photino

The photino is the hypothetical fermionic superpartner of the photon in supersymmetric theories of particle physics. In models with unbroken supersymmetry, gauge bosons have fermionic partners, and the photino would be a spin-1/2 neutral particle. In realistic models where supersymmetry is broken, the photino is not observed as a separate mass eigenstate.

After electroweak symmetry breaking, the photino mixes with the superpartners of the Z boson (zino) and the neutral Higgs bosons (higgsinos) to form the neutralinos, a set of neutral, mass-eigenstate fermions. Consequently, the photino per se is not typically found as an independent particle in low-energy spectra; rather, its components contribute to the neutralino states. The lightest neutralino is often considered a leading candidate for the lightest supersymmetric particle (LSP) in many models, and a photino-like neutralino can play a role in that context.

Interactions of the photino are governed by electroweak couplings through its mixing with bino, wino, and higgsino components. Its detectability depends on the degree of mixing and the mass spectrum of the superpartners. If a photino-like neutralino is the LSP, it can be a dark matter candidate with weak-scale interactions, potentially detectable in collider experiments or direct detection searches, though no experimental evidence has yet confirmed its existence.

In current literature, the term “photino” is often superseded by references to neutralinos, which are the physical, mass-eigenstate fermions arising from photino, zino, and higgsino mixing. The photino remains a useful historical or descriptive concept within the broader framework of supersymmetry.