The neutrino is the most ethereal of particles. Tens of billions of them emanating from nuclear reactions in the sun’s core pass through every square centimetre of Earth’s surface each second without notice. They have vanishingly small masses, a trillion times smaller than the top quark, and oscillate weirdly between their three flavours – electron, muon and tau – as they travel.
Since the first direct detection of a neutrino from a nuclear power plant in 1956, a vast and varied experimental programme employing reactor, solar, accelerator, atmospheric, cosmic and geological neutrino sources has grown up to explore its still-mysterious nature.
The latest issue of CERN Courier describes the state of the art in experimental neutrino physics, including recent results from the Tokai-to-Kamioka (T2K) facility in Japan that hint at differences in the way neutrinos and antineutrinos oscillate. It also celebrates the key role being played by Europe in contributing to a globally coordinated programme of neutrino research via the CERN Neutrino Platform.
Established in 2013, the CERN Neutrino Platform has enabled significant European participation in the US Long-Baseline Neutrino Facility, which will see neutrinos sent 1300 km from Fermilab in Chicago to the Deep Underground Neutrino Experiment (DUNE) in South Dakota, and in T2K, which sends neutrinos from Japan’s J-PARC accelerator facility to the Super-Kamiokande detector 295 km away. DUNE, T2K and its successor, the Hyper-Kamiokande project, will refine physicists’ understanding of neutrino oscillations, while a series of shorter baseline experiments are exploring the existence of a possible fourth, “sterile” neutrino.
For the US-based programme, the CERN Neutrino Platform has provided a large-scale demonstration of DUNE’s kilotonne-scale liquid-argon time-projection chambers (TPCs), with the construction and operation of two large-scale single- and dual-phase prototypes. The single-phase ProtoDUNE detector, which has recently completed two years of continuous recording of high-quality data, paves the way for the first DUNE module. At over 70 000 tonnes, the full DUNE detector will be the largest ever deployment of liquid-argon technology, which was first proposed by former CERN Director-General Carlo Rubbia in 1977 and serves as both target and tracker for neutrino interactions.
The first large-scale liquid-argon detector, ICARUS, has also been completely refurbished via the CERN Neutrino Platform. ICARUS was one of two detectors (along with OPERA) at Gran Sasso National Laboratory in Italy that studied neutrinos generated by CERN’s Super Proton Synchrotron (SPS) between 2006 and 2012. The refitted detector was shipped to the US in 2017 and is about to take data at Fermilab’s short-baseline neutrino facility.
For neutrino projects in Japan, the CERN Neutrino Platform has participated in the development of the BabyMIND magnetic spectrometer and upgrades to T2K’s “near-detector”, ND280. This detector, which was built inside the magnet from the UA1 experiment at CERN’s SPS, is crucial for understanding the neutrino flux prior to oscillations – one of the main measurement uncertainties at T2K and, in the future, at Hyper-Kamiokande. Independently, the SPS Heavy Ion and Neutrino Experiment (NA61/SHINE) at CERN has also contributed to a better understanding of T2K data, and has an important role to play in the future neutrino physics programmes in the US and Japan.
The 2020 update of the European strategy for particle physics, which was released on 19 June, recommends that Europe, and CERN through its neutrino platform, should continue to support neutrino projects in Japan and the US for the benefit of the worldwide neutrino community. “Experimental neutrino physics is back in town at CERN, and it looks like it is there to stay,” says Albert de Roeck, leader of the CERN EP-Neutrino group.