Mean Squared Bellman Error

Mean Squared Bellman Error: $L(\varphi ,\mathcal{D})=\underset{(s,a,r,s^{\prime },d)\sim \mathcal{D}}{E}\left[(Q_{\varphi }(s,a)-\left(r+\gamma (1-d){\max }_{a^{\prime }}{Q}_{\varphi }(s^{\prime },a^{\prime })\right){)}^2\right]$