η(1475) and <Emphasis Type="Italic">f</Emphasis><Subscript>1</Subscript>(1420) resonances in the decay processes <Emphasis Type="Italic">J</Emphasis>/ψ → γ(ρρ, γρ<Superscript>0</Superscript>, γϕ) and in γγ* collisions

A scenario that removes the contradiction between the suppression of the η(1475) → γγ decay width and the strong coupling of η(1475) to the ρρ, ωω, and γρ⁰ channels and which leads to a nontrivial prediction for the manifestation of η(1475) in γγ*(Q ²) collisions is considered. Data on the dependence of the cross section for the reaction γγ*(Q ²) → K[`(K)]pK\bar K\pi on the photon virtuality in the energy range 1.35–1.55 GeV are explained here by the production of an η(1475) resonance in contrast to their standard interpretation in terms of the f <sub>1</sub>(1420) resonance. Experimental verification of the present explanation requires determining the spin-parity of resonance contributions, R, in the reactions γγ*(Q <sup>2</sup>) R → R → K[`(K)]pK\bar K\pi and J/ψ → γR → γ(γρ⁰, γϕ).     

Contributions of qqqq\bar{q} Components to Axial Charges of Proton and N(1440)

The axial charges of the proton and N(1440) are studied in the framework of an extended constituent quark model (CQM) includingqqqq\bar{q} components. The cancellation between the contributions of qqq components and qqqq\bar{q} components gives a natural explanation to the experimental value of the proton axial charge, which can not be well reproduced in the traditional CQM even after the SU(6) \otimes O(3) symmetry breaking is taken into account. The experimental value of axial charge pins down the proportion of the qqqq\bar{q} component in the proton to about 20%, which is consistent with the ones given by the strong decay widths and helicity amplitudes. Besides, an axial charge for N(1440) about 1 is predicted with 30\% qqqq\bar{q} component, which is obtained by the strong and electromagnetic decays.     

Photoproduction of η -mesons off protons

Photoproduction of η -mesons was studied with the Crystal-Barrel detector at ELSA for photon energies in the range from 0.75 to 3GeV. Total and differential cross-sections are presented. The η mesons are detected in two decay modes, in η → 2γ and in η → 3π⁰ → 6γ . The cross-sections vary slowly as functions of energy and η production angle suggesting that only few resonances make significant contributions to the cross-section. For photon energies above 1.5GeV, the differential cross-sections show a strong forward peak due to meson exchanges in the t channel. A comparison of the η -photoproduction cross-sections with SAID and MAID as well as with a partial-wave analysis performed on our data in combination with other data sets shows good agreement. The Bonn-Gatchina partial-wave analysis suggests that the largest contribution to η photoproduction for energies below 3GeV proceeds via three resonances, the well-known N(1535)S ₁₁ , via N(1720)P ₁₃ , and a newly suggested N(2070)D ₁₅ .     

On the role of the final-state interactions in rare <Emphasis Type="Italic">B</Emphasis> decays

The effects of final-state interactions (FSI) in hadronic B decays are investigated. The model for FSI, based on Regge phenomenology of high-energy hadronic interactions, is proposed. It is shown that this model explains the pattern of phases in matrix elements of B → ππ and B → ρρ decays. These phases play an important role for CP violation in B decays. The most precise determination of the unitarity triangle angle α from B _d → ρπ decays is performed. The relation between CP asymmetries in B → Kπ decays is discussed. It is emphasized that the large-distance FSI can explain the structure of polarizations of the vector mesons in B decays and other puzzles like a very large branching ratio of the B decay to $ \bar \Xi $ \bar \Xi _c Λ _c .     

Analysis of angular distributions in γN → πeta;N

Angular distributions in the final state of π⁰η photoproduction on nucleons are considered. As a formal base the familiar isobar model is used in which the π⁰ηN state is a product of the resonance decay into ηΔ(1232) and πS ₁₁(1535) channels. One of the principal assumptions used is that in the actual energy region the reaction is dominated by a single resonance state. The developed formalism can serve as a tool for testing spin and parity of that resonance.     

