arXiv subjects
Publications and source records attributed to Pierre van Baal.
In previous work constant magnetic field strength solutions for SU(2) gauge theory on a torus were found, which somewhat surprisingly turned out to be classically stable. This was called marginal stability, as moving along one of its zero-modes, two of the stable modes turn unstable. Here we investigate the stability under quantum fluctuations in the domain where the solutions possess the marginal stability at the classical level.
We construct all instantons in the \nlsig\ on a cylindrical space-time, known not to exist on a finite time interval. The scale parameter, $ρ$, is related to the boundary condition in time. This may cure the $ρ\rightarrow0$ divergent instanton gas, through a proper inclusion of in and out states in the path integral.
The cooling algorithm for saddle points presented in ref. [1] is generalized to obtain static classical solutions of the SU(2)-Higgs field theory in the limit of infinite Higgs self-coupling. The sphaleron energy obtained via this algorithm is ${\cal E}_{\rm sph}= 5.08(7)\,M_W/α_W$ to be compared with 5.0707 obtained in the variational approach [2].
For SU(2) gauge theory on the three-sphere we focus on a subspace of modes of the gauge field that contains the tunnelling paths and the sphalerons and on which the energy functional is degenerate to second order in the fields. The ultimate goal is to study the $θ$-dependence of the low-lying states for this model by imposing boundary conditions on the fundamental domain.
Laplacian gauge fixing was introduced to find a unique representative of the gauge orbit, which on the lattice could be implemented by a ``finite'' algorithm. What was still lacking was a perturbative formulation of this gauge, which will be presented here. However, renormalizability is still to be demonstrated. For torodial and spherical geometries a detailed comparison with the Landau (or Coulomb) gauge will be made.
We describe a new cooling algorithm for SU(2) lattice gauge theory. It has any critical point of the energy or action functional as a fixed point. In particular, any number of unstable modes may occur. We also provide insight in the convergence of the cooling algorithms. A number of solutions will be discussed, in particular the sphalerons for twisted and periodic boundary conditions which are important for the low-energy dynamics of gauge theories. For a unit cubic volume we find a sphaleron energy of resp. $\cE_s=34.148(2)$ and $\cE_s=72.605(2)$ for the twisted and periodic case. Remarkably, the magnetic field for the periodic sphaleron satisfies at all points $\Tr B_x^2=\Tr B_y^2=\Tr B_z^2$.
We present results related to the search of SU(2) instantons on a geometry $[0,L]^3\times [0,T]$ obtained using over-improved cooling with fixed boundary conditions. We also introduce a criterion for finding a sphaleron at the top of the energy barrier of the instanton path.
For SU(2) gauge theories on the three-sphere we analyse the Gribov horizon and the boundary of the fundamental domain in the 18 dimensional subspace that contains the tunnelling path and the sphaleron and on which the energy functional is degenerate to second order in the fields. We prove that parts of this boundary coincide with the Gribov horizon with the help of bounds on the fundamental modular domain.
Lattice artefacts are used, through modified lattice actions, as a tool to find the largest instantons in a toroidal geometry [0,L]^3X[0,T] for T to infinity. It is conjectured that the largest instanton is associated with tunnelling through a sphaleron. Existence of instantons with at least 8 parameters can be proven with the help of twisted boundary conditions in the time direction. Numerical results for SU(2) gauge theory obtained by cooling are presented to demonstrate the viability of the method.
In this contribution to the proceedings we will describe some of the details for constructing the Gribov horizon and the boundary of the fundamental modular domain, when restricting to some low energy modes of pure SU(2) gauge theory in a spherical spatial geometry. The fundamental domain is a one-to-one representation of the set of gauge invariant degrees of freedom, in terms of transverse gauge fields. Boundary identifications are the only remnants of the Gribov copies.
It will be described how to uniquely fix the gauge using Coulomb gauge fixing, avoiding the problem of Gribov copies. The fundamental modular domain, which represents a one-to-one representation of the set of gauge invariant degrees of freedom, is a bounded convex subset of the trans- verse gauge fields. Boundary identifications are the only remnants of the Gribov copies, and carry all the information about the topology of the Yang-Mills configuration space. Conversely, the known topology can be shown to imply that (on a set of measure zero on the boundary) some points of the boundary coincide with the Gribov horizon. For the low-lying energies, wavefunctionals can be shown to spread out "across" certain parts of these boundaries. This is how the topology of Yang-Mills configuration space has an essential influence on the low-lying spectrum, in a situation where these non- perturbative effects are not exponentially suppressed. This write-up is a short summary, with adequate references, where details on most of the material I have presented can be found. However, not published before, is a new observation concerning Henyey's gauge copies.