neutralinos

Neutralinos are hypothetical fermions predicted by supersymmetric extensions of the Standard Model. They arise as neutral mass eigenstates that are mixtures of the neutral gauginos (bino and wino) and the neutral higgsinos, produced when the fermionic superpartners of gauge and Higgs bosons mix in the neutral sector.

In the Minimal Supersymmetric Standard Model (MSSM) and many of its extensions, there are four neutralinos, denoted χ̃^0_i with i = 1 to 4. The lightest neutralino, χ̃^0_1, is often the lightest supersymmetric particle (LSP) in models with R-parity conservation, making it a leading candidate for dark matter. The precise composition—whether bino-, wino-, or higgsino-like—governs its couplings, production rates, and cosmological behavior.

Neutralinos interact primarily through weak interactions and through their mixing with higgsinos and gauginos. They can couple to Z and Higgs bosons depending on their eigenstate composition, and heavier supersymmetric particles may decay in cascades down to the LSP, producing missing energy signals in detectors.

The neutralino mass matrix is 4×4, depending on parameters such as M1 (bino mass), M2 (wino mass), μ (Higgsino mass parameter), and tanβ. These parameters determine the mass spectrum and mixing, which in turn influence collider signatures and dark matter properties.

Phenomenology includes collider searches for events with missing transverse energy, direct detection experiments probing elastic scattering off nuclei, and indirect searches for annihilation products. As of now, there is no confirmed experimental discovery of neutralinos. In the NMSSM, the addition of a singlino can further enrich the neutralino sector and its phenomenology. If the lightest neutralino is stable, it can contribute to the observed dark matter relic density under appropriate conditions.