Consider the Lagrangian density in (2.84) for three non-identical Hermitian fields. Calculate the lowest order differential cross section in the center of mass for ϕ1(p1)ϕ2(p2)→ϕ1(p3)ϕ2(p4) as a function of s and the CM scattering angle θ for the special case m1=m2=m3.