The Stern-Gerlach experiment for the measurement of electron spins is represented by the observable $\hat{S} = \hat{\sigma} \cdot \hat{n} = \hat{\sigma}_x \cdot n_x + \hat{\sigma}_y \cdot n_y + \hat{\sigma}_z \cdot n_z$, where $\hat{\sigma}$ are the Pauli spin matrices
$(\hat{\sigma}_x) = \begin{pmatrix} 0 & 1 \\ 1 & 0 \end{pmatrix}$, $(\hat{\sigma}_y) = \begin{pmatrix} 0 & -i \\ i & 0 \end{pmatrix}$, $(\hat{\sigma}_z) = \begin{pmatrix} 1 & 0 \\ 0 & -1 \end{pmatrix}$,
and $\hat{n} = (\sin\vartheta \cos\varphi, \sin\vartheta \sin\varphi, \cos\vartheta)$ is the orientation of the Stern-Gerlach apparatus in spherical coordinates. Calculate the eigenvalues $s_{\pm}$ and normalized eigenstates $|s_{\pm}\rangle$ of $\hat{S}$ prepared by the apparatus. Interpret the special case $\vartheta = 0, \varphi = 0$.
Useful identities:
$\sin\vartheta = 2\sin(\vartheta/2)\cos(\vartheta/2)$; $\cos\vartheta = 2\cos^2(\vartheta/2) - 1 = 1 - 2\sin^2(\vartheta/2)$