Normalized solutions for a class of fractional Choquard equations with mixed nonlinearities

In this paper we study the following fractional Choquard equation with mixed nonlinearities: $$\left\{ \begin{array}{l} (-\Delta)^s u = \lambda u + \alpha \left( I_\mu * |u|^q \right) |u|^{q-2} u + \left( I_\mu * |u|^p \right) |u|^{p-2} u, \quad x \in \mathbb{R}^N, \\[4pt] \displaystyle \int_{\mathbb{R}^N} |u|^2 \,\mathrm{d}x = c^2 > 0. \end{array} \right.$$ Here $N > 2s$, $s \in (0,1)$, $\mu \in (0, N)$, and the exponents satisfy $$\frac{2N - \mu}{N} < q < p < \frac{2N - \mu}{N - 2s},$$ while $\alpha > 0$ is a sufficiently small parameter, $\lambda \in \mathbb{R}$ is the Lagrange multiplier associated with the mass constraint, and $I_\mu$ denotes the Riesz potential. We establish existence and multiplicity results for normalized solutions and, in addition, prove the existence of ground state normalized solutions for $\alpha$ in a suitable range.