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QED processes at electron-positron colliders are considerd. We present differential cross-sections for large--angle Bhabha scattering, annihilation into muons and photons. Radiative corrections in the first order are taken into account exactly. Leading logarithmic contributions are calculated in all orders by means of the structure-function method. An accuracy of the calculation can be estimated about 0.2%. [ABSTRACT FROM AUTHOR]

We study some aspects of the strong coupling dynamics of Dirichlet six branes, anti- six branes, and orientifold planes by using the equivalence of type IIA string theory and M-theory on S1. In the strong coupling limit there exists static configuration of brane and anti-brane at arbitrary separation, suspended in an external magnetic field. The mass of the open string stretched between the brane and the anti-brane approaches a finite positive value even when the branes coincide. Similar result is obtained for a Dirichlet six brane on top of an orientifold six plane. We also derive the anomalous gravitational interaction on the brane and the orientifold plane from M-theory. [ABSTRACT FROM AUTHOR]

We continue our study of heterotic/type-I duality in D < 10 dimensions. We consider the heterotic and type-I theories compactified on tori to lower dimensions. We calculate the special (``BPS-saturated'') ℱ4 and ℛ4 terms in the effective one-loop heterotic action. These terms are expected to be non-perturbatively exact for D > 4. The heterotic result is compared with the associated type-I result. In D < 9 dimensions, the type-I theory has instanton corrections due to D1 instantons. In D = 8 we use heterotic/type-I duality to give a simple prescription of the D-instanton calculation on the type-I side. We allow arbitrary Wilson lines and show that the D1-instanton determinant is the affine character-valued elliptic genus evaluated at the induced complex structure of the D1-brane world-volume. The instanton result has an expansion in terms of Hecke operators that suggests an interpretation in terms of an SO(N) matrix model of the D1-brane. The total result can be written in terms of generalized prepotentials, revealing an underlying holomorphic structure. In D < 8 we calculate again the heterotic perturbative thresholds and show that they agree with the D1-instanton calculation using the rules derived in D = 8. [ABSTRACT FROM AUTHOR]

We show that QCD observables defined as infrared- and collinear-safe, according to the usual Sterman-Weinberg criteria, can nevertheless be infinite at accessible points inside phase space, to any finite order of perturbation theory. Infrared finiteness is restored after resummation of divergent terms to all orders. The resulting characteristic structure, which we call a Sudakov shoulder, represents an interesting new class of QCD predictions. [ABSTRACT FROM AUTHOR]

Processes of electron--positron annihilation into charged pions and kaons are considered. Radiative corrections are taken into account exactly in the first order and within the leading logarithmic approximation in higher orders. A combined approach for accounting exact calculations and electron structure functions is used. An accuracy of the calculation can be estimated about 0.2%. [ABSTRACT FROM AUTHOR]

If the gravitino is light and all the other supersymmetric particles are heavy, we can consider the effective theory describing the interactions of its goldstino components with ordinary matter. To discuss the model-dependence of these interactions, we take the simple case of spontaneously broken supersymmetry and only two chiral superfields, associated with the goldstino and a massless matter fermion. We derive the four-point effective coupling involving two matter fermions and two goldstinos, by explicit integration of the heavy spin-0 degrees of freedom in the low-energy limit. Surprisingly, our result is not equivalent to the usual non-linear realization of supersymmetry, where a pair of goldstinos couples to the energy-momentum tensor of the matter fields. We solve the puzzle by enlarging the non-linear realization to include a second independent invariant coupling, and we show that there are no other independent couplings of this type up to this order in the low-energy expansion. We conclude by commenting on the interpretation of our results and on their possible phenomenological implications. [ABSTRACT FROM AUTHOR]

We discuss the relation between the Ramond-Ramond charges of D-branes and the topology of Chan-Paton vector bundles. We show that a topologically nontrivial normal bundle induces RR charge and that the result fits in perfectly with the proposal that D-brane charge is the topology of the Chan-Paton bundle, regarded as an element of K-theory. [ABSTRACT FROM AUTHOR]

We show how certain non-perturbative superpotentials W̃(Σ), which are the two-dimensional analogs of the Seiberg-Witten prepotential in 4d, can be computed via geometric engineering from 4-folds. We analyze an explicit example for which the relevant compact geometry of the 4-fold is given by ℙ1 fibered over ℙ2. In the field theory limit, this gives an effective U(1) gauge theory with N = (2,2) supersymmetry in two dimensions. We find that the analog of the SW curve is a K3 surface, and that the complex FI coupling is given by the modular parameter of this surface. The FI potential itself coincides with the middle period of a meromorphic differential. However, it only shows up in the effective action if a certain 4-flux is switched on, and then supersymmetry appears to be non-perturbatively broken. This can be avoided by tuning the bare FI coupling by hand, in which case the supersymmetric minimum naturally corresponds to a singular K3. [ABSTRACT FROM AUTHOR]

Covariant field equations of M-fivebrane in eleven dimensional curved superspace are obtained from the requirement of κ-symmetry of an open supermembrane ending on a fivebrane. The worldvolume of the latter is a (6|16) dimensional supermanifold embedded in the (11|32) dimensional target superspace. The κ-symmetry of the system imposes a constraint on this embedding, and a constraint on a modified super 3-form field strength on the fivebrane worldvolume. These constraints govern the dynamics of the M-fivebrane. [ABSTRACT FROM AUTHOR]

Extremal black holes in M-theory compactification on M × S1 are microscopically represented by fivebranes wrapping P × S1, where M is a Calabi-Yau threefold and P is a four-cycle in M. Additional spacetime charges arise from momentum around the S1 and expectation values for the self-dual three-form field strength in the fivebrane. The microscopic entropy of the fivebrane as a function of all the charges is determined from a two-dimensional (0,4) sigma model whose target space includes the fivebrane moduli space. This entropy is compared to the macroscopic formula. Precise agreement is found for both the tree-level and one-loop expressions. [ABSTRACT FROM AUTHOR]