  This letter investigates the reconfigurable intelligent surface (RIS)-aided
massive multiple-input multiple-output (MIMO) systems with a two-timescale
design. First, the zero-forcing (ZF) detector is applied at the base station
(BS) based on instantaneous aggregated CSI, which is the superposition of the
direct channel and the cascaded user-RIS-BS channel. Then, by leveraging the
channel statistical property, we derive the closed-form ergodic achievable rate
expression. Using a gradient ascent method, we design the RIS passive
beamforming only relying on the long-term statistical CSI. We prove that the
ergodic rate can reap the gains on the order of
$\mathcal{O}\left(\log_{2}\left(MN\right)\right)$, where $M$ and $N$ denote the
number of BS antennas and RIS elements, respectively. We also prove the
striking superiority of the considered RIS-aided system with ZF detectors over
the RIS-free systems and RIS-aided systems with maximum-ratio combining (MRC).
