goldstino

The goldstino is the fermionic Goldstone particle associated with spontaneous breaking of global supersymmetry. When supersymmetry is spontaneously broken, a massless fermion arises as the Goldstone mode of the broken symmetry. In four-component language this is often described as a massless Majorana (or equivalently a Weyl) fermion whose couplings are governed by the scale of SUSY breaking.

In global supersymmetry, the goldstino is a physical, massless fermion. Its interactions with ordinary matter originate from the supercurrent and are suppressed by the SUSY-breaking scale, typically denoted by F. The strength of these couplings grows with lower F and diminishes as F increases, reflecting the energy scale at which SUSY is broken.

If supersymmetry is local rather than global, the goldstino is absorbed by the gravitino through the super-Higgs mechanism. This process imparts mass to the gravitino, with the gravitino mass m3/2 approximately given by m3/2 ≈ F/(√3 M_Pl), where M_Pl is the Planck mass. In this scenario there is no independent light goldstino particle in the low-energy spectrum; instead, the goldstino’s degrees of freedom become the longitudinal polarization of the massive gravitino. Nevertheless, the low-energy interactions of the gravitino at energies above its mass can be described by the same goldstino-like couplings.

Phenomenologically, goldstino dynamics are central in models with gauge-mediated SUSY breaking, where a light gravitino (and thus a light effective goldstino) can be the lightest superpartner. Many collider and astrophysical signatures involve sparticles decaying to gravitino/goldstino plus Standard Model particles, producing characteristic missing-energy signals. The goldstino has not been observed to date.