't Hooft symbol

The Template:'t Hooft symbol is a rank-3 tensor which mixes the properties of both the Kronecker delta and the Levi-Civita symbol. It was introduced by Gerard 't Hooft in 1976 in order to compute the properties of a dilute gas of instantons, specifically BPST instantons. It is a collection of numbers which allows one to express the generators of the SU(2) Lie algebra in terms of the generators of the Lorentz group Lie algebra.

Definition

${\eta }_{\mu \nu }^a$ is the 't Hooft symbol: $${\eta }_{\mu \nu }^a=\{\begin{array}{cc}{ϵ}^{a\mu \nu }& \mu ,\nu =1,2,3\\ -{\delta }^{a\nu }& \mu =4\\ {\delta }^{a\mu }& \nu =4\\ 0& \mu =\nu =4\end{array}$$ Where ${\delta }^{a\nu }$ and ${\delta }^{a\mu }$ are instances of the Kronecker delta, and ${ϵ}^{a\mu \nu }$ is the Levi–Civita symbol.

In other words, the symbols are defined by

($a=1,2,3;\mu ,\nu =1,2,3,4;{ϵ}_{1234}=+1$)

$$\begin{array}{cc}{\eta }_{a\mu \nu }& ={ϵ}_{a\mu \nu 4}+{\delta }_{a\mu }{\delta }_{\nu 4}-{\delta }_{a\nu }{\delta }_{\mu 4}\\ {\overline{\eta }}_{a\mu \nu }& ={ϵ}_{a\mu \nu 4}-{\delta }_{a\mu }{\delta }_{\nu 4}+{\delta }_{a\nu }{\delta }_{\mu 4}\end{array}$$ where the latter are the anti-self-dual 't Hooft symbols.

Matrix form

In matrix form, the 't Hooft symbols are $${\eta }_{1\mu \nu }=\left[\begin{array}{cccc}0& 0& 0& 1\\ 0& 0& 1& 0\\ 0& -1& 0& 0\\ -1& 0& 0& 0\end{array}\right],{\eta }_{2\mu \nu }=\left[\begin{array}{cccc}0& 0& -1& 0\\ 0& 0& 0& 1\\ 1& 0& 0& 0\\ 0& -1& 0& 0\end{array}\right],{\eta }_{3\mu \nu }=\left[\begin{array}{cccc}0& 1& 0& 0\\ -1& 0& 0& 0\\ 0& 0& 0& 1\\ 0& 0& -1& 0\end{array}\right],$$ and their anti-self-duals are the following: $${\overline{\eta }}_{1\mu \nu }=\left[\begin{array}{cccc}0& 0& 0& -1\\ 0& 0& 1& 0\\ 0& -1& 0& 0\\ 1& 0& 0& 0\end{array}\right],{\overline{\eta }}_{2\mu \nu }=\left[\begin{array}{cccc}0& 0& -1& 0\\ 0& 0& 0& -1\\ 1& 0& 0& 0\\ 0& 1& 0& 0\end{array}\right],{\overline{\eta }}_{3\mu \nu }=\left[\begin{array}{cccc}0& 1& 0& 0\\ -1& 0& 0& 0\\ 0& 0& 0& -1\\ 0& 0& 1& 0\end{array}\right].$$

Properties

They satisfy the self-duality and the anti-self-duality properties: $${\eta }_{a\mu \nu }=\frac{1}{2}{ϵ}_{\mu \nu \rho \sigma }{\eta }_{a\rho \sigma },{\overline{\eta }}_{a\mu \nu }=-\frac{1}{2}{ϵ}_{\mu \nu \rho \sigma }{\overline{\eta }}_{a\rho \sigma }$$

Some other properties are$${\eta }_{a\mu \nu }=-{\eta }_{a\nu \mu },$$ $${ϵ}_{abc}{\eta }_{b\mu \nu }{\eta }_{c\rho \sigma }={\delta }_{\mu \rho }{\eta }_{a\nu \sigma }+{\delta }_{\nu \sigma }{\eta }_{a\mu \rho }-{\delta }_{\mu \sigma }{\eta }_{a\nu \rho }-{\delta }_{\nu \rho }{\eta }_{a\mu \sigma }$$ $${\eta }_{a\mu \nu }{\eta }_{a\rho \sigma }={\delta }_{\mu \rho }{\delta }_{\nu \sigma }-{\delta }_{\mu \sigma }{\delta }_{\nu \rho }+{ϵ}_{\mu \nu \rho \sigma },$$ $${\eta }_{a\mu \rho }{\eta }_{b\mu \sigma }={\delta }_{ab}{\delta }_{\rho \sigma }+{ϵ}_{abc}{\eta }_{c\rho \sigma },$$ $${ϵ}_{\mu \nu \rho \theta }{\eta }_{a\sigma \theta }={\delta }_{\sigma \mu }{\eta }_{a\nu \rho }+{\delta }_{\sigma \rho }{\eta }_{a\mu \nu }-{\delta }_{\sigma \nu }{\eta }_{a\mu \rho },$$ $${\eta }_{a\mu \nu }{\eta }_{a\mu \nu }=12,{\eta }_{a\mu \nu }{\eta }_{b\mu \nu }=4{\delta }_{ab},{\eta }_{a\mu \rho }{\eta }_{a\mu \sigma }=3{\delta }_{\rho \sigma }.$$

The same holds for $\overline{\eta }$ except for

$${\overline{\eta }}_{a\mu \nu }{\overline{\eta }}_{a\rho \sigma }={\delta }_{\mu \rho }{\delta }_{\nu \sigma }-{\delta }_{\mu \sigma }{\delta }_{\nu \rho }-{ϵ}_{\mu \nu \rho \sigma }.$$

and $${ϵ}_{\mu \nu \rho \theta }{\overline{\eta }}_{a\sigma \theta }=-{\delta }_{\sigma \mu }{\overline{\eta }}_{a\nu \rho }-{\delta }_{\sigma \rho }{\overline{\eta }}_{a\mu \nu }+{\delta }_{\sigma \nu }{\overline{\eta }}_{a\mu \rho }.$$

Furthermore, ${\eta }_{a\mu \nu }{\overline{\eta }}_{b\mu \nu }=0$ due to different duality properties.

See also

• Instanton
• 't Hooft anomaly
• 't Hooft–Polyakov monopole
• 't Hooft loop