Horn

The 8 GeV protons sent to a beryllium target produces pions. These particles, which are charged particles that can be controlled, eventually decay into neutrinos, which are electrically neutral and can’t be controlled. The horn is a solenoidal magnetic field generated from a high-current carrying device that focuses the pions. All other secondary particles get bent away by the horn due to variations in their masses and energies. The horn was designed to select the positive pions with the prescribed energies experimenters wanted. This is how the neutrons are “directed” to the experiment’s detector.