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Suppose we have two 4-components Dirac spinors, that is two non commuting objects, $\psi$ and $\chi$. We know that:

$ \bar{\psi} \chi= - \chi \bar{\psi} $

$ \bar{\psi} = \psi^{\dagger} \gamma_0 $

$\dagger=T*$

The question is the following. What is $( \bar{\psi} \chi)^{T } $ ? For $T$ I mean taking the transpose, $\dagger$ is the adjoint, $*$ the complex conjugate.

Explicitly, is $ (\bar{\psi} \chi )^{T }=\chi^{T }\bar{\psi} ^{T } $ or $ (\bar{\psi} \chi )^{T }=- \chi^{T }\bar{\psi} ^{T } $ ?

And what can we say about $( \bar{\psi} \chi)^{\dagger} $? Is $ (\bar{\psi} \chi )^{\dagger}=\chi^{\dagger}\bar{\psi} ^{\dagger} $ or $ (\bar{\psi} \chi )^{\dagger}=- \chi^{\dagger}\bar{\psi} ^{\dagger} $ ?

Please, make the explanation clear, not only giving the correct answer.

Xenomorph
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Rexcirus
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2 Answers2

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There're a few things that should be clarified. First of all, are you talking about classical spinors?

If they are classical spinors, then they take values in Grassmann-odd numbers, and so its transpose should be

\begin{align} (\bar{\psi}\chi)^{T}&=(\psi^{\dagger}\gamma^{0}\chi)^{T} \\ &=\left[(\psi^{\ast})^{T}\gamma^{0}\chi\right]^{T} \\ &=-\bar{\psi}\chi, \end{align}

where in the last step, one has

\begin{align} \bar{\psi}\chi=(\psi^{\ast})^{T}\gamma^{0}\chi &=\sum_{i=0}^{3}\sum_{j=0}^{3}(\psi^{\ast})_{i}(\gamma^{0})_{ij}(\chi)_{j} \\ &=-\sum_{i=0}^{3}\sum_{j=0}^{3}(\chi)_{j}(\psi^{\ast})_{i}(\gamma^{0})_{ij} \\ &=-\sum_{i=0}^{3}\sum_{j=0}^{3}(\chi)_{i}(\psi^{\ast})_{j}(\gamma^{0})_{ji} \\ &=-\sum_{i=0}^{3}\sum_{j=0}^{3}(\chi)_{i}\left[(\gamma^{0})^{T}\right]_{ij}(\psi^{\ast})_{j} \\ &=-\chi^{T}(\gamma^{0})^{T}\psi^{\ast}=-\chi^{T}\bar{\psi}^{T}\equiv-(\bar{\psi}\chi)^{T}. \end{align}

Now, the problem has to do with the involution of the Grassmann algebra $\Lambda_{\infty}$. To be specific, suppose you have a bunch of anti-commuting elements $\theta_{i}$, i.e. $$\theta_{i}\theta_{j}+\theta_{j}\theta_{i}=0.$$ Then, each element (also known as the supernumber) in $\Lambda_{\infty}$ takes the form $$z=z_{B}+\sum_{k=1}^{\infty}\frac{1}{k!}C_{i_{1}\dots i_{k}}\,\theta_{i_{1}}\cdots\theta_{i_{k}},$$ where $z_{B}\in\mathbb{C}$ is its body part, and $C_{i_{1}\dots i_{k}}\in\mathbb{C}$ is the coefficient of its soul part. The involution (complex conjugation) $\ast$ in $\Lambda_{\infty}$ is defined as an action satisfying the following rules:
$1.\quad(\theta_{i})^{\ast}=\theta_{i}$.
$2.\quad(\alpha z)^{\ast}=\bar{\alpha}z^{\ast}$, where $\alpha\in\mathbb{C}$.
$3.\quad(z+w)^{\ast}=z^{\ast}+w^{\ast}$.
$4.\quad(zw)^{\ast}=w^{\ast}z^{\ast}$.
$5.\quad(z^{\ast})^{\ast}=z$.

Back to your question, the classical Dirac spinors should take values in the infinite dimensional $\Lambda_{\infty}$. So you would expect

\begin{align} (\bar{\psi}\chi)^{\dagger}&=\left[(\bar{\psi}\chi)^{T}\right]^{\ast}=\left[\chi^{T}\bar{\psi}^{T}\right]^{\ast} \\ &=\left(\sum_{i=0}^{3}\sum_{j=0}^{3}(\chi)_{i}\left[(\gamma^{0})^{T}\right]_{ij}(\psi^{\ast})_{j}\right)^{\ast} \\ &=\sum_{i=0}^{3}\sum_{j=0}^{3}\left[(\chi)_{i}\left[(\gamma^{0})^{T}\right]_{ij}(\psi^{\ast})_{j}\right]^{\ast} \\ &=\sum_{i=0}^{3}\sum_{j=0}^{3}(\psi)_{j}\left[(\gamma^{0})^{\dagger}\right]_{ij}(\chi^{\ast})_{i} \\ &=\sum_{i=0}^{3}\sum_{j=0}^{3}(\psi)_{j}(\gamma^{0})_{ij}(\chi^{\ast})_{i} \\ &=-\sum_{i=0}^{3}\sum_{j=0}^{3}(\chi^{\ast})_{i}(\gamma^{0})_{ij}(\psi)_{j}\equiv-\chi^{\dagger}\gamma^{0}\psi=-\bar{\chi}\psi. \end{align}

Notice that in the last step one also has $\bar{\psi}^{\dagger}=(\psi^{\dagger}\gamma^{0})^{\dagger}=\gamma^{0}\psi$, and so you should obtain

$$(\bar{\psi}\chi)^{\dagger}=-\chi^{\dagger}\bar{\psi}^{\dagger}.$$

Xenomorph
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EDIT: The correct answer is at: https://physics.stackexchange.com/a/458455/50207

Leaving the (wrong) answer below as it can be instructive.


I've clarified my doubt. The expression

$ (\bar{\psi} \chi )^{T }=\chi^{T }\bar{\psi} ^{T } $

is simply the definition of trasposition (the same argument for the adjoint), independently on the nature of the objects in the parenthesis. What is subtle is that, the following is true:

$ \bar{\psi} \chi =-\chi^{T }\bar{\psi} ^{T } =-(\bar{\psi} \chi )^{T }$

The minus in the first step arise only due to the anticommuting nature of the variables.

Rexcirus
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