Effective chiral restoration in the rho'-meson in lattice QCD

In simulations with dynamical quarks it has been established that the ground state rho in the infrared is a strong mixture of the two chiral representations (0,1)+(1,0) and (1/2,1/2)_b. Its angular momentum content is approximately the 3S1 partial wave which is consistent with the quark model. Effective chiral restoration in an excited rho-meson would require that in the infrared this meson couples predominantly to one of the two representations. The variational method allows one to study the mixing of interpolators with different chiral transformation properties in the non-perturbatively determined excited state at different resolution scales. We present results for the first excited state of the rho-meson using simulations with n_f=2 dynamical quarks. We point out, that in the infrared a leading contribution to rho'= rho(1450) comes from (1/2,1/2)_b, in contrast to the rho. Its approximate chiral partner would be a h_1(1380) state. The rho' wave function contains a significant contribution of the 3D1 wave which is not consistent with the quark model prediction.Comment: 4 pp, a few short remarks have been added, a reference updated. To appear in PR

Gedanken Worlds without Higgs: QCD-Induced Electroweak Symmetry Breaking

To illuminate how electroweak symmetry breaking shapes the physical world, we investigate toy models in which no Higgs fields or other constructs are introduced to induce spontaneous symmetry breaking. Two models incorporate the standard SU(3)_c x SU(2)_L x U(1)_Y gauge symmetry and fermion content similar to that of the standard model. The first class--like the standard electroweak theory--contains no bare mass terms, so the spontaneous breaking of chiral symmetry within quantum chromodynamics is the only source of electroweak symmetry breaking. The second class adds bare fermion masses sufficiently small that QCD remains the dominant source of electroweak symmetry breaking and the model can serve as a well-behaved low-energy effective field theory to energies somewhat above the hadronic scale. A third class of models is based on the left-right--symmetric SU(3)_c x SU(2)_L x SU(2)_R x U(1)_{B-L} gauge group. In a fourth class of models, built on SU(4)_{PS} x SU(2)_L x SU(2)_R gauge symmetry, lepton number is treated as a fourth color. Many interesting characteristics of the models stem from the fact that the effective strength of the weak interactions is much closer to that of the residual strong interactions than in the real world. The Higgs-free models not only provide informative contrasts to the real world, but also lead us to consider intriguing issues in the application of field theory to the real world.Comment: 20 pages, no figures, uses RevTeX; typos correcte

Chiral Perturbation Theory for τ→ρπντ\tau \to \rho \pi\nu_\tau, τ→K∗πντ\tau \to K^* \pi \nu_\tau, and τ→ωπντ\tau \to \omega \pi \nu_\tau

We use heavy vector meson $SU(2)_L \times SU(2)_R$ chiral perturbation theory to predict differential decay distributions for $\tau \rightarrow \rho \pi \nu_\tau$ and $\tau \rightarrow K^* \pi \nu_\tau$ in the kinematic region where $p_V \cdot p_\pi/m_V$ (here $V = \rho$ or $K^*$) is much smaller than the chiral symmetry breaking scale. Using the large number of colors limit we also predict the rate for $\tau \rightarrow \omega \pi \nu_\tau$ in this region (now $V = \omega$). Comparing our prediction with experimental data, we determine one of the coupling constants in the heavy vector meson chiral Lagrangian.Comment: 14 pages, latex 2e. We include the decay of the tau into the omega, pi minus and the tau neutrino, and extract a value for the coupling constant g2, using experimental dat

On the π\pi and KK as qqˉq \bar q Bound States and Approximate Nambu-Goldstone Bosons

We reconsider the two different facets of $\pi$ and $K$ mesons as $q \bar q$ bound states and approximate Nambu-Goldstone bosons. We address several topics, including masses, mass splittings between $\pi$ and $\rho$ and between $K$ and $K^*$, meson wavefunctions, charge radii, and the $K-\pi$ wavefunction overlap.Comment: 15 pages, late

