Technical Design Report for large-scale neutrino detectors prototyping and phased performance assessment in view of a long-baseline oscillation experiment

In June 2012, an Expression of Interest for a long-baseline experiment (LBNO, CERN-SPSC-EOI-007) has been submitted to the CERN SPSC and is presently under review. LBNO considers three types of neutrino detector technologies: a double-phase liquid argon (LAr) TPC and a magnetised iron detector as far detectors. For the near detector, a high-pressure gas TPC embedded in a calorimeter and a magnet is the baseline design. A mandatory milestone in view of any future long baseline experiment is a concrete prototyping effort towards the envisioned large-scale detectors, and an accompanying campaign of measurements aimed at assessing the systematic errors that will be affecting their intended physics programme. Following an encouraging feedback from 108th SPSC on the technology choices, we have defined as priority the construction and operation of a $6\times 6\times 6$m$^3$ (active volume) double-phase liquid argon (DLAr) demonstrator, and a parallel development of the technologies necessary for large magnetised MIND detectors. The $6\times 6\times 6$m$^3$ DLAr is an industrial prototype of the design proposed in the EoI and scalable to 20 kton, 50~kton or more. It is to be constructed and operated in a controlled laboratory and surface environment with test beam access, such as the CERN North Area (NA). Its successful operation and full characterisation will be a fundamental milestone, likely opening the path to an underground deployment of larger detectors. The response of the DLAr demonstrator will be measured and understood with an unprecedented precision in a charged particle test beam (0.5-20 GeV/c). The exposure will certify the assumptions and calibrate the response of the detector, and allow to develop and to benchmark sophisticated reconstruction algorithms, such as those of 3-dimensional tracking, particle ID and energy flow in liquid argon. All these steps are fundamental for validating the correctness of the physics performance described in the LBNO EoI. We anticipate that a successful operation of the double-phase \six DLAr demonstrator and its campaign exposure to a charged particle beam, will provide very important and vital feedback for long baseline programmes, and in general for the field. It will represent a never-achieved milestone for LAr detectors. Its design specifically addresses and represents a concrete step towards an extrapolation of the technology to very large masses in the tens of kton range, such as the one considered and studied for several years within the EU FP7 funded LAGUNA/LAGUNA-LBNO design studies. The parameters of the demonstrator will be directly scalable and the components mass-produceable. Long drift paths will be assessed on a large scale. As requested by SPSC, we submit a Technical Design Report, in view of a realisation of the facility and an exposure to the charged particle beam before the LHC LS2.