Liquid argon TPCs are a relatively new technology: although the idea has been around for decades, the number of currently operating LAr TPCs is quite small, and the TPCs planned for the DUNE far detector will be more than ten times the volume of the largest currently operating (ICARUS). Therefore, it is worth exploring design ideas in detail before committing to a final DUNE design. The ProtoDUNE project is designed to test both the "standard" single-phase design and an alternative dual-phase setup in the CERN H4 testbeam, which provides a mixture of electrons, muons, pions, kaons and protons at a selected (within 5–7%) momentum in the range 0.5 to 7 GeV. Either positive or negative particles can be selected: for negatively charged particles, the beam is mostly electrons and pions, with 1–2% π–, ~1% antiprotons, and a K– fraction that rises from 0 at low momentum to 3.4% at 7 GeV, while for positive charges there is (unsurprisingly) a much higher proton fraction, 10–20% except at the very lowest momenta.
Clearly ProtoDUNE is not a neutrino experiment in itself. The main aims of the ProtoDune project, as stated in the ProtoDUNE-SP Technical Design Report, are to:
- prototype the production and installation procedures for the single-phase far detector design;
- validate the design from the perspective of basic detector performance—this can be achieved with cosmic-ray data;
- accumulate large samples of test-beam data to understand/calibrate the response of the detector to different particle species;
- demonstrate the long-term operational stability of the detector as part of the risk mitigation program ahead of the construction of the first 10-kt far detector module.
The Sheffield group is working on the single-phase detector, ProtoDUNE-SP (CERN experiment NP04). There is also a dual-phase prototype, ProtoDUNE-DP (CERN NP02) which will pursue similar goals for the dual-phase design, in which the ionisation electrons are drifted out of the liquid into an overlying layer of gaseous argon, in which a more intense electric field can be used to amplify the produced charge by avalanching. This technique is also used by the liquid xenon dark matter detector LZ, in which Sheffield is also involved; however, Sheffield is not participating in ProtoDUNE-DP. Note that as the DUNE far detector will consist of four independent modules, the choice of single vs dual phase is not either/or: it would be entirely possible to have, say, two of each. As the single-phase technology is more mature, the current plan is for the first module to be single-phase.
ProtoDUNE-SP at Sheffield
ProtoDUNE-SP is similar in design to SBND, with a central cathode plane assembly (CPA) and two sets of anode plane assemblies (APAs) at the ends of the cryostats. As this is a larger detector, roughly 11 m × 11 m × 11 m, there are three APAs per side instead of two. As with SBND, Sheffield has undertaken to deliver the steel APA frames, which are being manufactured by Portobello-RMF Ltd.
Obviously, the physics goals of both ProtoDUNE detectors involve the reconstruction and analysis of charged particle tracks and showers from the testbeam. The mixture of electrons and pions should give both showers (from the electrons) and tracks (from the pions), and as the beam momentum is pre-selected it will be possible to test methods for momentum/energy estimation, such as the multiple scattering method described in the SBND section. Sheffield has existing expertise from SBND and the DUNE 35-ton prototype, and will contribute to this team effort.
In addition, the ProtoDUNE detectors will not be deep underground and will collect a substantial sample of cosmic-ray data. Vitaly Kudryavtsev of Sheffield is a specialist in this field, and therefore the Sheffield group will focus in particular on understanding this cosmic sample through simulation, analysis and tuning reconstruction algorithms, with the goal of improving reconstruction and particle identification for cosmic-ray events in the DUNE far detector. DUNE itself will of course have proportionately fewer cosmic events, because of its deep underground site, but cosmic-ray muons are useful for calibration purposes, and some classes of muon-induced events may be significant backgrounds for some DUNE physics analyses, in particular the search for proton decay.