Evidence for Dual Superconductivity of Qcd Ground State

No reliable analytic approach exists to QCD at large distances. The usual perturbative quantization leads to an S matrix which is not Borel summable. For reasons which are not understood the perturbative expansion works anyhow at small distances, where a few terms correctly describe experiments. It fails at large distances, where the coupling is large, in particular in describing confinement of colour. Attempts have been made to describe the degrees of freedom relevant to confinement by effective models. Particularly attractive from the theoretical point of view, is the possibility that vacuum behaves as a dual superconductor. Dual Meissner effect would accordingly produce confinement by constraining the chromoelectric field into Abrikosov flux tubes, with energy proportional to their length. The mechanism is appealing because it relies on a symmetry property. Superconductivity is a Higgs mechanism, by which a charged field acquires a non zero v.e.v., the order parameter in the Landau Ginzburg free energy. The ground state has no definite charge, the U(1) related to charge conservation being spontaneously broken. For QCD magnetic charges should condense in the confined phase, and break some magnetic U(1) symmetry. A dual order parameter, a disorder parameter in the language of statistical mechanics, would then describe this change of symmetry. Only a non perturbative quantization, like lattice, can help in cheking if the above mechanism is at work. The simplest strategy to do that consists of two steps