# Berezin integral of a Grassmann field

Consider a time dependent Grassmann field i.e. $$\theta(t)$$. Now, consider the following Berezin integral $$\int [\mathcal{D}\theta] ~\prod_{t}\dot{\theta}\tag{1}$$ where $$\dot{\theta}$$ time derivative of $$\theta$$ and $$[\mathcal{D}\theta]$$ is a functional measure over the Grassmann field, something like a path integral measure and a product integral of $$\dot{\theta}$$. Now, what does the above integral evaluate to? One way I can think of evaluating the integral is that by recognizing that $$\dot{\theta} = \delta(\dot{\theta})$$ as $$\dot{\theta}^2 = 0$$. Therefore, we have $$\int [\mathcal{D}\theta] ~\prod_{t}~\dot{\theta} = \int [\mathcal{D}\theta]~\prod_{t} ~\delta(\dot{\theta}) = \det({\partial_t}).\tag{2}$$ Is my evaluation of the integral correct?

• If you have $\theta (t)$ then you have infinitlely many Grassmann variables (one for each $t$) and so the product of infinitelly many in $D[\theta]$. You will threfore need infinitely many of them in the integrand to get a non-zero answer. Sep 23, 2021 at 11:25
• @mikestone oops, good point. Fixed that. Sep 23, 2021 at 16:42

Let's discretize time $$t$$. In other words, assume that we have $$n$$ Grassmann-odd variables $$\theta^1, \ldots, \theta^n$$. In this language OP's differential operator $$\partial_t$$ is replaced with some Grassmann-even matrix $$A^i{}_j$$. Define $$\theta^{\prime i}= \sum_{j=1}^nA^i{}_j \theta^j,$$ so that $$\frac{\partial}{\partial\theta^j} ~=~\sum_{i=1}^n A^i{}_j\frac{\partial}{\partial\theta^{\prime i}}.$$ Then a straightforward calculation shows that OP's path-integral (1) is replaced with$$^1$$ \begin{align} \int\!d\theta^n \ldots d\theta^1~\theta^{\prime 1}\ldots \theta^{\prime n}~=~&\ldots~=~ \int\! d\theta^n \ldots d\theta^1~\delta(\theta^{\prime 1})\ldots \delta(\theta^{\prime n})\cr ~=~&\ldots~=~\det(A),\end{align} which indeed is a discrete version of OP's claim (2).
$$^1$$Recall that Berezin integration $$\int\!d\theta^n \ldots d\theta^1 ~=~\frac{\partial}{\partial\theta^n}\ldots \frac{\partial}{\partial\theta^1}$$ is the same as differentiation!