Neutralino

In supersymmetry, the neutralino is a hypothetical particle. There are four neutralinos that are fermions and are electrically neutral, the lightest of which is typically stable. They are typically labeled N͂0
1 (the lightest), N͂0
2, N͂0
3 and N͂0
4 (the heaviest) although sometimes is also used when is used to refer to charginos. These four states are mixtures of the bino and the neutral wino (which are the neutral electroweak gauginos), and the neutral higgsinos. As the neutralinos are Majorana fermions, each of them is identical to its antiparticle. Because these particles only interact with the weak vector bosons, they are not directly produced at hadron colliders in copious numbers. They would primarily appear as particles in cascade decays of heavier particles (decays that happen in multiple steps) usually originating from colored supersymmetric particles such as squarks or gluinos.

In R-parity conserving models, the lightest neutralino is stable and all supersymmetric cascade-decays end up decaying into this particle which leaves the detector unseen and its existence can only be inferred by looking for unbalanced momentum in a detector.