Can a CES parameter be negative?
To elaborate on my comment above, whether there's a "restriction" on the possible values of $a$ depends on your assumptions about the production function.

If you assume that all inputs into the production function contribute positively to the final output, then it must be that $a \in (0, 1)$.

Since you estimated the parameter, you might want to check the confidence interval of the estimated parameter. You can do a one-sided test if $a &lt; 1$, for example.