2. \( (\mathbf{2 4 \%}) \) Let \( X_{1}, \cdots, X_{n} \) be iid with pdf
\[
f(x ; \theta)=\frac{1}{\sqrt{2 \pi}} e^{-\frac{1}{2}(x-\theta)^{2}}, \quad x \in(-\infty, \infty) .
\]
(a) Show that \( Y=\sum_{i=1}^{n} X_{i} \) is sufficient and complete for \( \theta \).
(b) Find the UMVUE of \( \theta^{2} \).