neutralino

Neutralino is the neutral fermionic superpartner of the neutral electroweak gauge bosons and neutral Higgs bosons in supersymmetric theories that conserve R-parity. In the Minimal Supersymmetric Standard Model (MSSM), four neutralinos arise as mass eigenstates, denoted χ̃1^0, χ̃2^0, χ̃3^0, χ̃4^0, ordered by increasing mass. Each neutralino is a quantum mixture of the bino B̃, the neutral wino W̃^3, and the two neutral higgsinos H̃_u^0 and H̃_d^0.

Their masses and compositions depend on MSSM parameters: the gaugino masses M1 and M2, the Higgsino mass parameter μ, and tanβ, the ratio of Higgs vacuum expectation values. The neutralino mass matrix is diagonalized to give the mass eigenstates, whose couplings to Standard Model particles vary with their bino, wino, or higgsino content.

If R-parity is conserved, the lightest neutralino χ̃1^0 is stable and is a leading candidate for dark matter in many SUSY scenarios. Depending on composition, neutralinos can be bino-like, wino-like, higgsino-like, or mixed, with different annihilation cross sections and relic densities.

Phenomenology involves neutralinos as weakly interacting massive particles. They can be produced in high-energy collisions, leaving events with missing energy. Direct detection experiments seek elastic scattering of neutralinos on nuclei, while indirect detection searches look for annihilation products such as gamma rays or antimatter.

Experimental status remains open: no confirmed discovery of neutralinos. LHC searches constrain supersymmetric parameter space, and direct and indirect detection experiments have set upper limits on scattering and annihilation rates. Viable regions of parameter space often involve particular compositions and mass ranges, including coannihilation and resonance effects in the early universe.