Taichiro Kugo

Bio: Taichiro Kugo is an academic researcher from Kyoto University. The author has contributed to research in topics: Gauge theory & Supergravity. The author has an hindex of 33, co-authored 73 publications receiving 4269 citations.

Local Covariant Operator Formalism of Non-Abelian Gauge Theories and Quark Confinement Problem

Abstract: A manifestly covariant and local canonical operator formalism of non-Abelian gauge theories is presented in its full detail. This formalism, applicable to Yang-Mills theories as well as to gravity, not only provides us a transparent understanding in the scattering theoretical aspects, but also makes it possible to discuss other important problems directly related to the (Heisenberg) operators and the state vectors: As for the former, the physical S-matrix unitarity is proved quite generally on the basis of the representation of the algebra of the BRS charge, and asymptotic field analysis is explicitly performed for some examples. As for the latter, the problems of observables and the well-definedness of charge operators are discussed and clear results are obtained, where the locality and covariance of the formalism are indispensable. Observables are shown to be invariant under the BRS transformation as well as the unbroken global gauge groups. By analyzing the structure of “Maxwell” equations in YM theories, the converse of the Higgs theorem is found to hold. This turns out to lead to a remarkably simple criterion of quark confinement in QCD. The present formalism is found useful also for the U(1) problem and the charge universality proof in the Weinberg-Salam model. General theory of indefinite metric quantum fields is developed to some extent.

Is the ρ Meson a Dynamical Gauge Boson of Hidden Local Symmetry

Abstract: We suggest that the $\ensuremath{\rho}$ meson is a dynamical gauge boson of a hidden local symmetry in the non-linear chiral Lagrangian. The origin of the $\ensuremath{\rho}$-meson mass is understood as the Higgs mechanism of the hidden local symmetry. The low-energy dynamics of $\ensuremath{\rho}$, $\ensuremath{\pi}$, and matter fields, including the Kawarabayashi-Suzuki-Riazuddin-Fayyazuddin relation and $\ensuremath{\rho}$-coupling universality, is consistently described in this new framework. The electromagnetic interaction can be introduced in a unique manner, which gives a successful explanation of $\ensuremath{\rho}$ dominance of the photon coupling.

Non-Abelian Anomaly and Vector Mesons as Dynamical Gauge Bosons of Hidden Local Symmetries

Abstract: We present general solutions to the Wess-Zumino anomaly equation which incorporate vector mesons as dynamical gauge bosons of the hidden local symmetry in the nonlinear chiral Lagrangian. In contrast to the previous attempts to introduce the vector mesons, our formalism enables one to treat consistently and systematically various processes associated with pseudoscalar mesons (Nambu-Goldsone bosons) and vector mesons (dynamical gauge bosons); it is automatic in this framework that the on-shell amplitudes of photons and pseudoscalar mesons such as π^0 →2γ and γ→3 π which are fixed by the low energy theorems, are not affected no matter how the vector mesons participate in those processes. We further show that the “complete vector meson dominance” hypothesis of photon couplings is invalid in either π^0 →2γ or γ→3π process.

Topological quantum field theory

Abstract: A twisted version of four dimensional supersymmetric gauge theory is formulated. The model, which refines a nonrelativistic treatment by Atiyah, appears to underlie many recent developments in topology of low dimensional manifolds; the Donaldson polynomial invariants of four manifolds and the Floer groups of three manifolds appear naturally. The model may also be interesting from a physical viewpoint; it is in a sense a generally covariant quantum field theory, albeit one in which general covariance is unbroken, there are no gravitons, and the only excitations are topological.

Low energy hadron physics in holographic QCD

Abstract: We present a holographic dual of four-dimensional, large N_c QCD with massless flavors. This model is constructed by placing N_f probe D8-branes into a D4 background, where supersymmetry is completely broken. The chiral symmetry breaking in QCD is manifested as a smooth interpolation of D8 - anti-D8 pairs in the supergravity background. The meson spectrum is examined by analyzing a five-dimensional Yang-Mills theory that originates from the non-Abelian DBI action of the probe D8-brane. It is found that our model yields massless pions, which are identified with Nambu-Goldstone bosons associated with the chiral symmetry breaking. We obtain the low-energy effective action of the pion field and show that it contains the usual kinetic term of the chiral Lagrangian and the Skyrme term. A brane configuration that defines a dynamical baryon is identified with the Skyrmion. We also derive the effective action including the lightest vector meson. Our model is closely related to that in the hidden local symmetry approach, and we obtain a Kawarabayashi-Suzuki-Riazuddin-Fayyazuddin-type relation among the couplings. Furthermore, we investigate the Chern-Simons term on the probe brane and show that it leads to the Wess-Zumino-Witten term. The mass of the \eta' meson is also considered, and we formulate a simple derivation of the \eta' mass term satisfying the Witten-Veneziano formula from supergravity.

Instantons in QCD

Abstract: The authors review the theory and phenomenology of instantons in quantum chromodynamics (QCD). After a general overview, they provide a pedagogical introduction to semiclassical methods in quantum mechanics and field theory. The main part of the review summarizes our understanding of the instanton liquid in QCD and the role of instantons in generating the spectrum of light hadrons. The authors also discuss properties of instantons at finite temperature and how instantons can provide a mechanism for the chiral phase transition. They give an overview of the role of instantons in some other models, in particular low-dimensional sigma models, electroweak theory, and supersymmetric QCD.