What is the difference between local and global symmetries in terms of properties of the particle spectrum?

Question

I know that a global symmetry implies the presence of a conserved charge but how it does affect the particle spectrum? and in this sense what is the difference with a gauge symmetry?

Answer 1

1. Gauge symmetry is not symmetry, it is redundancy in description of field in QFT. It remove unphysical degrees of freedom, that we need to have clearly Lorentz-invariant formulation. So gauge symmetry doesn't affect spectrum.

2. Global symmetry may be spontaneously broken by vacuum expectation value or be unbroken. In second case all particles sit in representation of global group. In first case appear Goldstone bosons (# of g.b. = # broken generators), which signal about symmetry breaking.