# A question in QFT of Weinberg.

#### ndung

The Goldstone's theorem was demonstrated base on the breaking of global symmetry of Lagrangian of scalar fields.Then why we know the existence of Goldstone bosons in fermion sector where the breaking symmetry happend for Lagrangian of fermion field?Example why we know the existence of pion when chiral symmetry SU(2)xSU(2) breaking to subgroup SU(2) of u and d quarks?

#### ndung

In scalar field case the vacum expectation value is nonzero when the symmetry is broken.But in fermion field case the vacum expectation value must be zero.So how do we extend the Goldstone theorem from scalar case to fermion case?