# Auxiliary Grassmann variables in supergeometry

I was reading on super geometry and how it is used to model fermions and supersymmetry in classical field theory.

In texts like [1] or [2] the authors introduced auxiliary Grassmann odd variables to get anticommuting fields.

Take for example a super field $$\phi : \mathbb{R}^{m|1} \rightarrow \mathbb{R}^{n|0}$$ mapping a super spacetime to some configuration space. It can be written as $$\phi = x + \theta \, \psi$$ where $$x$$ should be a bosonic field, $$\psi$$ a fermionic one and $$\theta$$ is the odd cordinate on $$\mathbb{R}^{m|1}$$.

In [1] it is argued that in this formulation $$\psi$$ is not an anticommuting (odd) quantity. To fix this we actually need to take $$\phi$$ as a map $$\phi : \mathbb{R}^{m|1} \times \mathbb{R}^{0|L}\rightarrow \mathbb{R}^{n|0} \;.$$ We denote with $$\eta$$ the odd coordinates of $$\mathbb{R}^{0|L}$$ and they serve as auxiliary Grassmann variables. $$x$$ is now even in those and $$\psi$$ odd. Therefore $$\psi$$ is now indeed an anticommuting field.

I have two questions regarding those auxiliary variables $$\eta$$:

1. Do they have an “intuitive” or geometric interpretation or are they just formalism that in the end give us anticommuting fields?

2. How do we get rid of them when calculating observables? I know that we Berezin integrate over the super spacetime coordinates $$\theta$$ when calculating the action for example. Do we do the same for the $$\eta$$?

[1] Hélein, Frédéric. “An Introduction to Supermanifolds and Supersymmetry”, p. 15. (He introduces the term “super manifold with flesh” for adding auxiliary variables)

[2] end of section 3 of nlab on Supergeometry. (Here the Dirac field serves as an example)

1. The odd "flesh/fuzz" $$\eta$$ can be viewed as a realization of the functor of points. In a nutshell it is introduced to be able to consider Grassmann-odd indeterminates, viz. souls.
• ${}$ See the link. – Qmechanic Dec 15 '18 at 12:32