nLab Feynman slash notation

Idea

In the context of spin geometry, the Feynman slash notation is a notation popular in physics texts, specifically in quantum field theory, where it was introduced by Richard Feynman, for the Clifford algebra-element associated with a given vector.

Abstractly, given an inner product space $(V, \langle -\rangle)$ with associated Clifford algebra $Cl_{(V,\langle -,-\rangle)}$, then there is a canonical linear map

$$f\;\colon\;V \longrightarrow Cl_{(V,\langle -,-\rangle)}$$

such that $f(v)\cdot f(v) = \langle v,v\rangle \cdot 1 \in Cl_{(V,\langle -,-\rangle)}$.

Now let $\left( x^\mu\right)_{\mu = 1}^{dim(V)}$ be a linear basis for $V$, so that every vector $A \in V$ may be expanded, using the Einstein summation convention, as

$$A = A_\mu x^\mu \,,$$

and let $\left( \gamma^\mu \coloneqq f(x^\mu)\right)$ be the corresponding generators of the Clifford algebra, then this map is given by

$$A \mapsto A_\mu \gamma^\mu \,.$$

This particular class of instances of the Einstein summation convention on the right is further abbreviated with a slash as

$$\slash{A} \coloneqq A_\mu \gamma^\mu \,.$$

This is the Feynman slash notation.

Similarly in quantum field theory, the Dirac operator on Minkowski spacetime may be expanded as $i \gamma^\mu \frac{\partial}{\partial x^\mu}$ and this is then similarly abbreviated as

$$i \slash{\partial} \coloneqq i \gamma^\mu \frac{\partial}{\partial x^\mu}$$

Related concepts

• Einstein summation convention

References

• Wikipedia, Feynman slash notation