Simon Rodriguez

Chiral model for q ¯ q and q q ¯ q q mesons

We point out that the spectrum of pseudoscalar and scalar mesons exhibits a quasidegenerate chiral nonet in the energy region around 1.4 GeV whose scalar component has a slightly inverted spectrum. Based on the empirical linear rising of the mass of a hadron with the number of constituent quarks which yields a mass around 1.4 GeV for tetraquarks, we conjecture that this quasichiral nonet arises from the mixing of a chiral nonet composed of tetraquarks with conventional q-barq states. We explore this possibility in the framework of a chiral model assuming a tetraquark chiral nonet around 1.4 GeV with chiral symmetry realized directly. We stress that U{sub A}(1) transformations can distinguish q-barq from tetraquark states, although it cannot distinguish specific dynamics in the later case. We find that the measured spectrum is consistent with this picture. In general, pseudoscalar states arise as mainly q-barq states but scalar states turn out to be strong admixtures of q-barq and tetraquark states. We work out also the model predictions for the most relevant couplings and calculate explicitly the strong decays of the a{sub 0}(1450) and K{sub 0}*(1430) mesons.

UA(1) SYMMETRY BREAKING AND THE SCALAR SECTOR OF QCD

It is shown that most of the unusual properties of the lowest lying scalar (and pseudoscalar) mesons can be understood, at the qualitative and quantitative level, on the basis of the breakdown of the UA(1) symmetry coupled to the vacuum expectation values of scalars by the spontaneous breaking of chiral symmetry.

A chiral model for bar{q}q and bar{q}bar{q}qq

We point out that the spectrum of pseudoscalar and scalar mesons exhibits a quasidegenerate chiral nonet in the energy region around 1.4 GeV whose scalar component has a slightly inverted spectrum. Based on the empirical linear rising of the mass of a hadron with the number of constituent quarks which yields a mass around 1.4 GeV for tetraquarks, we conjecture that this quasichiral nonet arises from the mixing of a chiral nonet composed of tetraquarks with conventional q-barq states. We explore this possibility in the framework of a chiral model assuming a tetraquark chiral nonet around 1.4 GeV with chiral symmetry realized directly. We stress that U{sub A}(1) transformations can distinguish q-barq from tetraquark states, although it cannot distinguish specific dynamics in the later case. We find that the measured spectrum is consistent with this picture. In general, pseudoscalar states arise as mainly q-barq states but scalar states turn out to be strong admixtures of q-barq and tetraquark states. We work out also the model predictions for the most relevant couplings and calculate explicitly the strong decays of the a{sub 0}(1450) and K{sub 0}*(1430) mesons.

mesons

We develop a second order formalism for massive spin 1/2 fermions based on the projection over Poincare invariant subspaces in the ((1/2),0)+(0,(1/2)) representation of the homogeneous Lorentz group. Using the U(1){sub em} gauge principle we obtain a second order description for the electromagnetic interactions of a spin 1/2 fermion with two free parameters, the gyromagnetic factor g and a parameter {xi} related to odd-parity Lorentz structures. We calculate Compton scattering in this formalism. In the particular case g=2, {xi}=0, and for states with well-defined parity, we recover Dirac results. In general, we find the correct classical limit and a finite value r{sub c}{sup 2} for the forward differential cross section, independent of the photon energy and of the value of the parameters g and {xi}. The differential cross section vanishes at high energies for all g, {xi} except in the forward direction. The total cross section at high energies vanishes only for g=2, {xi}=0. We argue that this formalism is more convenient than Dirac theory in the description of low energy electromagnetic properties of baryons and illustrate the point with the proton case.

Second order formalism for spin12fermions and Compton scattering

We study the two photon decay of neutral scalars below 1.5 GeV in the context of a recently proposed chiral model for bar{q}q and bar{q}bar{q}qq states. We find good agreement with experimental results for the a_{0}(980)->gamma gamma. Our calculations for f_{0}(980)->gamma gamma shows that further work is necessary in order to understand the structure of this meson. The model predicts Gamma(a_{0}(1450)->gamma gamma)=0.16+/-0.10KeV, Gamma(sigma->gamma gamma)=0.47+/-0.66 KeV, Gamma(f(1370)->gamma gamma)=0.07+/-0.15 KeV, Gamma(f(1500)->gamma gamma)=0.74+/-0.78 KeV.

