J.Cao: JUNO, a Multi-Purpose Neutrino Observatory
JUNO stands for Giant underground neutrino observatory. It is a 20kTon liquid scintillator detector with 3% energy resolution, placed 700 meters underground. It has rich physics possibilities: a precise study of reactor neutrinos, a watch for supernovae, a study of geoneutrinos, solar and atmospheric neutrinos, and exotic searches. Looking at inverse beta decay from reactor antineutrinos in the 4 MeV range, JUNO is located at a baseline where the survival probability is the lowest.
To determine the mass hierarchy one needs a proper baseline, in the 45 to 60 km range; all reactors should have equal baseline to maximize the sensitivity. Energy resolution is also crucial, and 100k events require a 20kTon detector looking at the 35 GW reactors. There are several reactors under construction. By 2020 one expects a power of 26.6GW.
The detector has 40 m diameter. An acrylic tank with diameter of 36 meters surrounded by a steel tank. Instrumented with 18000 20-inch PMT tubes. Juno should receive about 60 reactor neutrino events per day, 10 to 1000 solar neutrinos per day (depending on the threshold), 5k events in 10 seconds for a SN1987A-class supernova, and 1-2 geoneutrinos per day, plus several atmospheric ones. 250 muons from cosmic ray per day are also expected.
The sensitivity to the mass hierarchy reaches 4 standard deviations in 6 years. Comparing to the sensitivity of other experiments, it is competitive in schedule and complementary in the physics case.
Precision measurements can be done on solar neutrino parameters. The unitarity of the PMNS matrix can be probed to 1%, more precise than the CKM matrix elements.
Juno will be a superb supernova neutrino detector, allowing to disentangle almost all neutrino types. Also possible to study past core-collapse fluxes.
For geo-neutrinos, Juno will add a factor of 20 statistics with respect to KamLAND and Borexino, but there are huge reactor neutrino backgrounds, so the measurement will need accurate reactor neutrino spectra measurements. The signal to noise is of the order of 5%.
The detector is still in design and prototyping phase. For the liquid scintillator, the challenges are to optimize the fluors concentration to increase the light yield, and increase the transparency, while reducing the radioactive contaminants. A high quantum-efficiency Photomultiplier tube design is underway, with 20-inch tubes. These will be produced by Hamamatsu or Photonics. The veto detectors will reduce the cosmic muon flux, which has a rate of 0.0031 Hz/m^2 and an average energy above 200 GeV.
The civil construction will take the next two years, and in parallel the detector component production will take place. Data taking is expected in 2020. There is a strong component of the collaboration from Italy, Germany, and other european institutions (23 in total), plus 28 in Asia.