The recent discovery of the Higgs particle has left us in no doubt that the Standard Model (SM) of particle physics is correct in very large part.
However, another slightly less recent discovery, that of neutrino mass, is in tension with the SM's successes. In order for the Weak force to be properly described in the theory,
neutrinos were assigned exactly zero mass: however it has been proved that while their masses are very small, they are definitely not zero. Over the last few decades, particle physicists mounted a number of experiments looking for, and finally measuring, a quantity known as Charge Parity (CP) violation in sub-atomic particles called quarks. This manifests itself as a difference in interaction rates between matter and antimatter particles and the reason for looking for this difference was an attempt to explain what happened to the anti-matter which must have existed in the early universe. The simple explanation was that as the Universe expanded, transitions between matter and anti-matter were stopped at some energy density and over time, the small excess of matter that had been produced led to the total annihilation of all the antimatter, leaving just a small amount of matter which is what we see around us in the Universe.
However, the discovery of neutrino mass gives rise to the possibility of a mechanism which can explain the matter that we see.
The overarching goal of this proposal was to demonstrate that a giant water Cherenkov detector could be built at a fraction of the cost, in order to allow enough detector mass to measure the CP violating angle (at a later time).
The novel detector concept was named CHIPS, for Cherenkov detectors In PitS which was located in the Wentworth 2W flooded taconite pit near Aurora, Minnesota. This pit intersects the NuMI neutrino beam. The concept pushes on the costs of the detector by using the natural body of water to support the detector volume, avoiding a very strong and costly mechanical structure. It was designed to use the water overburden to shield from cosmic rays, making use of the time window that the beam is delivered and the knowledge of the neutrinos direction, to avoid having to be positioned under a very large overburden of rock.
The new technical concept was that of a submerged cylinder, providing a light tight, and water tight barrier within which are mounted the PMTs. The internal water would be continuously circulated and cleaned making the water very transparent to the Cherenkov light being produced. The PMTs are traditional, and very well understood, low technical risk, photon detectors. They have been deployed in water Cherenkov detectors before but the technical challenge here is to combine the signals from so many PMTs under the water, and use local computer power also under the water to identify the neutrino events coming from the NuMI beam which arrive at a known time and within a very short time window. The electronic were then transported to shore, up to 300m away, via one fibre cable. This again provides very large cost savings compared to traditional detectors.
