Asymmetric reheating in the mirror model

In this paper, we propose a mechanism within the mirror model to address cosmological constraints on the number of relativistic degrees of freedom at low temperatures. The mechanism involves introducing a singlet scalar field and its mirror counterpart into the mirror model. The singlet scalar and its mirror partner acquire asymmetric vacuum expectation values, allowing the mirror singlet scalar having a small mass, below the mass of the Standard Model (SM) Higgs. The mirror singlet scalar decouples from the thermal cosmic bath while maintaining a high abundance. Its slow decay subsequently transfers a significant amount of entropy to the SM sector, increasing the SM sector temperature relative to the mirror sector. We demonstrate that the scalar potential’s parameter space supports this scenario within the allowed constraints. This mechanism effectively lowers the temperature ratio between the mirror and SM sectors, allowing the model to evade the cosmological constraints on the number of relativistic degree of freedom. At the same time, keV-scale mirror neutrinos emerge as viable warm dark matter candidates.