The phase diagram of the three-dimensional Z2 gauge Higgs system at zero and finite temperature

We study the effect of adding a matter field to the Z2 gauge model in three dimensions at zero and finite temperature. Up to a given value of the parameter regulating the coupling, the matter field produces a slight shift of the transition line without changing the universality class of the pure gauge theory, as seen by finite size scaling analysis or by comparison, in the finite temperature case, to exact formulas of conformal field theory. At zero temperature the critical line turns into a first-order transition. The fate of this kind of transition in the finite temperature case is discussed.Comment: 6 pages, 10 figures, based on a poster by AR and a talk by FG at Lattice2002(higgssusy

Dirac-Born-Infeld action from spontaneous breakdown of Lorentz symmetry in brane-world scenarios

In whatever Lorentz invariant theory, the presence of extended d-dimensional objects inside a higher dimensional bulk space-time, like for instance D-branes in string theories, induces a spontaneous breakdown of the Poincare' invariance of the bulk; however the effective action describing these extended objects should still respect this larger invariance through a non-linear realization of the full symmetry. Here the specific form of such a realization in presence of an electromagnetic field is uncovered. The Dirac-Born-Infeld type action for a D-brane turns out to be invariant under such transformations. Conversely it is explicitly demonstrated in some simple cases that the most general invariant action, as long as derivatives of the field strength (and second derivatives of the scalars) can be neglected, is a linear combination of terms of the Dirac-Born-Infeld type with different scale parameters. The string result is obtained if one further assumes that the theory depends upon a single length scale.Comment: 4 pages, no figures. v4: typos corrected; matches the published versio