Collision of domain walls and reheating of the brane universe

We study particle production at the collision of two domain walls in 5-dimensional Minkowski spacetime. This may provide the reheating mechanism of an ekpyrotic (or cyclic) brane universe, in which two BPS branes collide and evolve into a hot big bang universe. We evaluate a production rate of particles confined to the domain wall. The energy density of created particles is given as ρ ≈ 20g^-4 N_bm_n⁴ where g is a coupling constant of particles to a domain-wall scalar field, N_b is the number of bounces at the collision and m_n is the fundamental mass scale of the domain wall. It does not depend on the width d of the domain wall, although the typical energy scale of created particles is given by ω ∼ 1/d. The reheating temperature is evaluated as T_R ≈ 0.88̄N_b^1/4. In order to have the baryogenesis at the electro-weak energy scale, the fundamental mass scale is constrained as m _n ≥ 1.1 × 10⁷ GeV for ̄ ∼ 10 ^-5.