The LBNE collaboration is now quite a formidable enterprise with over 300 collaborators from more than 60 institutions including six national laboratories, and it is growing. The project is very ambitious with a program to reach unprecedented sensitivity and precision for addressing the neutrino mass hierarchy, CP violation in neutrino mixing, the value of the mixing parameters including precision measurement of θ13. While the primary goal of LBNE is to study neutrino oscillations it will also provide new capabilities to search for nucleon decay, observe neutrinos emitted by supernovae in our galaxy and beyond, and address other important topics in physics and astrophysics.
Under the leadership of its elected spokespersons, Bob Svoboda and Milind Diwan, the collaboration has gone through an exhaustive process to make a series of major decisions that affect the LBNE experimental configuration. The latest decision in this series, the selection of detector technology, took place over the last few weeks. Making these decisions has involved physics studies, analysis of the technical feasibility of various configurations and external reviews organized by the collaboration.
Last fall, after analyzing various options for the generation of the beam at Fermilab, the first major decision was reached. There was unanimity in selecting a beam configuration where the beam first climbs a hill before heading underground towards the Homestake Mine. This configuration makes construction easier, protects the aquifer and minimizes the depth of the near detector compared to the alternate configurations.
Another important decision has been the decision on the depth of the experiment. There is a very strong consensus that the physics reach and breadth of the program is greatly enhanced by building the facilities at a depth of 4850 feet at Homestake. This benefits the LBNE program but also will make it possible for other smaller but very important experiments to take place. The direct search for dark matter and the search for neutrino-less double beta decay could take place economically with the developments made for LBNE. Together with LBNE these experiments will be an important part of the future national physics program.
By far the toughest decision has been the selection of the technology for the detector. The decision is difficult for good reasons: both the liquid-argon TPC technology and the water Cerenkov technology are viable options with different strengths and weaknesses. Both involve a significant scale-up of existing technology, although the challenge is greater for the liquid-argon technology than for the water. On the other hand, liquid-argon technology offers great potential because of the extraordinarily detailed information on each neutrino event.
The decision-making process established by the collaboration for the technology choice called for the project manager to make the final recommendation before seeking the concurrence of the Fermilab director, the Laboratory Oversight Board and the DOE. In the presence of consensus the decision would have been relatively easy. As the established process developed, however, the decision turned out to be a very difficult one as explained in detail in the documentation for the technology choice.
The decision is to build LBNE with liquid-argon TPC technology. This constitutes the preferred alternative as we go into the CD-1 review. Back-up alternatives must be presented at the CD-1 review and we have a ready alternative with the water Cerenkov technology. Because a lot of work was done on the water Cerenkov technology we will make sure to capture and complete the important parts of that research effort. Most of the effort in the next few months, however, will go into studying all the aspects of the liquid-argon detectors and infrastructure. The LBNE experiment built on this technology will be a phenomenal achievement and one that will require the most demanding work from the laboratory and the collaboration, integrating everyone in this very ambitious enterprise.