Does matter matter for neutrino flavor?
The presence of matter in the neutrino path may also have an impact on flavor change. If it does, the flavor
count after travel would be altered. Some of these interactions are expected from the tiny number of oscillation
generated electron neutrinos, but extra interactions of muon or tau neutrinos with the Earth are non-standard and
are thus called non-standard interactions, or NSI for short. (The Earth is made up of regular matter—electrons,
protons and neutrons—and not of matter in muon or tau flavors.)