Masaki Murata

Multibrane solutions in open string field theory

A bstractWe study properties of a class of solutions of open string field theory which depend on a single holomorphic function F (z). We show that the energy of these solutions is well defined and is given by integer multiples of a single D-brane tension. Potential anomalies are discussed in detail. Some of them can be avoided by imposing suitable regularity conditions on F (z), while the anomaly in the equation of motion seems to require an introduction of the so called phantom term.

On Multibrane Solutions in Open String Field Theory

Since the original insight by Sen1) that the tachyon vacuum of open string field theory can be constructed in the so-called universal sector of the conformal field theory, and that it describes a state with no D-brane, it led naturally to the question whether there are other physically distinct solutions in this sector. In particular, whether there is a solution describing a state of multiple D-branes. The result of numerous numerical searches in level truncation was inconclusive if not negative, but there were good arguments based on vacuum string field theory which suggested otherwise.2),3) The starting point of the present work was a simple observation, that up to this date, there had been no computation of the energy for a class of analytic solutions of the open string field theory of the form∗∗∗)

Chiral currents and static properties of nucleons in holographic QCD

We analyze static properties of nucleons in the two flavor holographic QCD model of Sakai and Sugimoto described effectively by a five-dimensional U(2) Yang-Mills theory with the Chern-Simons term on a curved background. The baryons are represented in this model as a soliton, which at a time slice is approximately the Belavin-Polyakov-Shvarts-Tyupkin (BPST) instanton with a fixed size. First, we construct a chiral current in four dimensions from the Noether current of local gauge transformations which are nonvanishing on the boundaries of the extra dimension. We examine this chiral current for nucleons with quantized collective coordinates to compute their charge distribution, charge radii, magnetic moments, and axial-vector coupling. Most of the results are better close to the experimental values than in the Skyrme model. We discuss the problems of our chiral current, nonuniqueness of the local gauge transformation for defining the current, and its gauge-noninvariance.

Melting spectral functions of the scalar and vector mesons in a holographic QCD model

We investigate the finite-temperature spectral functions of heavy quarkonia by using the soft-wall anti-de Sitter/QCD model. We discuss the scalar, the pseudoscalar, the vector, and the axial-vector mesons and compare their qualitative features of the melting temperature and growing width. We find that the axial-vector meson melts earlier than the vector meson, while there appears only a slight difference between the scalar and pseudoscalar mesons, which also melt earlier than the vector meson.

Baryons and the Chern-Simons Term in Holographic QCD with Three Flavors

We study dynamical baryons in the holographic QCD model of Sakai and Sugimoto in the case of three flavors, with special interest in the construction of the Chern-Simons (CS) term. The baryon classical solution in this model is given by the BPST instanton, and we carry out the collective coordinate quantization of this solution. The CS term should give rise to a first-class constraint which selects baryon states with the correct spins. However, the original CS term written in terms of the CS 5-form does not do so. Considering this fact, we propose a new CS term that is gauge invariant and takes the form of an integral over a six-dimensional space whose boundary is the original five-dimensional spacetime of the holographic model. Collective coordinate quantization using our new CS term leads to the correct baryon states and their mass formula.

Gauge-Fixing Problem in Modified Cubic Superstring Field Theory

The purpose of our work is to write down a gauge fixed action for the modified cubic superstring field theory in the well-known Siegel gauge. In this theory, there are new gauge symmetries associated with the kernel of the picture changing operator. This gauge is fixed by restricting the space of fields by using the projection operator removing the kernel. We have found the gauge-fixed kinetic term for the Ramond string field. On the other hand, we have not yet succeeded in finding the one for the Neveu-Schwarz string field. It is to be desired to find the full solution to the master equation involving the interaction terms.

No-Ghost Theorem for Neveu-Schwarz String in 0-Picture via Similarity Transformation

A proof of the no-ghost theorem for Neveu-Schwarz string in 0-picture is outlined in a manner using a similarity transformation. It is shown that a nontrivial metric consistent with the BRST cohomology is needed to define a positive semidefinite norm in the physical Hilbert space. The one-to-one correspondence between physical states in 0-picture and those in the conventional (−1)-picture are confirmed. As a by-product, we find a new inverse picturechanging operator, which is noncovariant but has nonsingular operator product with itself. A possibility to construct a new gauge-invariant superstring field theory is discussed.

No-Ghost Theorem for Neveu-Schwarz String in 0-Picture

The no-ghost theorem for Neveu-Schwarz string is directly proved in 0-picture. The one-to-one correspondence between physical states in 0-picture and those in the conventional (-1)-picture are confirmed. It is shown that a non-trivial metric consistent with the BRST cohomology is needed to define a positive semi-definite norm in the physical Hilbert space. As a by-product, we find a new inverse picture changing operator, which is non-covariant but has non-singular operator product with itself. A possibility to construct a new gauge invariant superstring field theory is discussed.