  This work investigates the optimal control problem for reflected
McKean-Vlasov SDEs and the viscosity solutions to Hamilton-Jacobi-Bellman(HJB)
equations on the Wasserstein space in terms of intrinsic derivative. It follows
from the flow property of reflected McKean-Vlasov SDEs that the dynamic
programming principle holds. Applying the decoupling method and the heat kernel
estimates for parabolic equations, we show that the value function is a
viscosity solution to an appropriate HJB equation on the Wasserstein space,
where the characterization of absolutely continuous curves on the Wasserstein
space by the continuity equations plays an important role. To establish the
uniqueness of viscosity solution, we generalize the construction of a distance
like function initiated in Burzoni et al.(SICON, 2020) to the Wasserstein space
over multidimensional space and show its effectiveness to cope with HJB
equations in terms of intrinsic derivative on the Wasserstein space.
