The CERN Neutrinos to Gran Sasso facility is complementary to these experiments but is optimised to the identification of tau neutrinos into which the muon neutrinos are supposed to change. If tau neutrinos are detected, this would not only show directly what the neutrinos are oscillating into, but would also be the first time that this kind of neutrino has been seen. CERN makes the neutrinos by smashing high energy protons against a target. The large amount of energy produced in the crash results in the formation of multitudes of particles, including particles called pions. The pions are focused by magnets and allowed to decay in a beam pipe which points towards Gran Sasso.
Pions decay into muons and neutrinos, which both come out of the end of the pipe.
The muons, whose in flight decay provides more neutrinos, are stopped within 800 metres by iron and earth shields, leaving only the neutrinos to carry on to reach the detector. The pointing accuracy which can be obtained with well established methods already in use at CERN, is such that the beam on arrival at Gran Sasso can be positioned to within 40 metres. This is necessary because neutrinos interact so weakly with matter that the vast majority of them will fly straight through the detectors in the same way as they travelled unimpeded from CERN to Gran Sasso. Only about 2500 of those neutrinos will interact with a 1000 ton target. If the results from SuperKamiokande are correct, tens of the neutrinos interacting with the Gran Sasso detectors are expected to have transformed into tau neutrinos, although the exact number depends on what the neutrino mass turns out to be.