σ σ geometry is of the order of the square of the de Broglie wavelength σ σ geometry ≈ ≈ λ λ 2 = ( ℏ ℏ m r v ) 2 ∝ ∝ 1 ϵ ϵ where m r is the reduced mass of the system and ϵ ϵ is the center of mass energy of the system.