Measurement of η photoproduction on the proton from threshold to 1500 MeV

Beam asymmetry and differential cross-section for the reaction γp → ηp were measured from production threshold to 1500MeV photon laboratory energy. The two dominant neutral decay modes of the η -meson, η → 2γ and η → 3π⁰ , were analyzed. The full set of measurements is in good agreement with previously published results. Our data were compared with three models. They all fit satisfactorily the results but their respective resonance contributions are quite different. The possible photoexcitation of a narrow state N(1670) was investigated and no evidence was found.     

Mechanism of the production of the <Emphasis Type="Italic">a</Emphasis><Subscript>0</Subscript>(980) resonance in the γγ → π<Superscript>0</Superscript>η reaction

High-statistics Belle data on the γγ → π⁰η reaction have been analyzed in order to reveal the mechanism of two-photon production and the nature of the a ₀(980) resonance. The solution obtained for the γγ → π⁰η amplitude is in agreement with the predictions of the chiral theory for the πη-scattering length; with the strong coupling of the a ₀(980) resonance with the πη, K $ \bar K $ \bar K , and πη′ channels; and with the key role of the a ₀(980) → (K $ \bar K $ \bar K + π⁰η + π⁰η′) → γγ rescattering mechanisms in the a ₀(980) → γγ decay. This picture is much in favor of the q ² $ \bar q $ \bar q ² nature of a ₀(980) resonance and is consistent with the properties of its partners, σ₀(600) and f ₀(980) resonances, in particular, with those manifested in the γγ → ππ reactions. The important role of vector exchanges in the formation of the nonresonant background in the γγ → π⁰η reaction has been revealed. Preliminary information on the π⁰η → π⁰η reaction has been obtained.     

Scalar mesons in the decays η′ → 3π0 and η′ → π0π+π?

The decays η → 3π ⁰ and η → π ⁰ π ⁺ π ⁻ are considered within the isobar model. It is shown that, in order to explain the branching ratio and the shape of the Dalitz plot for the decay η′ → 3π ⁰, it is sufficient to take into account the contributions of the σ and a ₀ mesons. The inclusion of the σ meson is necessary for reproducing the shape of the distribution over the Dalitz plot. The branching ratio for the decay η′ → π ⁰ π ⁺ π ⁻ is obtained. The predictions for the distributions over the Dalitz plot for this decay are presented. These predictions depend strongly on model parameters.     

Confinement singularities in two-pion decays of mesons

We consider as an example the two-pion decay of the ρ meson, the ³ S ₁ q $ \bar q $ \bar q state of the constituent quarks—the decay being determined by the transition q $ \bar q $ \bar q → ππ contains information about confinement interactions. One can specify in this decay two types of transitions: (i) the bremsstrahlung radiation of a pion q → q + π (or $ \bar q $ \bar q → $ \bar q $ \bar q + π) with a subsequent fusion q $ \bar q $ \bar q → π, and (ii) the direct transition q $ \bar q $ \bar q → ππ. We demonstrate how in the amplitudes of the corresponding transitions the quark singularities have to disappear, i.e., what is the way the quark confinement at relatively short distances can be realized. We calculate and estimate the contributions of processes with bremsstrahlung radiation of the pion and of the direct transition q $ \bar q $ \bar q → ππ. The estimates demonstrate that the processes involving the direct transition q $ \bar q $ \bar q → ππ are necessary, but they cannot be determined unambiguously by the decay ρ(775) → ππ. We conclude that for the determination of the q $ \bar q $ \bar q → ππ transition more complete data on the resonance decays into the ππ channels are needed than those available at the moment.     

