Related papers: Fivebrane Instantons and $R^2$ couplings in $N=4$ …
In this lecture we summarize some recent work on the understanding of instanton effects in string theories with 16 supersymmetries. In particular, we consider F^4 couplings using the duality between the heterotic string on T^4 and type IIA…
We discuss three classes of solitonic solutions in string theory: instantons, monopoles and string-like solitons. Instantons may provide a nonperturbative understanding of the vacuum structure of string theory, while monopoles may appear in…
We investigate membrane and fivebrane instanton effects in type IIA string theory compactified on rigid Calabi-Yau manifolds. These effects contribute to the low-energy effective action of the universal hypermultiplet, in four dimensional…
A long-standing puzzle about the heterotic string has been what happens when an instanton shrinks to zero size. It is argued here that the answer at the quantum level is that an extra $SU(2)$ gauge symmetry appears that is supported in the…
We express the infinite sum of D-fivebrane instanton corrections to ${\cal R}^2$ couplings in ${\cal N}=4$ type I string vacua, in terms of an elliptic index counting 1/2-BPS excitations in the effective $Sp(N)$ brane theory. We compute the…
A detailed understanding of instanton effects for half-BPS couplings is pursued in theories with 16 supersymmetries. In particular, we investigate the duality between heterotic string on $T^4$ and type IIA on $K_3$ at the $T^4/Z_2$ orbifold…
String instanton effects in Higgs physics are discussed through a type IIA model based on T^{6}/(Z^{2}\times Z^{'2}) orentifold compactifaction. By inclusion of rigid E2-branes, the model exhibits a MSSM-like spectrum, as well as extra mu…
In this paper we begin revisiting the little string theories (LSTs) which govern the dynamics of the instantonic heterotic $E_8 \times E_8$ five-branes probing ALE singularities, building on and extending previous results on the subject by…
0-brane of type IIA string theory can be interpreted as a dimensional reduction of a gravitational wave in 11 dimensions. We observe that a similar interpretation applies also to the D-instanton background of type IIB theory: it can be…
Under the six-dimensional heterotic/type $IIA$ duality map, a solitonic membrane solution of heterotic string theory transforms into a singular solution of type $IIA$ theory, and should therefore be interpreted as a fundamental membrane in…
We study the cohomological physics of fivebranes in type II and heterotic string theory. We give an interpretation of the one-loop term in type IIA, which involves the first and second Pontrjagin classes of spacetime, in terms of…
We study the global structure of vacua of heterotic strings compactified on orbifolds $T^4/Z_N$ (N=2,3) in the presence of heterotic 5-branes. Gauge symmetry breaking associated with orbifold is described by instantons in the field theory.…
We discuss a string model where a conformal four-dimensional N=2 gauge theory receives corrections to its gauge kinetic functions from "stringy" instantons. These contributions are explicitly evaluated by exploiting the localization…
The (I)nstantonic strings are described as extended states occupying a flat (isometry) direction on the multiple M5-brane world-volume. These are constituted by 4-dimensional gauge instantons and 2-dimensional chiral axion. In this light we…
We analyze the Neveu-Schwarz fivebrane instanton in type IIA string theory compactifications on rigid Calabi-Yau threefolds, in the low-energy supergravity approximation. It there appears as a finite action solution to the Euclidean…
We derive the precise relation between level matching condition and fractional instanton numbers in six dimensional, abelian and supersymmetric orbifolds of E8 x E8 heterotic string theory. The fractional part of the two E8 instanton…
We analyze M5-instantons in F-theory, or equivalently D3-instantons with varying axio-dilaton, in the presence of 7-brane gauge groups. The chiral two-form on the M5-brane plays an important role, because it couples the M5-brane to vector…
We perform a detailed conformal field theory analysis of D2-brane instanton effects in four-dimensional type IIA string vacua with intersecting D6-branes. In particular, we explicitly compute instanton induced fermion two-point couplings…
T-duality is used to extract information on an instanton of zero size in the $E_8\times E_8$ heterotic string. We discuss the possibility of the appearance of a tensionless anti-self-dual non-critical string through an implementation of the…
The non-perturbative dynamics of quantum field theories is studied using theoretical tools inspired by string formalism. Two main lines are developed: the analysis of stringy instantons in a class of four-dimensional N=2 gauge theories and…