Existence and symmetry of normalized ground state to Choquard equation with local perturbation.

We study the Choquard equation with a local perturbation \begin{equation*} -\Delta u=\lambda u+(I_\alpha\ast|u|^p)|u|^{p-2}u+\mu|u|^{q-2}u,\ x\in \mathbb{R}^{N} \end{equation*} having prescribed mass \begin{equation*} \int_{\mathbb{R}^N}|u|^2dx=a^2. \end{equation*} For a $L^2$-critical or $L^2$-supercritical perturbation $\mu|u|^{q-2}u$, we prove existence and symmetry of normalized ground state, by using the mountain pass lemma, the Poho\v{z}aev constraint method, the Schwartz symmetrization rearrangement and some theories of polarization. In particular, our results cover the Hardy-Littlewood-Sobolev upper critical exponent case $p=(N+\alpha)/(N-2)$.