Shigeaki Yahikozawa

COMMENTS ON D-INSTANTONS IN C< 1 STRINGS

We suggest that the boundary cosmological constant ζ in c<1 unitary string theory be regarded as the one-dimensional complex coordinate of the target space on which the boundaries of world-sheets can live. From this viewpoint we explicitly construct analogues of D-instantons which satisfy Polchinskis “combinatorics of boundaries.” We further show that our operator formalism developed in the preceding articles is powerful in evaluating D-instanton effects, and also demonstrate for simple cases that these effects exactly coincide with the stringy nonperturbative effects found in the exact solutions of string equations.

General Operator Solutions and BRST Quantization of Superstrings in the pp-Wave with Torsion

We completely accomplish the canonically covariant quantization of Ramond-NeveuSchwarz (RNS) superstrings in the pp-wave background with a non-zero flux of the NS-NS antisymmetric two-form field. Here this flux is equivalent to a nonvanishing torsion. In this quantization, general operator solutions, which satisfy the entire equation of motion and all the canonical (anti)commutation relations, play an important role. The whole of covariant string coordinates and fermions can be composed of free modes. Moreover, employing covariant free-mode representations, we calculate the anomaly in the super-Virasoro algebra and determine the number of dimensions of spacetime and the ordering constant from the nilpotency condition of the BRST charge in the pp-wave background with the flux.

Covariant BRST Quantization of Closed Strings in the pp-Wave Background

We canonically quantize closed string theory in the pp-wave background with a non-zero flux of the three-form field strength by using the covariant BRST operator formalism. In this canonical quantization, we completely construct new covariant free-mode representations, for which it is particularly important to take account of the commutation relations of the zero mode of the light-cone string coordinate X − with other modes. All covariant string coordinates are composed of free modes. Moreover, employing these covariant string coordinates for the energy-momentum tensor, we calculate the anomaly in the Virasoro algebra and determine the number of dimensions of spacetime and the ordering constant from the nilpotency condition of the BRST charge in the pp-wave background.

Information metric on instanton moduli spaces in nonlinear sigma models.

We study the information metric on instanton moduli spaces in two-dimensional nonlinear sigma models. In the CP1 model, the information metric on the moduli space of one instanton with the topological charge Q=k(k > or =1) is a three-dimensional hyperbolic metric, which corresponds to Euclidean anti-de Sitter space-time metric in three dimensions, and the overall scale factor of the information metric is 4k(2)/3; this means that the sectional curvature is -3/4k(2). We also calculate the information metric in the CP2 model.

GENERAL OPERATOR SOLUTIONS AND BRST QUANTIZATION OF SUPERSTRINGS IN THE pp-WAVE WITH TORSION

We completely accomplish the canonically covariant quantization of RNS superstrings in the pp-wave background with torsion which is equivalent to the NS-NS flux. In this quantization, we construct general operator solutions which satisfy all the equations of motion and all the canonical (anti)commutation relations. Because the general operator solutions can be composed of only free modes, we also call them free-mode representations. Using the covariant free-mode representations, we calculate the anomaly in the super-Virasoro algebra and determine the number of dimensions of spacetime and ordering constant from the nilpotency condition of the BRST charge in the NS-NS pp-wave.

Can we construct the enhançon configuration with D2-D6 branes shell?

The stability of the shell of wrapped D6-branes on K3 is investigated from the point of view of supergravity. We first construct an effective energy-momentum tensor for the shell under the reasonable conditions and show that supersymmetric solutions satisfy Israel’s junction conditions at arbitrary radius of the shell. Next we study the perturbation of the whole system including the self-gravity of the shell. It is found that in spite of the existence of wrapped D6-branes with negative tension, there is no eigenmode whose frequencies of the shell and the fields are imaginary numbers, at any radius of the shell. Furthermore, when the radius of the shell is less than the enhancon radius, resonances are produced, and this indicates a kind of “instability” of the system. This can even classically explain why the shell is constructed at the enhancon radius.

A model of membrane with fractional spin and unusual statistics

We present a model of membrane with fractional world-volume spin and exotic statistics. The model is constructed by coupling the bosonic membrane to the world-volume U(1) Chern-Simons term. Dirac’s method for the constrained system is used to quantize this theory and to show that the membrane has exotic statistics. We also evaluate finite size effects and clarify the relationship between a conserved charge in space-time and an angular-momentum operator on a world-volume.