 Protons collide with a graphite target, producing mesons, mainly pions and kaons, which are then focused by a set of three magnetic horns and directed into a tunnel called the decay volume.Depending on the horns polarity, either positive or negative particles are focused.Positive pions and kaons decay mainly into and , forming a muon neutrino beam, while negative pions and kaons decay mainly into and , forming a muon antineutrino beam.