
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
Given that $F_{1}$ and $F_{2}$ are the left and right foci of the ellipse $C$: $\dfrac{x^{2}}{a^{2}} + \dfrac{y^{2}}{b^{2}} = 1$ ($a > b > 0$), and point $P(1, \dfrac{\sqrt{2}}{2})$ is on the ellipse, with $\sqrt{2} |PF_{1}|$, $|F_{1}F_{2}|$, $\sqrt{2} |PF_{2}|$ forming an arithmetic sequence.

$(1)$ Find the standard equation of the ellipse $C$;  
$(2)$ Given a moving line $n$ passing through point $F_{2}$ and intersecting the ellipse $C$ at points $A$ and $B$, is there a fixed point $Q$ on the $x$-axis such that $\overrightarrow{QA} \cdot \overrightarrow{QB} = -\dfrac{7}{16}$ always holds? If it exists, find the coordinates of point $Q$; if not, explain why.

Present the answer in LaTex format: \boxed{Your answer}
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