Dirac algebra (contraction gamma matrices)

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Discussion Overview

The discussion revolves around the contraction of gamma matrices in Dirac algebra, specifically the expression \(\gamma^{\mu}_{ab}\gamma_{\mu \,\alpha\beta}\). Participants explore the nature of the indices involved and seek a general formula for this contraction.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses a desire for a general formula for the contraction of gamma matrices, noting the complexity introduced by the mixing of dotted and undotted indices.
  • Another participant questions the reason for the different types of indices present in the expression.
  • A third participant clarifies that the first gamma matrix is sandwiched between two spinors, which contributes to the index structure.
  • A later reply suggests that the Fierz identities may be relevant, indicating that they express the product of two Dirac matrices in terms of matrices in a different configuration.

Areas of Agreement / Disagreement

Participants do not reach a consensus on a general formula for the contraction of gamma matrices, and multiple perspectives on the nature of the indices and relevant identities are presented.

Contextual Notes

The discussion does not resolve the underlying assumptions regarding the properties of the gamma matrices or the specific conditions under which the Fierz identities apply.

IRobot
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I would like to have a general formula, and I am quite sure it must exist, for: \gamma^{\mu}_{ab}\gamma_{\mu \,\alpha\beta} but I didn't succeed at deriving it, or intuiting it, I am troubled by the fact that it must mix dotted and undotted indices.
 
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Why does it have different type of indices ?
 
Because the first gamma matrix is sandwiched between two spinors.
 
IRobot, I think what you are referring to are the Fierz identities, which express the product of two Dirac matrices in terms of matrices in the crossed channel. That is, (γμ)abμ)cd expressed in terms of (γμ)adμ)bc. See http://gemma.ujf.cas.cz/~brauner/files/Fierz_transform.pdf for an exhaustive treatment!
 

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