Prolonged decay and CP asymmetry

The time evolution of unstable particles that occur in the expanding universe is investigated. The off-shell effect not included in the Boltzmann-like equation is important for the decay process when the temperature becomes much below the mass of unstable particle. When the off-shell effect is taken into account, the thermal abundance of unstable particles at low temperatures has a power law behavior of temperature T, (Γ/M)(T/M)^α+1 unlike the Boltzmann suppressed e^-M/T, with the power α related to the spectral rise near the threshold of the decay and with Γ the decay rate. Moreover, the relaxation time towards the thermal value is not governed by the exponential law; instead, it is the power law of time. The evolution equation for the occupation number and the number density of the unstable particle is derived, when both of these effects, along with the cosmic expansion, are included. We also critically examine how the scattering of thermal particles may affect the off-shell effect to the unstable particle. As an application showing the importance of the off-shell effect we compute the time evolution of the baryon asymmetry generated by the heavy X boson decay. It is shown that the out-of equilibrium kinematics previously discussed is changed; this change becomes considerable for large values of Γ/H ≫ 1 where H is the Hubble rate at the temperature equal to the X-boson mass, while we confirm the previous result for small values of Γ/H ≤ 1.