Masayasu Harada

Simple description of {pi}{pi} scattering to 1 GeV

Motivated by the 1/Nc expansion, we present a simple model of �� scattering as a sum of a current-algebra contact term and resonant pole exchanges. The model preserves crossing symmetry as well as unitarity up to 1.2 GeV . Key features include chiral dynamics, vector meson dominance, a broad low energy scalar (�) meson and a Ramsauer-Townsend mechanism for the understanding of the 980 MeV region. We discuss in detail the regularization (corresponding to rescattering effects) necessary to make all these nice features work.

Effects of symmetry breaking on the strong and electroweak interactions of the vector nonet.

Starting from a chiral-invariant and quark-line-rule-conserving Lagrangian of pseudoscalar and vector nonets we introduce first- and second-order symmetry breaking as well as quark-line-rule-violating terms and fit the parameters, at the tree level, to many strong and electroweak processes. A number of predictions are made. The electroweak interactions are included in a manifestly gauge-invariant manner. The resulting symmetry-breaking pattern is discussed in detail. Specifically, for the {open_quote}{open_quote}strong{close_quote}{close_quote} interactions, we study all the vector meson masses and {ital V}{r_arrow}{phi}{phi} decays, including isotopic spin violations. In the electroweak sector we study the {l_brace}{rho}{sup 0},{omega},{phi}{r_brace}{r_arrow}{ital e}{sup +}{ital e}{sup {minus}} decays, {l_brace}{pi}{sup +},{ital K}{sup +},{ital K}{sup 0}{r_brace} {open_quote}{open_quote}charge radii,{close_quote}{close_quote} {ital K}{sub {ital l}3} {open_quote}{open_quote}slope factor,{close_quote}{close_quote} and the overall {ital e}{sup +}{ital e}{sup {minus}}{r_arrow}{pi}{sup +}{pi}{sup {minus}} process. It is hoped that the resulting model may be useful as a reasonable description of low energy physics in the range up to about 1 GeV. {copyright} {ital 1996 The American Physical Society.}

The Pion Decay Constant and the ρ-Meson Mass at Finite Temperature in the Hidden Local Symmetry

We study the temperature dependence of the pion decay constant and ρ-meson mass in the hidden local symmetry model at one loop. Using the standard imaginary time formalism, we include the thermal effect of ρ meson as well as that of pion. We show that the pion gives a dominant contribution to the pion decay constant and ρ-meson contribution slightly decreases the critical temperature. The ρ-meson pole mass increases as T 4 /m 2 at low temperature dominated by the pion-loop effect. At high temperature, although the pion-loop effect decreases the ρ-meson mass, the ρ-loop contribution overcomes the pion-loop contribution and ρ-meson mass increases with temperature. We also show that the conventional parameter a is stable as the temperature increases.

Generalization of the Bound State Model

In the bound state approach the heavy baryons are constructed by binding, with any orbital angular momentum, the heavy meson multiplet to the nucleon considered as a soliton in an effective meson theory. We point out that this picture misses an entire family of states, labeled by a different angular momentum quantum number, which are expected to exist according to the geometry of the three-body constituent quark model (for N{sub C}=3). To solve this problem we propose that the bound state model be generalized to include orbitally excited heavy mesons bound to the nucleon. In this approach the missing angular momentum is {open_quotes}locked up{close_quotes} in the excited heavy mesons. In the simplest dynamical realization of the picture we give conditions on a set of coupling constants for the binding of the missing heavy baryons of arbitrary spin. The simplifications made include working in the large M limit, neglecting nucleon recoil corrections, neglecting mass differences among different heavy spin multiplets, and also neglecting the effects of light vector mesons. {copyright} {ital 1997} {ital The American Physical Society}

Solving the homogeneous Bethe-Salpeter equation

We study a method for solving the homogeneous Bethe-Salpeter equation. By introducing a {open_quote}{open_quote}fictitious{close_quote}{close_quote} eigenvalue {lambda} the homogeneous Bethe-Salpeter equation is interpreted as a linear eignevalue equation, where the bound state mass is treated as an input parameter. Using the improved ladder approximation with a constant fermion mass, we extensively study the spectrum of the fictitious eigenvalue {lambda} for the vector bound states and find the discrete spectrum for a vanishing bound state mass. We also evaluate the bound state masses by tuning the appropriate eigenvalue to be unity, and find massless vector bound states for specific values of the constant fermion masses. {copyright} {ital 1996 The American Physical Society.}

Hidden Structure in a Lagrangian for Hyperfine Splitting of the Heavy Baryons

Abstract We investigate the hyperfine splitting of the heavy baryons in the bound-state approach. We start with an ordinary relativistic Lagrangian which has been extensively used to discuss finite mass corrections to the heavy limit predictions. It turns out that the dominant contribution arises from terms which do not manifestly break the heavy spin symmetry. The actual heavy spin violating terms are uncovered by carefully performing a 1 M expansion of this Lagrangian.

Hyperfine splitting of low-lying heavy baryons

Abstract We calculate the next-to-leading order contribution to the masses of the heavy baryons in the bound-state approach for baryons containing a heavy quark. These 1 N C corrections arise when states of good spin and isospin are generated from the background solition of the light meson fields. Our study is motivated by the previously established result that light vector meson fields are required for this soliton in order to reasonably describe the spectrum of both the light and the heavy baryons. We note that the inclusion of light vector mesons significantly improves the agreement of the predicted hyperfine splitting with experiment. A number of aspects of this somewhat complicated calculation are discussed in detail.

Scaling behavior in soliton models

Abstract In the framework of chiral soliton models we study the behavior of static nucleon properties under rescaling of the parameters describing the effective meson theory. In particular we investigate the question of whether the Brown-Rho scaling laws are general features of such models. When going beyond the simple Skyrme model we find that restrictive constraints need to be imposed on the mesonic parameters in order to maintain these scaling laws. Furthermore, in the case when vector mesons are included in the model it turns out that the isoscalar form factor no longer scales according to these laws. Finally we note that, in addition to the exact scaling laws of the model, one may construct approximate local scaling laws , which depend of the particular choice of Lagrangian parameters.