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Composition | Elementary particle |
---|---|

Statistics | Fermionic |

Family | Fermion |

Interactions | Gravitation |

Status | Hypothetical |

Symbol | ^{}_{}G͂^{}_{} |

Antiparticle | Self |

Electric charge | 0 e |

Spin | 3/2 |

In supergravity theories combining general relativity and supersymmetry, the **gravitino** (^{}_{}G͂^{}_{}) is the gauge fermion supersymmetric partner of the hypothesized graviton. It has been suggested as a candidate for dark matter.

If it exists, it is a fermion of spin 3/2 and therefore obeys the Rarita–Schwinger equation. The gravitino field is conventionally written as *ψ _{μα}* with

Thus the gravitino is the fermion mediating supergravity interactions, just as the photon is mediating electromagnetism, and the graviton is presumably mediating gravitation. Whenever supersymmetry is broken in supergravity theories, it acquires a mass which is determined by the scale at which supersymmetry is broken. This varies greatly between different models of supersymmetry breaking, but if supersymmetry is to solve the hierarchy problem of the Standard Model, the gravitino cannot be more massive than about 1 TeV/c^{2}.

Murray Gell-Mann and Peter van Nieuwenhuizen intended the spin-3/2 particle associated with supergravity to be called the 'hemitrion', meaning 'half-3', however the editors of *Physical Review* were not keen on the name and instead suggested 'massless Rarita–Schwinger particle' for their 1977 publication.^{[1]}^{[2]} The current name of gravitino was instead suggested by Sidney Coleman and Heinz Pagels,^{[3]} although this term was originally coined in 1954 by Felix Pirani to describe a class of negative energy excitations with zero rest mass.^{[4]}

If the gravitino indeed has a mass of the order of TeV, then it creates a problem in the standard model of cosmology, at least naïvely.^{[5]}^{[6]}^{[7]}^{[8]}

One option is that the gravitino is stable. This would be the case if the gravitino is the lightest supersymmetric particle and R-parity is conserved (or nearly so). In this case the gravitino is a candidate for dark matter; as such gravitinos will have been created in the very early universe. However, one may calculate the density of gravitinos and it turns out to be much higher than the observed dark matter density.

The other option is that the gravitino is unstable. Thus the gravitinos mentioned above would decay and will not contribute to the observed dark matter density. However, since they decay only through gravitational interactions, their lifetime would be very long, of the order of *M _{pl}*

One possible solution to the cosmological gravitino problem is the split supersymmetry model, where the gravitino mass is much higher than the TeV scale, but other fermionic supersymmetric partners of standard model particles already appear at this scale.

Another solution is that R-parity is slightly violated and the gravitino is the lightest supersymmetric particle. This causes almost all supersymmetric particles in the early Universe to decay into Standard Model particles via R-parity violating interactions well before the synthesis of primordial nuclei; a small fraction however decay into gravitinos, whose half-life is orders of magnitude greater than the age of the Universe due to the suppression of the decay rate by the Planck scale and the small R-parity violating couplings.^{[9]}