Neutrinos
Neutrinos do not interact in particle detectors, and therefore escape undetected. Their presence can be inferred by the momentum imbalance of the visible particles in an event. In electron-positron colliders, both the neutrino momentum in all three dimensions and the neutrino energy can be reconstructed. Neutrino energy reconstruction requires accurate charged particle identification. In colliders using hadrons, only the momentum transverse to the beam direction can be determined.
Read more about this topic: Particle Identification