Narrow Results

By means of unitarized Chiral Perturbation Theory it is possible to obtain a remarkable description of meson–meson scattering amplitudes up to 1.2 GeV, and generate poles associated to scalar and vector resonances. Since Chiral Perturbation Theory is the QCD low energy effective theory, it is then possible to study its large-N_c limit where states are easily identified. The vectors thus generated follow closely a behavior, whereas the light scalar poles follow the large-N_c behavior expected for a dominant tetraquark or two-meson structure.

A collisional model for hadron resonances appearing in hadron collisions is proposed. The given approach leads to a simple explanation of the scalar sector below 1 GeV with correct predictions for masses and dominant decay modes.

A collective treatment of the I=0 scalar mesons below 2 GeV [σ(550), f₀(980), f₀(1370), f₀(1500) and f₀(1710)] in a nonlinear chiral Lagrangian framework that is constrained by the mass and the partial decay widths of the I=1/2,1 scalars [κ(900), , a₀(980) and a₀(1450)] is presented. The sub-structure of these states in terms of two and four quark components, as well as a glueball component is explored, and its correlation with the mass of f₀(1370) is studied. Consistency with the available experimental data suggests that the σ(550) is dominantly a nonstrange four-quark state, whereas the sub-structure of other I=0 states are sensitive to the input mass of f₀(1370). This investigation estimates the scalar glueball mass in the range 1.47–1.64 GeV.

We have analysed properties of two resonances a₀(980) and a₀(1450) using the πη and coupled channel model. Although the forces in the scalar-isovector channel are attractive the a₀(980) resonance cannot be interpreted as a kaon-antikaon bound state within our model.

We present a quark-quark interaction for the complete study of the meson spectra, from the light to the heavy sector. We compare the quark model predictions against well-established experimental data. This allows to identify discrepancies between quark model results and experiment that may signal physics beyond conventional hadron spectroscopy.

Despite the apparent simplicity of meson spectroscopy there are some states which cannot be accommodated in the usual structure. Among them there are either exotic states as the X(1600) or the recently measured charm states and X(3872) and some of the light scalar mesons. In this work we present a possible description of these states in terms of tetraquarks.

The KLOE experiment at the Frascati ϕ-factory DAΦNE has collected about 0.5 fb^-1 of data till the end of the year 2002. These data allow to perform a wide physics program, ranging from the physics of charged and neutral kaons to radiative ϕ-decays. Results are presented for the K_L lifetime and the semileptonic processes K_S,L→πeν. From the light meson spectroscopy program, results on the decays ϕ→f₀(980)γ, a₀(980)γ as well as ϕ→ηγ, η′γ are presented.

We present a short review of the current understanding of the scalar meson sector, with special attention to the problem of the low-mass σ. The dispersion relation N/D-method used for the restoration of the low-energy ππ (IJ^PC = 00⁺⁺)-wave amplitude is discussed. The low-energy ππ amplitude was determined from the data in the energy region 280–500 MeV and it was sewn with the previously found K-matrix solution for the region 450–1950 MeV. The N/D-amplitude has a pole on the second sheet of the complex-s plane, near the ππ threshold at , that corresponds to the low-mass σ-meson. We discuss the hypothesis that this pole may be related to the confinement forces, thus being the eyewitness of confinement.

The production of pairs has been investigated at ANKE (COSY-Jülich) in the reaction at excitation energies Q of 47.4 and 104.7 MeV. Partial wave analyses of the invariant mass and angular spectra show the dominance of the a₀-channel ( configuration). In the non-isospin selective pn → dK⁺K^- reaction the scalar production is strongly suppressed with respect to ϕ vector mesons. A measurement of the isoscalar production in the isospin selective reaction dd → α K⁺K^- has been performed in April 2006.

The novel point of this analysis is a direct use of the Reggeon exchange technique for the description of the reactions πN → two mesons + N at large energies of the initial pion. This approach allows us to describe simultaneously distributions over M (invariant mass of two mesons) and t (momentum transfer squared to nucleons). Making use of this technique, the following resonances (as well as corresponding bare states), produced in the πN→π⁰π⁰N reaction, are studied: f₀(980), f₀(1300) [f₀(1370) in PDG notation], f₀(1200–1600), f₀(1500), f₀(1750), f₂(1270), f₂(1525), f₂(1565), f₂(2020), f₄(2025). Adding data for the reactions (at rest, from liquid H₂) →π⁰π⁰π⁰, π⁰π⁰η, π⁰ηη and (at rest, from gaseous H₂) →π⁰π⁰π⁰, π⁰π⁰η, π⁰ηη, we have performed a simultaneous K matrix fit of two-meson spectra in all these reactions. The results of combined fits to the above-listed isoscalar f_J states and to isovector ones, a₀(980), a₀(1475), a₂(1320), are presented.

