Neutrino physics requires baselines both big and small, and neutrinos both artificial and astrophysical. One of the most prominent experiments of the past two decades is Tokai-to-Kamioka (T2K), which observes electron–neutrino appearance in an accelerator-produced muon–neutrino “superbeam” travelling coast to coast across Japan. To squeeze systematics in their hunt for leptonic CP violation, the collaboration recently brought online an upgraded near detector.
“The upgraded detectors are precision detectors for a precision-physics era,” says international co-spokesperson Kendall Mahn (Michigan State). “Our current systematic constraint is at the level of a few percent. To make progress we need to be able to probe regions we’ve not probed before.”
T2K studies the oscillations of 600 MeV neutrinos that have travelled 295 km from the J-PARC accelerator complex in Tokai to Super-Kamiokande – a 50 kton gadolinium-doped water-Cherenkov detector in Kamioka that has also been used to perform seminal measurements of atmospheric neutrino oscillations and constrain proton decay. Since the start of data taking in 2010, the collaboration made the first observation of the appearance of a neutrino flavour due to quantum-mechanical oscillations and the most precise measurement of the θ23 parameter in the neutrino mixing matrix. As well as placing limits on sterile-neutrino oscillation parameters, the collaboration has constrained a wide range of the parameters that describe neutrino interactions with matter. The uncertainties of such measurements typically limit the precision of fits to the fundamental parameters of the three-neutrino paradigm, and constraining neutrino-interaction systematics is the main purpose of near detectors in superbeam experiments such as T2K and NOvA, and the future ones Hyper-Kamiokande and DUNE.
T2K’s near-detector upgrade improves the acceptance and precision of particle reconstruction for neutrino interactions. A new fine-grained “SuperFGD” detector (see pink rectangle, left, on “New and improved” image) serves as the target for neutrino interactions in the new experimental phase. Comprised of two million 1 cm3 cubes of scintillator strung with optical fibres, SuperFGD lowers the detection threshold for protons ejected from nuclei to 300 MeV/c, improving the reconstruction of neutrino energy. Two new time-projection chambers flank it above and below to more closely mimic the isotropic reconstruction of Super-Kamiokande. Finally, six new scintillator planes suppress particle backgrounds from outside the detector by measuring time of flight.
Following construction and testing at CERN’s neutrino platform, the new detectors were successfully integrated in the experiment’s global DAQ and slow-control system. The first neutrino-beam data with the fully upgraded detector was collected in June, with the collaboration also benefitting from an upgraded neutrino beam with 50% greater intensity. Beam intensity is set to increase further in the coming years, in preparation for commissioning the new 260 kton Hyper-Kamiokande water Cherenkov detector. Cavern excavation is underway in Kamioka, with first data-taking planned for 2027.
But much can already be accomplished in the new phase of the T2K experiment, says the team. As well as improving precision on θ23 and another key mixing parameter Δm223, and refining the theoretical models used in neutrino generators, T2K will improve its fit to δCP, the fundamental parameter describing CP violation in the leptonic sector. Measuring its value could shed light on the question of why the universe is dominated by matter.
“T2K’s current best fit to δCP is –1.97,” says Mahn. “We expect to be able to observe leptonic CP violation at 3σ significance if the true value of δCP is –π/2.”
