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Homework Help: Gamma matrices

  1. Sep 24, 2012 #1
    1. The problem statement, all variables and given/known data
    Solve the equation. What is it's trace?

    2. Relevant equations
    [STRIKE]k[/STRIKE] γμ γ5 [STRIKE]o[/STRIKE] γ[itex]\nu[/itex] γ5

    3. The attempt at a solution
    I dont think this is reduced enough.

    trace: just got rid of gamma5 with anticommutation.
    Last edited: Sep 24, 2012
  2. jcsd
  3. Sep 24, 2012 #2


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    You haven't posted an equation, you've posted an expression. You can't solve an expression.

    I see the same index, [itex]\nu[/itex], 3 times in this term, so it is not valid according to the rules of the Einstein summation convention.
  4. Sep 24, 2012 #3
    My bad, you re right it is an expression. I should have said evaluate the expression or simplify or something similar.

    As for index [itex]\nu[/itex]. Doesn't [STRIKE]o[/STRIKE] =γ[itex]\nu[/itex]o?
    Would really appreciate some help here.
  5. Sep 24, 2012 #4


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    So the problem is to simplify the expression [strike]k[/strike] γμ γ5 [strike]o[/strike] γν γ5, and then find the trace? If so, are you given a particular space-time and metric (or Lagrangian)? Are [itex]k_{\nu}[/itex] and [itex]o_{\nu}[/itex] arbitrary vectors, or do they have some meaning here?

    Do you mean [itex]{\not}{o}=\gamma^{\nu}o_{\nu}[/itex], where [itex]o_{\nu}[/itex] is some covariant vector in your spacetime? If so, then realize that there is an implied summation over the index [itex]\nu[/itex] in the term [itex]\gamma^{\nu}o_{\nu}[/itex], according to the Einstein summation convention. This makes the [itex]\nu[/itex] in this term a so-called "dummy" index which can be replaced with any other Greek index.

    When you have something like [itex]{\not}{k}{\not}{o}[/itex] and you want to write it in terms of the Dirac matrices and the covariant vectors, according to the Einstein summation convention, you should use a different index for each implied sum, so that it is clear which terms belong to which summation. For example, [itex]{\not}{k}{\not}{o}=\gamma^{\nu}k_{\nu}\gamma^{\nu}o_{\nu}[/itex] is meaningless and incorrect, but [itex]{\not}{k}{\not}{o}=\gamma^{\mu}k_{\mu}\gamma^{\nu}o_{\nu}[/itex] is correct and consistent with the Einstein summation convention. Likewise, you wouldn't write [itex]\gamma_{\nu}{\not}{k}=\gamma_{\nu} \gamma^{\nu} k_{\nu}[/itex], but rather you would use a dummy index that is not already used in the term like [itex]\mu[/itex], and write [itex]\gamma_{\nu}{\not}{k}=\gamma_{\nu} \gamma^{\mu} k_{\mu}[/itex]

    Now, that said, if this sort of index notation is not immediately clear to you, you should almost certainly brush up on your mathematics before trying to study spinors and Dirac matrices.
    Last edited: Sep 24, 2012
  6. Sep 25, 2012 #5
    kγ and oν are just arbitrary vectors.

    I think I kinda get this notation now.

    As for mathematics behind I couldn't agree more. The problem is we (the students) first saw this kind of notation when actually studying scattering of Dirac particles and didn't really get much (or any) of the math behind it.

    But thank you for clearing things out a bit.
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