Starting from the Lagrangian L=−∂μϕa∂μϕa/2−μ2ϕaϕa/2−λ(ϕaϕa)2/4!and taking μ2→−∞,λ→∞such that <ϕaϕa>is finite and droping any infinite mass fields, I want to get the Lagrangian L=−f2∂μσa∂μσa/2
where a=1,2,3,4, f a constant with units of mass and we also have the constraint Σσaσa=1. Could you give me any hint on how I should start?