Question: Why is the neutrino beam 295 km long and under the mountains?

Pierre Lasorak answered on 12 Nov 2014:

Hi
That’s a good question.
So the distance is carefully chosen because we want to see a typical effect that happen only at this distance. We want to count the number of neutrinos we see and compare it with the number of neutrinos we created near at the JPARC. If the number are not matching, then there are some weird things happening. And in fact it does! This is what we called neutrino oscillations. If we had put the detector 2 times further then we would recover the number of neutrinos we created…
Why under the mountains is another question actually. This is because we want to use the rock as a shelter from the shower of particles that comes continuously on us. Otherwise we would see to much things in the detector and we wouldn’t be able to make the distinction between neutrino/other particles.