Hadrons and broken symmetries with WASA-at-COSY

The WASA Detector Facility is an internal experiment at the cooler synchrotron (COSY) in Jülich, Germany. The COSY accelerator provides proton and deuteron beams with momenta up to 3.7 GeV/c giving access to hadron physics including the strange quark sector. The physics program with the WASA detector involves hadron dynamics and hadron structure. Key experiments address fundamental symmetries and symmetry violations via the study of rare and not-so-rare meson decays. From the very first production run, results on the Dalitz plot slope parameter in the isospin violating η → 3π ⁰ decay have been obtained. The 3π ⁰ final state is also used to study meson production mechanisms. Investigations of other decay modes of the η-meson address C, P, and T symmetries and combinations. Higher orders in chiral perturbation theory are probed with the η → π ⁰ decay. The status and plans for studying hadron structure with Dalitz decays of mesons are presented.     

The model-independent analysis indications for spectroscopy of scalar mesons. Proof for the K0* (900)

In a model-independent approach the data on ππ → ππ, K $ \bar K $ \bar K , ηη, ηη′ in the I ^G J ^PC = 0⁺0⁺⁺ channel and on the Kπ scattering in the $ I\left( {J^P } \right) = \frac{1} {2}\left( {0^ + } \right) $ I\left( {J^P } \right) = \frac{1} {2}\left( {0^ + } \right) channel are analyzed jointly for studying the status and QCD nature of the f ₀- and the K*₀-mesons. It is shown that in the 1500-MeV region, there are two states, wide (interpreted as a glueball) and narrow (q $ \bar q $ \bar q ). In the Kπ-scattering data analysis, the proof for the K*₀(900) is given.     

The cusp effect in <Emphasis Type="Italic">η</Emphasis>′→<Emphasis Type="Italic">η</Emphasis><Emphasis Type="Italic">π</Emphasis><Emphasis Type="Italic">π</Emphasis> decays

Strong final-state interactions create a pronounced cusp in η′→η π ⁰ π ⁰ decays. We adapt and generalize the non-relativistic effective field theory framework developed for the extraction of π π scattering lengths from K→3π decays to this case. The cusp effect is predicted to have an effect of more than 8% on the decay spectrum below the π ⁺ π ⁻ threshold.     

η, h￠ \eta{^\prime} → πl+l- in a chiral unitary approach

The decays η, → π⁺π^- l ⁺ l ^- (with l = e,μ are investigated within a chiral unitary approach which combines the chiral effective Lagrangian with a coupled-channels Bethe-Salpeter equation. Predictions for the decay widths and spectra are given.     

Photoproduction of η -mesons off nuclei for Eγ ? 2.2 GeV

The photoproduction of η -mesons off ¹²C , ⁴⁰Ca , ⁹³Nb , and ^nat Pb nuclei has been measured with a tagged photon beam with energies between 0.6 and 2.2GeV. The experiment was performed at the Bonn ELSA accelerator with the combined setup of the Crystal Barrel and TAPS calorimeters. It aimed at the in-medium properties of the S ₁₁(1535) nucleon resonance and the study of the absorption properties of nuclear matter for η -mesons. Careful consideration was given to contributions from ηπ final states and secondary production mechanisms of η -mesons, e.g. from inelastic πN reactions of intermediate pions. The analysis of the mass number scaling shows that the nuclear absorption cross-section for η -mesons is constant over a wide range of the η momentum. The comparison of the excitation functions to data off the deuteron and to calculations in the framework of a BUU model show no unexplained in-medium modifications of the S ₁₁(1535) .     

a 0 + (980)-resonance production in the reaction pp → dπ + η close to the K\bar K threshold

The reaction pp → dπ ⁺ η has been measured at a beam energy of T _p = 2.65 GeV (p _p = 3.46 GeV/c) using the ANKE spectrometer at COSY-Julich. The missing-mass distribution of the detected dπ ⁺ pairs exhibits a peak around the η mass on top of a strong background of multipion pp → dπ ⁺(nπ) events. The differential cross section d ⁴ σ/dΩ_d dΩ _π ⁺ dp _d dp _π ⁺ for the reaction pp → dπ ⁺ η has been determined model independently for two regions of phase space. Employing a dynamical model for the a ₀ ⁺ production allows one then to deduce a total cross section of σ(pp → da ₀ ⁺ → dπ ⁺ η) = 1.1 ± 0.3_stat ± 0.7_syst μb for the production of π ⁺ η via the scalar a ₀ ⁺ (980) resonance and σ(pp → dπ ⁺ η) = 3.5 ± 0.3_stat ± 1.0_syst μb for the nonresonant production. Using the same model as for the interpretation of recent results from ANKE for the reaction , the ratio of the total cross sections is , which is in agreement with branching ratios in the literature. The text was submitted by the authors in English.     

