In a recent publication [Wurdack et al., Nat. Comm. 14:1026 (2023)], it was
shown that in microcavities containing atomically thin semiconductors
non-Hermitian quantum mechanics can lead to negative exciton polariton masses.
We show that mass-sign reversal can occur generally in radiative resonances in
two dimensions (without cavity) and derive conditions for it (critical
dephasing threshold etc.). In monolayer transition-metal dichalcogenides, this
phenomenon is not invalidated by the strong electron-hole exchange interaction,
which is known to make the exciton massless