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An admissible estimator is one for which there is no estimator $\delta$ satisfyingfor every $\theta$, with strict inequality for at least one $\theta$. For $p\ge 3$, the James–Stein estimatorhas riskIt strictly dominates $X$, and hence

If $a>1$, compare withIts bias is the negative of the original bias, so the squared-bias terms agree, whileIts risk is therefore strictly smaller for every $\theta$. Thus
${\hat{\theta }}_{a,b}$ is not an admissible estimator.