The observation of a difference between the oscillation probability of neutrinos and antineutrinos would be an unequivocal signal of a violation of CP symmetry.

IRFU is heavily involved in a variety of experiments on the many facets of neutrino physics. In this context, the institute has contributed to the pioneering T2K experiment since its inception, as well as to the preparation of its future successors, Hyper-Kamiokande and DUNE. These experiments should definitively conclude on the existence of CP symmetry violation in the neutrino sector. Several important results have been obtained since the T2K experiment was launched in 2010, including the first indications of a possible CP symmetry violation in 2020 in neutrino oscillation, and a better understanding of the interaction of neutrinos with atomic nuclei. 

T2K is now entering its second phase, which is based on improvements to the beam and the ND280 near detector (see Figure 1). The goals are to improve both the statistical and systematic uncertainties in the measurements. In particular, the near detector provides crucial information on the beam before oscillation, and on the interactions of neutrinos with matter. This information, combined with data from the Super-Kamiokande far detector, enables precise analyses of neutrino oscillation.

Renewal of the ND280 beam and detector

The neutrino beam is produced in several stages: protons from the J-PARC accelerator are directed at a target, producing hadrons, in particular pions, which then create neutrinos or antineutrinos as they decay. The J-PARC infrastructure has undergone a major upgrade, increasing the power of the proton beam from 450 kW to 710 kW, as well as improving the instruments governing the production of the associated neutrino beam. This upgrade will increase the number of neutrinos produced by around 50%.

To take full advantage of this increase in statistics, an improvement in the control of the relevant systematic uncertainties is required. This objective will be achieved by modifying the upstream part of the ND280 near detector. These modifications include firstly, the addition of a new highly granular scintillator, the "SuperFGD" (designed by a collaboration between Japanese, American, and European laboratories) inside which neutrino interactions take place and which detects the resulting particles around the interaction point. Another major improvement to the ND280 detector involves the integration of two new horizontal time-projection chambers to measure particles produced at high angles, the new 'High-Angle TPCs' (Time Projection Chambers), produced as part of a partnership between IFRU and several European laboratories. Finally, six new scintillator planes for measuring the time-of-flight of outgoing particles will be incorporated into this new version of the near detector (designed and manufactured by European laboratories). They will improve the rejection of background particles originating from outside the detector. This rejuvenation of the near detector will lead to better reconstruction of the particles produced during neutrino interactions, by increasing the angular acceptance of the detector, the detection threshold for the particle pulse, and the accuracy of detection.

IFRU's crucial role in the development of wide-angle TPCs

The IRFU teams made a major contribution to the ND280's first TPCs, which have been in operation since the early days of T2K. These old TPCs play a major role in the experiment, but are not capable of reconstructing particles at large angles relative to the axis of the neutrino beam. The IRFU teams therefore invested heavily in the new High-Angle TPCs, read by detectors based on Micromegas technology with an encapsulated resistive anode (ERAM) and instrumented with a compact, lightweight field cage (see Figure 2). In ERAMs, a resistive layer is deposited on a segmented anode to spread the charge over several adjacent pads. This method improves spatial resolution (< 800 μm) for a given segmentation (1 cm2), making it possible to achieve accuracy of between 4% and 10% on the pulse of charged particles with a minimum of readout channels. In addition, this approach enhances the stability of Micromegas and protects the electronics against electrical discharge events. The IRFU teams, split between the DPhP and DEDIP groups, are actively involved in the HA-TPC project, particularly in the design, production and testing of the Micromegas detectors. Their arrangement on the new HA-TPC and the associated readout electronics were also designed and optimised at IRFU, while the production and testing of a larger number of modules took place at CERN.

Compared with conventional Micromegas technology, this new resistive Micromegas technology makes it possible to detect signals not only on the pad (small tile) directly receiving avalanche electrons, but also on neighbouring pads. The combined analysis of these signals enables the trajectory of the particle to be reconstructed much more accurately. However, this new reconstruction procedure is much more complex to implement than for conventional Micromegas technology: the IRFU teams have also set up precise and comprehensive analyses to characterise the response of the detectors with a view to reconstructing the charged particles in the HA-TPCs.

Prospects

The deployment of the first HA-TPC is a success. The first neutrino interactions have been observed in the new ND280 near detector since the start-up of the enhanced neutrino beam (see Figure 3).  The installation of the last HA-TPC is scheduled for May 2024, and the next period of beam data collection is planned for June 2024.

The expected future harvest of new data from T2K will make it possible to refine the constraints on CP violation, with the aim of reaching a statistical significance level of 3σ (and later 5σ in the future HyperK-Kamiokande experiment, which will use the same beam and the same near detector as T2K). In addition, it will enable the three-neutrino oscillation paradigm to be tested more accurately, with the aim of achieving accuracy of better than 2% for the parameters governing the disappearance of muon neutrinos. At the same time, it will contribute to a deeper understanding of the interactions between neutrinos and matter, which are crucial if next-generation oscillation experiments are to achieve a 5σ measurement of CP violation and determine the mass order of neutrinos.

Contacts 

Samira Hassani

Sara Bolognesi