Strong Decays of QQˉQ \bar Q Mesons

We present a detailed study of the two-body strong decays of light mesons. Both the space part and the spin-flavor-color part of the wave functions are generated algebraically and closed forms are obtained for all decays. Experimental deviations from our systematics are seen to be suggestive of both missing mesons and exotic QCD configurations.Comment: 24 pages (+6 figures, available from the authors), LATEX file, Yale preprint YCTP-N18-9

Comments on Diquarks, Strong Binding and a Large Hidden QCD Scale

We present arguments regarding diquarks possible role in low-energy hadron phenomenology that escaped theorists' attention so far. Good diquarks, i.e. the $0^{+}$ states of two quarks, are argued to have a two-component structure with one of the components peaking at distances several times shorter than a typical hadron size (a short-range core). This can play a role in solving two old puzzles of the 't Hooft 1/N expansion: strong quark mass dependence of the vacuum energy density and strong violations of the Okubo-Zweig-Iizuka (OZI) rule in the quark-antiquark $0^\pm$ channels. In both cases empiric data defy 't Hooft's 1/N suppression. If good diquarks play a role at an intermediate energy scale they ruin 't Hoofts planarity because of their mixed-flavor composition. This new scale associated with the good diquarks may be related to a numerically large scale discovered in [V. Novikov, M. Shifman, A. Vainshtein and V. Zakharov, Nucl. Phys. B 191, 301 (1981)] in a number of phenomena mostly related to vacuum quantum numbers and $0^\pm$ glueball channels. If SU(3)$_{\rm color}$ of bona fide QCD is replaced by SU(2)$_{\rm color}$, diquarks become well-defined gauge invariant objects. Moreover, there is an exact symmetry relating them to pions. In this limit predictions regarding good diquarks are iron-clad. If passage from SU(2)$_{\rm color}$ to SU(3)$_{\rm color}$ does not lead to dramatic disturbances, these predictions remain qualitatively valid in bona fide QCD.Comment: 18 pages, 3 figures; journal version, minor change

Unified description of light- and strange-baryon spectra

We present a chiral constituent quark model for light and strange baryons providing a unified description of their ground states and excitation spectra. The model relies on constituent quarks and Goldstone bosons arising as effective degrees of freedom of low-energy QCD from the spontaneous breaking of chiral symmetry. The spectra of the three-quark systems are obtained from a precise variational solution of the Schr\"odinger equation with a semirelativistic Hamiltonian. The theoretical predictions are found in close agreement with experiment.Comment: 9 pages, including 2 figure

Nonperturbative Light-Front QCD

In this work the determination of low-energy bound states in Quantum Chromodynamics is recast so that it is linked to a weak-coupling problem. This allows one to approach the solution with the same techniques which solve Quantum Electrodynamics: namely, a combination of weak-coupling diagrams and many-body quantum mechanics. The key to eliminating necessarily nonperturbative effects is the use of a bare Hamiltonian in which quarks and gluons have nonzero constituent masses rather than the zero masses of the current picture. The use of constituent masses cuts off the growth of the running coupling constant and makes it possible that the running coupling never leaves the perturbative domain. For stabilization purposes an artificial potential is added to the Hamiltonian, but with a coefficient that vanishes at the physical value of the coupling constant. The weak-coupling approach potentially reconciles the simplicity of the Constituent Quark Model with the complexities of Quantum Chromodynamics. The penalty for achieving this perturbative picture is the necessity of formulating the dynamics of QCD in light-front coordinates and of dealing with the complexities of renormalization which such a formulation entails. We describe the renormalization process first using a qualitative phase space cell analysis, and we then set up a precise similarity renormalization scheme with cutoffs on constituent momenta and exhibit calculations to second order. We outline further computations that remain to be carried out. There is an initial nonperturbative but nonrelativistic calculation of the hadronic masses that determines the artificial potential, with binding energies required to be fourth order in the coupling as in QED. Next there is a calculation of the leading radiative corrections to these masses, which requires our renormalization program. Then the real struggle of finding the right extensions to perturbation theory to study the strong-coupling behavior of bound states can begin.Comment: 56 pages (REVTEX), Report OSU-NT-94-28. (figures not included, available via anaonymous ftp from pacific.mps.ohio-state.edu in subdirectory pub/infolight/qcd