Chiara R. Nappi

Open Strings in Background Gauge Fields

Some properties of open bosonic strings in a background abelian gauge field are investigated. We derive the equations of motion and effective action for the electromagnetic field, and discuss the effect of the field on the string spectrum and partition function.

Loop corrections to superstring equations of motion

Abstract We extend to the O(32) superstring our program of renormalizing string theory by cancelling BRST anomalies between different genus worldsheets. We calculate all the anomalies arising to lowest nontrivial loop order and in general background fields. At tree level, string consistency requires the background fields to satisfy equations of motion which ensure the conformal invariance of the underlying sigma model. We implement loop anomaly cancellation by adding to these equations appropriate loop-order source terms which necessarily break conformal invariance. The procedure leads to consistent loop-corrected equations of motion which can be derived from a spacetime effective action. The loop-order terms in this action incorporate a number of crucial aspects of spacetime physics, including the interaction of the photon with the graviton and the interactions of the antisymmetric tensor field with the gauge field which are responsible for spacetime gauge anomaly cancellation. We take these results as compelling evidence that loop-corrected string theory is associated with sigma models which are not conformally invariant in the usual sense. An extended version of conformal invariance, whose deep structure has yet to be uncovered, presumably incorporates string loop physics.

Adding Holes and Crosscaps to the Superstring

A hole or crosscap in the world sheet of a superstring creates the eigenstate of the quantized boundary conditions. This observation makes the calculation of open string loop divergences almost trivial. We confirm that the O(32) theory is finite. In other theories, the divergences induce BRST anomalies. Cancelling them against the tree-level BRST anomalies of the sigma model gives the loop-corrected equations of motion without ambiguities.

Wess-Zumino-Witten model based on a nonsemisimple group.

We present a conformal field theory which desribes a homogeneous four dimensional Lorentz-signature space-time. The model is an ungauged WZW model based on a central extension of the Poincaré algebra. The central charge of this theory is exactly four, just like four dimensional Minkowski space. The model can be interpreted as a four dimensional monochromatic plane wave. As there are three commuting isometries, other interesting geometries are expected to emerge via O(3, 3) duality. ⋆ Research supported in part by the Ambrose Monell Foundation. † Research supported in part by NSF Grant PHY91-06210. In this paper, we will describe a new example of a homogeneous (but not isotropic) four dimensional space-time – with Lorentz signature – that can be constructed from an ungauged WZW model. The model has c (or in the supersymmetric case ĉ) equal to four, so it can be directly substituted for four dimensional Minkowski space and regarded as a solution of any more or less realistic string theory. It describes a special case of a monochromatic plane wave with more than the usual symmetry. The model is a WZW model based on a certain non-semi-simple Lie algebra of dimension four. The algebra has the following explicit description: [J, Pi] = ǫijPj [Pi, Pj ] = ǫijT [T, J ] = [T, Pa] = 0 (1) This algebra is a central extension of the 2D Poincaré algebra to which it reduces if one sets T = 0. We will call the corresponding simply-connected group R. In general, given a Lie algebra with generators TA (here TA = P1, P2, J, T ), and structure constants f AB (so [TA, TB] = f D ABTD), to define a WZW model, one needs a bilinear form ΩAB in the generators TA, which is symmetric (ΩAB = ΩBA), invariant f ABΩCD + f D ACΩBD = 0 (2) and non-degenerate (so that there is an inverse matrix Ω obeying ΩΩBC = δ C). ‡ Then the WZW action on a Riemann surface Σ is

String Loop Corrections to beta Functions

Abstract We study the problem of finding the beta functions, and the associated spacetime effective action, for interacting open and closed strings propagating in background fields. String loop divergences play a crucial role in this problem. Cancelling them against sigma model divergences gives a consistent set of loop-corrected beta functions, which can be derived from a simple generalization of the string-tree-level effective action. This suggests the existence of new string theories which are conformally invariant only after all world sheets have been summed.

A Relation between approaches to integrability in superconformal Yang-Mills theory

We make contact between the infinite-dimensional non-local symmetry of the type-IIB superstring on AdS5 × S5 and a non-abelian infinite-dimensional symmetry algebra for the weakly coupled superconformal gauge theory. We explain why the planar limit of the one-loop dilatation operator is the hamiltonian of a spin chain, and show that it commutes with the g2N = 0 limit of the non-abelian charges.

Superstring model building

We consider constraints on superstring theories compactified on Calabi-Yau manifolds arising from the proton lifetime, neutrino masses, sin 2 θ w , and the requirement of perturbative unification. All possible low-energy gauge groups are studied with variable number of generations. Several three-generation models pose no immediate problem. Four-generation models need an intermediate scale. We suggest a natural mechanism to generate it.

Stabilization of Chiral Solitons via Vector Mesons

We introduce vector mesons in a chiral lagrangian. The ω meson stabilizes the chiral solitons. We compute the static properties of baryons in this model.

Yangian symmetry in D = 4 superconformal Yang-Mills theory

We will discuss an integrable structure for weakly coupled superconformal Yang-Mills theories, describe certain equivalences for the Yangian algebra, and fill a technical gap in our previous study of this subject.

The Quark Content of the Proton

Abstract The quark content of the proton is studied using the quark scalar bilinear, q q, as a probe. An analysis of hadron masses and the sigma term suggests an unexpectedly large signal for s s. This is seen to be reasonable within both the Skyrme model and the bag model. The interpretation of the quark content is discussed in both models.