We consider four dimensional quantum field theories which have a continuous manifold of inequivalent exact ground states – a moduli space of vacua. Classically, the singular points on the moduli space are associated with extra massless particles. Quantum mechanically these singularities can be smoothed out. Alternatively, new massless states appear there. These may be the elementary massless pa...

We identify a large class of one-loop amplitudes for massless particles that can be constructed via unitarity from tree amplitudes, without any ambiguities. One-loop amplitudes for massless supersymmetric gauge theories fall into this class; in addition, many non-supersymmetric amplitudes can be rearranged to take advantage of the result. As applications, we construct the one-loop amplitudes fo...

We discuss rescattering effects that can be measured in e+e− annihilation to three jets through a single gauge boson, by using triple product (“event handedness”) correlations of the Z (γ∗) polarization with jet momenta. The gauge boson polarization may be produced either by polarized beams or through the natural polarization (left-right asymmetry) of the Z. QCD rescattering does not generate t...

We study the existence of long-lived meta-stable supersymmetry breaking vacua in gauge theories with massless quarks, upon the addition of extra massive flavors. A simple realization is provided by a modified version of SQCD with Nf,0 < Nc massless flavors, Nf,1 massive flavors and additional singlet chiral fields. This theory has local meta-stable minima separated from a runaway behavior at in...

We describe few aspects of the quantum symmetries of some massless two-dimensional field theories. We discuss their relations with recent proposals for the factorized scattering theories of the massless PCM1 and O(3)θ=π sigma models. We use these symmetries to propose massless factorized S-matrices for the su(2) sigma models with topological terms at any level, alias the PCMk models, and for th...

Recently, it has been shown that the massless Schwinger model, namely two-dimensional massless spinor electrodynamics, may be solved exactly through the bosonisation of the massless fermion without having recourse to any gauge fixing procedure. With this background in mind, and the possible resolution of massless scalar electrodynamics in two-dimensional Minkowski spacetime compactified into th...