From oscillation experiments we know that neutrinos have non-zero masses: The SuperKamiokande-Experiment in Japan (1998) and the Sudbury Neutrino Obervatory (2002) were the two experiments presenting the first compelling evidence that the flavor of neutrinos can change when travelling through space and thus that neutrinos have mass. In 2015 they were awarded the Nobel prize.
The neutrinos show up in three flavors, electron-, muon-, and tauon-neutrinos that are quantumechanical superpositions of the three neutrino mass eigenstates m1, m2, and m3. These mass eigenstates can be in principle determined by astrophysical observations or via experiments involving low energy weak decays, like Tritium decay or electron capture, or the neutrinoless double beta decay. Currently, only upper limits for the Neutrino masses are known: the Tritium decay experiment KATRIN established that the effective neutrino mass must be less than 1 eV, combined neutrinoless double beta decay experiments calculated the upper limit for the effective Majorana mass to be below 0,07 - 0,16 eV.
Whereas absolute neutrino masses are still unknown, two squared neutrino mass differences ∆m212 and |∆m312|, (∆mij2 = mi2 − mj2 ), are known with increasing precision from neutrino oscillation experiments. From this results it is clear that m1 and m2 are close to each other, but m3 either weighs much more or much less than the other two. The open question of the correct mass ordering of the three eigenstates is known as the neutrino mass ordering problem.
As the challenging efforts to measure the absolute neutrino masses are ongoing, a swift solution of the mass ordering is fundamental for neutrino physics. To pursue the task, an exact modelling of the fluxes of atmoshperic as well as reactor neutrinos is crucial as a first step, a task that is central to project N02. While the JUNO experiment in China is currently under construction, IceCube-Gen2 is approved and the IceCube insert PINGU is planned. Both experiments, JUNO and IceCube, employ two very different but complementary paths towards the aim of contributing to a conclusice evidence to the mass ordering problem.