A new study has demonstrated that plasma effects suppress dark matter production via the collisional "freeze-in" mechanism, a finding with significant implications for the viability of sterile neutrinos as dark matter candidates. While plasma effects were previously known to inhibit oscillatory "freeze-in," their role in collisional "freeze-in" was less clear, leading to conflicting results in the literature, particularly concerning sterile neutrino production.

Researchers utilized first-principles nonequilibrium Kadanoff-Baym equations, formulated in a flavor-covariant manner, to establish a general result. They determined that plasma effects also reduce the efficiency of collisional "freeze-in." Specifically, the vacuum mixing angle, a key parameter in the collision rate, must be replaced by its plasma-corrected counterpart. This correction implies a suppression in the production of dark matter particles.

Applying this result to sterile neutrinos, a popular dark matter candidate, leads to a strong conclusion: sterile neutrinos cannot be dark matter in the keV-MeV mass range if produced from the Standard Model plasma. This finding closes a potential avenue for dark matter production in this mass range and reinforces the need to explore other models or mechanisms to explain the nature of dark matter.