Effective Lagrangians and IG(JPC)=1?(1?+) exotic resonance state in the VP and the PP channel

For I ^G (J ^PC)=1⁻(1⁻⁺) exotic waves in the reactions ρπ → ρπ, ρπ → ηπ, ρπ → η′π, and ρπ → and in allied reactions, a model is constructed that satisfies the conditions of unitarity and analyticity and which employs, as an input, an “anomalous” nondiagonal VPPP interaction relating the ρπ, ηπ, η′π, and channels. The possibility of obtaining, within this simple field-theoretical model, a resonance behavior of the I ^G(J ^PC)=1⁻(1⁻⁺) amplitudes corresponding to the {10} − and {8} representations of the SU(3) group and their mixing is demonstrated explicitly in the mass range 1.3–1.6 GeV, where data of present-day experiments suggest a rich exoticism. __________ Translated from Yadernaya Fizika, Vol. 65, No. 3, 2002, pp. 579–588. Original Russian Text Copyright ? 2002 by Achasov, Shestakov.     

Electromagnetic corrections in <Emphasis Type="Italic">η</Emphasis>→3<Emphasis Type="Italic">π</Emphasis> decays

We re-evaluate the electromagnetic corrections to η→3π decays at next-to-leading order in the chiral expansion, arguing that effects of order e ²(m _u −m _d ) disregarded so far are not negligible compared to other contributions of order e ² times a light-quark mass. Despite the appearance of the Coulomb pole in η→π ⁺ π ⁻ π ⁰ and cusps in η→3π ⁰, the overall corrections remain small.     

Search for the f1(1285) → π+π-π0 decay with the VES detector

The isospin-violating decay f ₁(1285) → π⁺π^-π⁰ has been studied at the VES facility. This study is based on the statistics acquired in π^- Be interactions at 27, 36.6 and 41 GeV/c in the diffractive reaction π^- N → (f ₁π^-)N . The f ₁(1285) → π⁺π^-π⁰ decay is observed. The preliminary ratio of decay probabilities BR(f ₁(1285) → π⁺π^-π⁰) to BR(f ₁(1285) → ηπ⁺π^-)^. BR(η → γγ) is ∼ 2% . Original article based on material presented at HADRON 2007.     

Analysis of the decays ψ′ → J/ψπ+π? and η′c → ηcπ+π? with the heavy-quark symmetry

In this article, we study the decays ψ′ → J/ψπ ⁺ π ⁻ and η′_c → η _c π ⁺ π ⁻ by taking into account the chiral symmetry breaking effects, the final-state interactions and the heavy-quark symmetry. We can confront the predictions of the η′_c → η _c π ⁺ π ⁻ decay width and differential decay width with the experimental data in the future, and obtain powerful constraints on the chiral breaking effects and the final-state interactions, and test the heavy-quark symmetry.     

Excitation of the Roper resonance in single- and double-pion production in nucleon-nucleon collisions⋆

In most investigations the Roper resonance is sensed only very indirectly via complex partial-wave analyses. We find indications for its excitation in the invariant nπ⁺ mass spectrum of the pp → npπ⁺ reaction at M ≈ 1360 MeV with a width of ≈ 150 MeV . The values fit very favorably to the most recent phase shift results as well as to the observations at BES. In the near-threshold two-pion production pp → ppπ⁰π⁰ , where the Roper excitation and its subsequent decays via the routes N ^* → Δπ → Nππ and N ^* → Nσ are the only dominant processes, we find its direct decay into the Nσ channel to be the by far dominating decay process --in favor of a monopole excitation of the Roper resonance.