Overview

The gravitino is a hypothesized elementary particle arising when supersymmetry is combined with general relativity (supergravity). It is the proposed superpartner of the graviton and has fermionic character with spin 3/2. No experimental detection of the gravitino exists; its properties depend strongly on the mechanism that breaks supersymmetry and on the details of the underlying theory.

Key characteristics

As a spin-3/2 field, the gravitino carries both gravitational and fermionic attributes. In many minimal models it is a Majorana particle, though more exotic constructions can yield different quantum numbers. The field decomposes into helicity components: helicity ±3/2 modes behave like a gravitational superpartner, while the helicity ±1/2 components are related to the goldstino — the would-be Nambu–Goldstone fermion associated with supersymmetry breaking — which becomes part of the gravitino by the super-Higgs mechanism.

Role in particle physics

In supersymmetric models with conserved R-parity, the gravitino can be the lightest supersymmetric particle (LSP) and thus a dark matter candidate. If heavier, it can act as a metastable state that decays into lighter superpartners plus standard model particles. Collider signatures linked to a gravitino often involve large missing energy and, depending on the next-to-lightest supersymmetric particle (NLSP), displaced decays or long-lived charged tracks.

Cosmological implications

Gravitinos have major consequences for cosmology. Thermal and nonthermal production in the early universe can determine their abundance. Late gravitino decays may alter Big Bang nucleosynthesis yields or inject energy into the cosmic plasma, so observations of light element abundances and the cosmic microwave background place strong model-dependent constraints. In some scenarios a very light gravitino could behave as warm dark matter, while a heavier, stable gravitino could serve as cold dark matter.

Phenomenology and constraints

  • Production mechanisms include scatterings in the thermal bath and decays of heavier particles after inflation.
  • Cosmological data constrain reheating temperatures and supersymmetry-breaking scales to avoid overproduction or harmful late decays.
  • Direct detection is effectively impossible because interactions are suppressed by the Planck scale; indirect signatures come from collider events and astrophysical observations.

History and significance

The gravitino emerged from the development of supergravity in the 1970s as the fermionic counterpart to the graviton. Its theoretical role links particle physics, gravity, and early-universe cosmology; studying its possible properties helps shape viable models of supersymmetry breaking, dark matter, and inflationary reheating. Confirmation would have profound implications, while current experimental and observational limits guide model building.