Gluon confinement, i-particles and BRST soft breaking

Abstract: A few issues on gluon confinement are addressed with the help of a renormalizable gauge model obtained by introducing a replica of the Faddeev-Popov action and a soft breaking of the BRST symmetry. Confinement turns out to be encoded in the spectral properties of the corresponding correlation functions. While the propagators of the elementary fields have no interpretation in terms of physical excitations, examples of local composite operators whose two-point correlation functions possess a spectral representation with positive spectral density can be introduced. These composite operators turn out to be left invariant by the BRST transformations, a feature which has strong consequences on their renormalizability properties. Moreover, they display a direct interpretation in terms of i-particles [1], which are the unphysical modes corresponding to a confining propagator of the Gribov type. A possible way to take into account the effects of the Gribov copies is also outlined.