  While conventional semiconductor technology relies on the manipulation of
electrical charge for the implementation of computational logic, additional
degrees of freedom such as spin and valley offer alternative avenues for the
encoding of information. In transition metal dichalcogenide (TMD) monolayers,
where spin-valley locking is present, strong retention of valley chirality has
been reported for MoS$_2$, WSe$_2$ and WS$_2$ while MoSe$_2$ shows anomalously
low valley polarisation retention. In this work, chiral selectivity of MoSe$_2$
cavity polaritons under helical excitation is reported with a polarisation
degree that can be controlled by the exciton-cavity detuning. In contrast to
the very low circular polarisation degrees seen in MoSe$_2$ exciton and trion
resonances, we observe a significant enhancement of up to 7 times when in the
polaritonic regime. Here, polaritons introduce a fast decay mechanism which
inhibits full valley pseudospin relaxation and thus allows for increased
retention of injected polarisation in the emitted light. A dynamical model
applicable to cavity-polaritons in any TMD semiconductor, reproduces the
detuning dependence through the incorporation of the cavity-modified exciton
relaxation, allowing an estimate of the spin relaxation time in MoSe$_2$ which
is an order of magnitude faster than those reported in other TMDs. The valley
addressable exciton-polaritons reported here offer robust valley polarised
states demonstrating the prospect of valleytronic devices based upon TMDs
embedded in photonic structures, with significant potential for
valley-dependent nonlinear polariton-polariton interactions.
