Scientists make high-intensity neutrino beams by starting with bunches of protons from a bottle of hydrogen gas. They accelerate each bunch to nearly the speed of light and smash it onto a target, usually made of graphite or beryllium. The protons shatter the target’s atomic nuclei and produce new particles, including short-lived pions and kaons. Pions and kaons very often decay producing neutrinos.
Because neutrinos are electrically neutral, it is impossible to steer them towards the detector. So to make a collimated neutrino beam, the pions and kaons are focussed before they decay. This is accomplished with focusing horns, which produce strong, carefully aligned magnetic fields that redirect pions and kaons so that they all travel in the same direction. This ensures that the decay neutrinos also travel in the desired direction.
Along with the neutrinos, muon particles are produced in the pion/kaon decays. These muons are then stopped in thick blocks of aluminium, steel and concrete. The blocks, or "dumps", stop and absorb all particles except the ghost-like neutrinos, which pass through unaffected.