We discuss a possible interpretation of the scalar mesons f₀(980) and a₀(980) as hadronic molecules - bound states of K and mesons. Using a phenomenological Lagrangian approach we calculate strong as well as the electromagnetic decay properties of both scalars. The covariant and gauge invariant model, which also allows for finite size effects of the hadronic molecule, delivers results in good agreement with experimental data.

We review how the use of recent precise data on kaon decays together with forward dispersion relations (FDR) and Roy's equations allow us to determine the sigma resonance pole position very precisely, by using only experimental input. In addition, we present preliminary results for a modified set of Roy-like equations with only one subtraction, that show a remarkable improvement in the precision around the σ region. We also improve the matching between the parametrizations at low and intermediate energy of the S0 wave, and show that the effect of this on the sigma pole position is negligible.

The charge-exchange reaction π^-p → ω(780)ɸ(1020)n is studied with the VES setup. The (ωΦ) system is observed at relatively low background. Its invariant mass distribution peaks near threshold. The two-particles partial wave analyses shows that the J^PC = 0⁺⁺ state dominates. Comparison with scalar wave in ωω system at the comparable mass gives evidence for violation of the OZI rule. Another subject is a search for possible transition f₀(980) → a₀(980), which violates the isospin symmetry.

We construct the amplitudes of $\pi \eta$ photoproduction taking into account the effects of the $\pi \eta \text{–}K\bar{K}$ interchannel coupling. The idea of our model is to describe the scalar isovectors as dynamically produced in the final state while the initial stage of the reaction being described in terms of meson exchanges. Meson loops which arise this way include not only pseudoscalars but also vector mesons. These amplitudes are used to calculate the $S$-wave cross-sections and mass distributions in the $\pi \eta$ effective mass region corresponding to the scalar resonances $a_0(980)$ and $a_0(1450)$. The values we obtained for $a_0(980)$ are comparable with predictions of other models while the cross-section for $a_0(1450)$ is about an order of magnitude larger than prediction based on the quark model. We show that the amplitudes with loops containing vector mesons calculated in the on-shell approximation are not suppressed in contrast to amplitudes containing only pseudoscalar loops. We also estimate the cross-sections for the $P$- and $D$-waves in the $\pi \eta$ channel.

First radial excitations of the scalar and pseudoscalar meson nonets as well as their ground states are described as quark–antiquark bound systems in the framework of a nonlocal chiral quark model of the NJL type. Glueball states and their possible mixing with members of the multiplets are not considered.

Radial excitations are described by means of simple polynomial form factors in the momentum–space which are introduced into the four-quark vertices of the quark Lagrangian. The form factors are chosen in the regime of spontaneous chiral symmetry breaking so that the gap equations do not change their standard form, which provides fulfillment of the low energy theorems. All free parameters of the form factors are fixed by experimentally observed masses of pseudoscalar mesons. All masses of the scalar mesons are predicted.

The local six-quark 't Hooft interaction is introduced to solve the so-called U_A(1)-problem. The (four particle) singlet–octet mixing for the ground and radially excited states of scalar and pseudoscalar mesons is taken into account. The widths of the main strong decays of the first radial excitation of the scalar meson nonet are calculatedin satisfactory agreement with experimental data.

Classification of the considered meson nonets is given. It is shown that the meson states a₀(1450), f₀(1370), f_J(1710) and are the first radial excitations of a₀(980), f₀(400-1200), f₀(980), .

The isosinglet scalar mesons f₀(1710), f₀(1500), f₀(1370) and their mixing are studied. Two recent lattice results are employed as the starting point; one is the approximate SU(3) symmetry in the scalar sector above 1 GeV for the connected insertion part without annihilation, and the other is the scalar glueball mass at 1710 MeV in the quenched approximation. In the SU(3) symmetry limit, f₀(1500) becomes a pure SU(3) octet and is degenerate with a₀(1450), while f₀(1370) is mainly an SU(3) singlet with a slight mixing with the scalar glueball which is the primary component of f₀(1710). These features remain essentially unchanged even when SU(3) breaking is taken into account. The observed enhancement of ωf₀(1710) production over ϕf₀(1710) in hadronic J/ψ decays and the copious f₀(1710) production in radiative J/ψ decays lend further support to the prominent glueball nature of f₀(1710).

New results concerning the scalar mesons f₀(980) and σ(600) produced in the ϕ → ππγ decays are presented. They are obtained from an integrated luminosity of ~ 400 pb^-1 of KLOE data.

We study the quark structure of the sigma meson through the decay of D₁(2430) meson by constructing an effective Lagrangian for charmed mesons interacting with light mesons based on the chiral symmetry and heavy quark symmetry. Within the linear realization of the chiral symmetry, we include the P-wave charmed mesons (D₁(2430), D₀(2400)) as the chiral partners of (D., D), and the light scalar mesons as the chiral partner of the pseudoscalar mesons. In the light meson sector, both the and states are incorporated respecting their different U(1)_A transformation properties. We predict the D₁ → D_ππ decay width with two pions in the I = 0, l = 0 channel, which can be tested in the future experiment. We find that the width increases with the percentage of the content in the sigma meson.