Two photon decay of neutral scalars below 1.5 GeV in a chiral model for bar{q}q and bar{q}bar{q}qq states

Abstract We point out that meson spectrum indicates the existence of a degenerate chiral nonet in the energy region around 1.4 GeV with a slightly inverted spectrum with respect to a q ¯ q nonet. Based on this observation and the approximately linear rising of the mass of a hadron with the number of constituent quarks we conjecture the existence of a tetraquark chiral nonet in this energy region with chiral symmetry implemented directly. We realize this idea in a chiral model and take into account the mixing of the tetraquark chiral nonet with a conventional q ¯ q nonet. We find that the mass spectrum of mesons below 1.5 GeV is consistent with this picture. In general, pseudoscalar states arise as mainly q ¯ q states but scalar states turn out to be strong admixtures of q ¯ q and tetraquark states.

A chiral q¯q¯qq nonet?

We study the two photon decay of neutral scalars below 1.5 GeV in the context of a recently proposed chiral model for bar{q}q and bar{q}bar{q}qq states. We find good agreement with experimental results for the a_{0}(980)->gamma gamma. Our calculations for f_{0}(980)->gamma gamma shows that further work is necessary in order to understand the structure of this meson. The model predicts Gamma(a_{0}(1450)->gamma gamma)=0.16+/-0.10KeV, Gamma(sigma->gamma gamma)=0.47+/-0.66 KeV, Gamma(f(1370)->gamma gamma)=0.07+/-0.15 KeV, Gamma(f(1500)->gamma gamma)=0.74+/-0.78 KeV.

Two photon decay of neutral scalars below 1.5 GeV in a chiral model for q q and qqqq states

We study the two photon decay of neutral scalars below 1.5 GeV in the context of a recently proposed chiral model for bar{q}q and bar{q}bar{q}qq states. We find good agreement with experimental results for the a_{0}(980)->gamma gamma. Our calculations for f_{0}(980)->gamma gamma shows that further work is necessary in order to understand the structure of this meson. The model predicts Gamma(a_{0}(1450)->gamma gamma)=0.16+/-0.10KeV, Gamma(sigma->gamma gamma)=0.47+/-0.66 KeV, Gamma(f(1370)->gamma gamma)=0.07+/-0.15 KeV, Gamma(f(1500)->gamma gamma)=0.74+/-0.78 KeV.

Two photon decay of neutral scalars below 1.5 GeV in a chiral model for anti-q q and anti-qq qq states

We point out that the spectrum of pseudoscalar and scalar mesons exhibits a quasidegenerate chiral nonet in the energy region around 1.4 GeV whose scalar component has a slightly inverted spectrum. Based on the empirical linear rising of the mass of a hadron with the number of constituent quarks which yields a mass around 1.4 GeV for tetraquarks, we conjecture that this quasichiral nonet arises from the mixing of a chiral nonet composed of tetraquarks with conventional q-barq states. We explore this possibility in the framework of a chiral model assuming a tetraquark chiral nonet around 1.4 GeV with chiral symmetry realized directly. We stress that U{sub A}(1) transformations can distinguish q-barq from tetraquark states, although it cannot distinguish specific dynamics in the later case. We find that the measured spectrum is consistent with this picture. In general, pseudoscalar states arise as mainly q-barq states but scalar states turn out to be strong admixtures of q-barq and tetraquark states. We work out also the model predictions for the most relevant couplings and calculate explicitly the strong decays of the a{sub 0}(1450) and K{sub 0}*(1430) mesons.

Chiral model for q-barq and qq-barqq mesons

We point out that the spectrum of pseudoscalar and scalar mesons exhibits a quasidegenerate chiral nonet in the energy region around 1.4 GeV whose scalar component has a slightly inverted spectrum. Based on the empirical linear rising of the mass of a hadron with the number of constituent quarks which yields a mass around 1.4 GeV for tetraquarks, we conjecture that this quasichiral nonet arises from the mixing of a chiral nonet composed of tetraquarks with conventional q-barq states. We explore this possibility in the framework of a chiral model assuming a tetraquark chiral nonet around 1.4 GeV with chiral symmetry realized directly. We stress that U{sub A}(1) transformations can distinguish q-barq from tetraquark states, although it cannot distinguish specific dynamics in the later case. We find that the measured spectrum is consistent with this picture. In general, pseudoscalar states arise as mainly q-barq states but scalar states turn out to be strong admixtures of q-barq and tetraquark states. We work out also the model predictions for the most relevant couplings and calculate explicitly the strong decays of the a{sub 0}(1450) and K{sub 0}*(1430) mesons.