Status and Potential of JUNO
Vito Antonelli (left) discussed the JUNO experiment focusing on a restricted set of topics.
Since the value of theta_13 is relatively large, the corrections it produces are sensitive on the mass hierarchy “sign”. To see this one must look at the survival probability of electron antineutrinos, which is dependent on a contribution which is mass-hierarchy dependent. One may explicitate this by rewriting the decrease in survival probability by a phase dependent term that is opposite for the two hierarchies.
The value of baseline of JUNO (53km) was chosen to maximize its sensitivity to those effects. JUNO is a huge liquid scintillator tank, 20ktons of material, with 700m of rock overburden. Juno will study the inverse beta decay of antineutrino on protons, with the positron signal accompanied by a delayed neutrino capture signal.
The main requirements for JUNO were high statistics – not being very close to the source one needs a lot of mass. The key point for sensitivity to the wiggles in the rate is to have a very good energy resolution. A muon veto system was necessary to remove cosmogenic background.
As Juno looks at vacuum oscillations, it does not suffer from the uncertainty in the Earth density profile or the ambiguity in the CP violating phase. It also does not depend on the value of theta_13.
JUNO is a multi-purpose experiment, and it will be able to determine three mixing parameters with accuracy much improved with respect to currently known values; it will study supernova bursts and diffuse supernova neutrinos, as well as geoneutrinos and solar neutrinos. For mixing angles, the potential of JUNO is to go to subpercent accuracy. For supernova neutrinos, given a distance of 10 kparsecs, one expects thousands of events. For geoneutrinos, the main issue is to understand the radiogenic contributions from uranium and thorium to the total power of the Earth. This allows to distinguish different geological models. Here, for JUNO the big advantages will be its size and radiopurity. The signal will produce events between 1 and 2.5 MeV and they expect hundred of such events, well distinguished from backgrounds.
For solar model, JUNO will be able to look into the ratio of Berillium and Boron-8 fluxes, to study the metallicity problem. Antonelli showed spectra of the energy measurable by JUNO for an ideal radiopurity, allowing 7Be fluxes to be constrained. For the 8B flux, the main problem is to constrain the long-lived spallation radioisotopes.
The speaker then discussed the status of construction. The excavation work should be finished in a few months. A pilot plant has been installed to study the purification and the optical purity that can be obtained. Another issue concerns the photomultipliers, which were the object of the last presentation of Thursday. In addition to the large PMT’s, smaller ones will be interspersed and they will add redundancy and energy resolution.