Introduction to DUNE
The Deep Underground Neutrino Experiment (DUNE) is a next-generation long-baseline neutrino experiment in the USA, scheduled to start data-taking in the mid-2020s. DUNE has similar physics goals to Hyper-Kamiokande but uses a completely different strategy to achieve them. As a result, the DUNE and Hyper-Kamiokande projects are highly complementary: the combination of the two will be much more effective than either one alone.
Like all long-baseline neutrino experiments, DUNE has three principal components: a beamline, a near detector, and a far detector. The far detector will be located at the Sanford Underground Research Facility (SURF)—better known to neutrino physicists as the Homestake Mine, former home of the original Davis solar neutrino experiment. (The dark matter experiment LZ, in which the Sheffield Dark Matter group is participating, is also housed at SURF.) The near detector and neutrino beamline are at Fermilab, giving a baseline of 1300 km from the target to the far detector.
Components of DUNE
Unlike T2K and Hyper-K, DUNE is an on-axis experiment. It therefore sees a broad range of neutrino energies, peaking between 1 and 5 GeV as shown in the figure, as opposed to Hyper-K's relatively narrow energy range peaking around 600 MeV. However, the key variable for neutrino oscillation experiments is the ratio of baseline to neutrino energy, L/E: Hyper-K has an L/E value of 500 km/GeV, and DUNE's range of ~260–1300 km/GeV covers this value, showing that both are optimised for the same squared mass difference Δm213. The choice of an on-axis geometry has advantages and disadvantages: the absolute sensitivity to oscillation is reduced, because a smaller fraction of the beam is at the L/E value corresponding to peak oscillation probability (this is why T2K and NOνA, designed when we had only an upper limit to the value of the mixing angle θ13, both chose an off-axis geometry), but the ability to observe the oscillation over a range of L/E values helps to distinguish between different phenomena, e.g. the effects of CP violation and those of matter-enhanced oscillation. It does mean that accurate reconstruction of the neutrino energy is crucial to realising the physics potential of the experiment.
The DUNE far detector, as currently planned, consists of four independent LAr TPCs, each containing about 17 ktons of liquid argon. There are two possible technologies: single-phase, as used in all the large-scale LAr TPCs that have been built to date, and dual-phase, as used (with liquid xenon rather than liquid argon) in the LZ experiment. In dual-phase designs, the electrons are drifted out of the liquid and into an overlying layer of gaseous argon, which allows higher electric fields to be used and therefore greatly increases the gain. Both technologies are set to be tested over the next couple of years in the ProtoDUNE project at CERN, and both may eventually be used in DUNE—however, as the single-phase design is the one that has been proven in large LAr detectors, it is agreed that the first of the four modules will use this technology.
The DUNE near detector will be located a few hundred metres downstream of the target. Its design is not yet finalised, but the current reference design, loosely based on the NOMAD experiment (1995–1998) which ran in the CERN neutrino beam, calls for a straw-tube tracker and electromagnetic calorimeter running in a 0.4 T dipole magnetic field for momentum measurement and charge determination—an important capability since, as can be seen from the figure, the principal contaminant in the Fermilab νμ beam is ν̅μ (and vice versa). Alternative designs include a high-pressure gaseous argon TPC and a liquid argon TPC: these would have the advantage of using the same target nucleus as the far detector, thereby reducing near-far systematics.
The current plan is that the four modules of the far detector will be installed over the period 2024–2027, with the beam operational in 2026.
DUNE at Sheffield
The proposed DUNE far detectors will be by far the largest LAr TPCs ever constructed: at 760 tons, the ICARUS LAr TPC is less than one-twentieth the size of a single DUNE module. Consequently, an extensive R&D programme is needed to ensure that the detector design is appropriate and that testing procedures and assembly protocols are well thought out. This has been the main focus of DUNE work in Sheffield to date. For Sheffield involvement in SBND, which—though it has a well-motivated physics programme of its own—is in many respects a testbed for DUNE, and the ProtoDUNE-SP experiment at CERN, see the links menu. Here we will focus primarily on the 35-ton prototype and on physics studies.