In Zee's Quantum Field theory book he writes(adsbygoogle = window.adsbygoogle || []).push({});

[tex]\begin{align}\gamma^0 &= \begin{bmatrix}I & 0 \\ 0 & -I\end{bmatrix}=I \otimes \tau_3 \\ \gamma^i &= \begin{bmatrix}0 & \sigma^i \\ \sigma^i & 0\end{bmatrix}=\sigma^i \otimes \tau_2 \\ \gamma^5 &=\begin{bmatrix}0 & I \\ I & 0\end{bmatrix}=I \otimes \tau_1 \end{align} [/tex]

The Pauli matrices [itex]\sigma^i[/itex] I've seen. However, I have two questions

1) What are these [itex]\tau_i[/itex] matrices?

2) I'm not familiar with this [itex]\otimes[/itex] notation.

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# Otimes notation and tau matrices used in definition of gamma matrices?

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