Washout Dynamics and Baryon Asymmetry from Nonlinear Neutrino Physics: Nonlinear Neutrino Effects in the Thermal History of the Early Universe

We investigate the consequences of a nonlinear modification of the Standard Model Lagrangian of the form [Formula: see text] where [Formula: see text] parameterizes deviations from linear field dynamics and [Formula: see text] sets the characteristic energy scale of the deformation. Such a modification induces small but systematic corrections to neutrino mass generation, washout rates, and CP-violating asymmetries within the seesaw and Weinberg-operator frameworks. We derive the corresponding modified Boltzmann equations governing the evolution of heavy-neutrino abundance and lepton asymmetry, showing that the nonlinear term enhances or suppresses washout depending on the sign of [Formula: see text]. The resulting lepton asymmetry determines the final baryon asymmetry after sphaleron conversion, yielding constraints on the allowed parameter space of ([Formula: see text]). At the theoretical level, the nonlinearity modifies the ultraviolet behavior of loop integrals, potentially acting as a mild regulator or enhancement of divergences depending on the sign of [Formula: see text]. The deformation of the Lagrangian induces logarithmic temperature dependence in neutrino masses. This effect is small at late times but can be substantial in the high-temperature early Universe, where it directly influences washout rates and the efficiency of leptogenesis. Thus, neutrino masses must be treated as temperature-dependent dynamical quantities, not fixed constants, in cosmological analyses.