Z2 electric strings and center vortices in SU(2) lattice gauge theory
  We study the representations of SU(2) lattice gauge theory in terms of sums over the worldsheets of center vortices and Z2 electric strings, i.e. surfaces which open on the Wilson loop. It is shown that in contrast to center vortices the density of electric Z2 strings diverges in the continuum limit of the theory independently of the gauge fixing, however, their contribution to the Wilson loop yields physical string tension due to non-positivity of their statistical weight in the path integral, which is in turn related to the presence of Z2 topological monopoles in the theory. 
M. I. Polikarpov and P. V. Buividovich