From N=2 Fermionic Strings to Superstrings?

I review the covariant quantization of the critical $N{=}2$ fermionic string with and without a global ${\bf Z}_2$ twist. The BRST analysis yields massless bosonic and fermionic vertex operators in various ghost and picture number sectors, as well as picture-changers and their inverses, depending on the field basis chosen for bosonization. Two distinct GSO projections exist, one (untwisted) retaining merely the known bosonic scalar and its spectral-flow partner, the other (twisted) yielding two fermions and one boson, on the massless level. The absence of interactions in the latter case rules out standard spacetime supersymmetry. In the untwisted theory, the $U(1,1)$-invariant three-point and vanishing four-point functions are confirmed at tree level. I comment on the $N{=}2$ string field theory, the integration over moduli and the realization of spectral flow.Comment: 15 pages, latex, no figures, macros included, 52 kb Talk at Symposium in Wendisch-Rietz, 8/9

Time-Space Noncommutative Abelian Solitons

We demonstrate the construction of solitons for a time-space Moyal-deformed integrable U(n) sigma model (the Ward model) in 2+1 dimensions. These solitons cannot travel parallel to the noncommutative spatial direction. For the U(1) case, the rank-one single-soliton configuration is constructed explicitly and is singular in the commutative limit. The projection to 1+1 dimensions reduces it to a noncommutative instanton-like configuration. The latter is governed by a new integrable equation, which describes a Moyal-deformed sigma model with a particular Euclidean metric and a magnetic field.Comment: 1+10 page

On the BRST Cohomology of N=2 Strings

We analyze the BRST cohomology of the critical N=2 NSR string using chiral bosonization. Picture-changing and spectral flow is made explicit in a holomorphic field basis. The integration of fermionic and U(1) moduli is performed and yields picture- and U(1) ghost number-changing insertions into the string measure for n-point amplitudes at arbitrary genus and U(1) instanton number.Comment: Talk at ``Strings '95'', March 95; 2 page

Noncommutative Instantons and Solitons

I explain how to construct noncommutative BPS configurations in four and lower dimensions by solving linear matrix equations. Examples are instantons in D=4 Yang-Mills, monopoles in D=3 Yang-Mills-Higgs, and (moving) solitons in D=2+1 Yang-Mills-Higgs. Some emphasis is on the latter as a showcase for the dressing method.Comment: 11 pages; talk presented at the 27th Johns Hopkins Workshop in Goteborg and at the 36th International Symposium Ahrenshoop in Berlin, both in August 200

Noncommutative Sine-Gordon Model

As I briefly review, the sine-Gordon model may be obtained by dimensional and algebraic reduction from 2+2 dimensional self-dual U(2) Yang-Mills through a 2+1 dimensional integrable U(2) sigma model. I argue that the noncommutative (Moyal) deformation of this procedure should relax the algebraic reduction from U(2)->U(1) to U(2)->U(1)xU(1). The result are novel noncommutative sine-Gordon equations for a pair of scalar fields. The dressing method is outlined for constructing its multi-soliton solutions. Finally, I look at tree-level amplitudes to demonstrate that this model possesses a factorizable and causal S-matrix in spite of its time-space noncommutativity.Comment: 6 pages, 4 figures; talk at the XIIIth International Colloquium Integrable Systems and Quantum Groups in Prague 17-19 June 2004, and at the 37th International Symposium Ahrenshoop on the Theory of Elementary Particles in Berlin-Schm"ockwitz 23-27 August 200

Supersymmetric noncommutative solitons

I consider a supersymmetric Bogomolny-type model in 2+1 dimensions originating from topological string theory. By a gauge fixing this model is reduced to a supersymmetric U(n) chiral model with a Wess-Zumino-Witten-type term in 2+1 dimensions. After a noncommutative extension of the model, I employ the dressing method to construct explicit multi-soliton configurations on noncommutative R^{2,1|2N}.Comment: 13 pages, 2 figures; talk given during "Noncommutative Spacetime Geometries" at Alessandria, March 2007, and "Noncommutative Geometry and Physics" at Orsay, April 200

Scattering of Noncommutative Solitons in 2+1 Dimensions

Interactions of noncommutative solitons in a modified U(n) sigma model in 2+1 dimensions can be analyzed exactly. Using an extension of the dressing method, we construct explicit time-dependent solutions of its noncommutative field equation by iteratively solving linear equations. The approach is illustrated by presenting bound states and right-angle scattering configurations for two noncommutative solitons.Comment: 1+10 pages; v2: 2 typos fixed, refs updated; v3: typos (signs, coefficients) correcte

Semiclassical Approach to Finite-N Matrix Models

We reformulate the zero-dimensional hermitean one-matrix model as a (nonlocal) collective field theory, for finite~$N$. The Jacobian arising by changing variables from matrix eigenvalues to their density distribution is treated {\it exactly\/}. The semiclassical loop expansion turns out {\it not\/} to coincide with the (topological) ${1\over N}$~expansion, because the classical background has a non-trivial $N$-dependence. We derive a simple integral equation for the classical eigenvalue density, which displays strong non-perturbative behavior around $N\!=\!\infty$. This leads to IR singularities in the large-$N$ expansion, but UV divergencies appear as well, despite remarkable cancellations among the Feynman diagrams. We evaluate the free energy at the two-loop level and discuss its regularization. A simple example serves to illustrate the problems and admits explicit comparison with orthogonal polynomial results.Comment: 27 pages / 3 figures (ps file fixed