Relatore: Dr Giuseppe Burgio - Universita' di Tubinga

Luogo:Sala Feynman

E-mail organizzatore: francesco.direnzo@fis.unipr.it

Abstract

We introduce a non-local topological order parameter for SU(2) Yang-

Mills theories discretized on a lattice, the center flux z. We show

that:

• Center vortex topological sectors, classified by π_1(SO(3))=Z2,

can only exist in the ordered phase of z;

• Depending on the dimension 2 ≤ d ≤ 4 and action chosen, z

exhibits a critical behaviour sharing striking features with

the Kosterlitz-Thouless type of transitions, although belonging

to a novel universality class;

• Such critical behaviour does not depend on the temperature T.

Yang-Mills theories can thus exist in two different continuum

phases, characterized by a topologically ordered or disordered

vacuum; this reminds of a quantum phase transition, albeit

controlled by the choice of symmetries and not by a physical

parameter.