Photoproduction of η and η' mesons off nucleons close to threshold

The importance of Born terms and resonance exchange for η and η' photoproduction off both the proton and neutron within U(3) baryon chiral perturbation theory is investigated. Low-lying resonances such as the vector mesons and J ^P = 1/2⁺, 1/2^- baryon resonances are included explicitly and their contributions together with the Born terms are calculated. The coupling constants of the resonances are determined from strong and radiative decays. We obtain reasonable agreement with experimental data near threshold. Received: 17 March 2000 / Accepted: 8 September 2000     

Dynamically generated resonances from the vector octet-baryon octet interaction

We study the interaction of vector mesons with the octet of stable baryons in the framework of the local hidden gauge formalism using a coupled-channels unitary approach. We examine the scattering amplitudes and their poles, which can be associated to known J ^P = 1/2^-, 3/2^- baryon resonances, in some cases, or give predictions in other ones. The formalism employed produces doublets of degenerate J ^P = 1/2^-, 3/2^- states, a pattern which is observed experimentally in several cases. The findings of this work should also be useful to guide present experimental programs searching for new resonances, in particular in the strange sector where the current information is very poor.     

���Ѧ� Interaction Leading to N* and ��* Resonances

We have performed a calculation for the three-body ????? system by using the fixed center approximation to Faddeev equations, taking the interaction between ?? and ??, ?? and ??, and ?? and ?? from the chiral unitary approach. We find several peaks in the modulus squared of the three-body scattering amplitude, indicating the existence of resonances, which can be associated to known I?=?1/2, 3/2 and J ^P ?=?1/2⁺, 3/2⁺ and 5/2⁺ baryon states.     

Non-diffractive production of meson resonances

We study the peripheral cross sections of resonances that cannot be produced by π-exchange. In particular, we concentrate on the four meson nonets expected as L = 1 quark states (i.e., the J^P = 0⁺π_N(980); J^P = 1⁺ A₁, B; J^P = 2⁺ A₂). We use SU(3), Regge poles, factorization, exchange degeneracy, pole extrapolation, and the vector-meson-photon analogy. We predict the cross sections in both photoproduction and non-diffractive hadronic reactions. In passing, we discuss the large unnatural-parity (B, K?Q_B) exchange contributions and even the possibility of studying ππ → πω while avoiding the B production background.     

Inclusive production cross sections of resonances in 32 GeV/c K+p interactions

The inclusive production of resonances is systematically studied in K⁺p interactions at 32 GeV/c. Total production cross sections are given for three baryon resonances, five vector and three 2⁺ tensor mesons. We also compare the central and fragmentation components of the total production cross sections with quark model predictions.     

A partial-wave analysis of the neutral 3π system produced in the reaction π−p → (π+π−π0) n at 12 and 15 GeV/c

Data on the charge-exchange reaction π^?p → (π⁺π^?π⁰)n have been taken at beam momenta of 12 and 15 GeV/c, using the CERN Omega Multiparticle Spectrometer. A partial-wave analysis has been made of the (3π)⁰ system. We observe both natural and unnatural spin-parity production. The natural parity states can be identified with established resonances. In addition a natural spin-parity enhancement is observed at a mass of about 2 GeV/c² with J^P = 4⁺ preferred. We have called this effect the A₂¹ (2030). The unnatural spin-parity production found is consistent with reggeized Deck model predictions. No unambiguous A₁ or A₃ production is observed.     

A study of 28Si(α, γ)32S resonances below Eα = 3.83 MeV

The γ-decays of eleven resonances in the ²⁸Si(α,γ)³²S reaction below E_α = 3.83 MeV have been studied using a large Ge(Li) detector. Results for branching ratios differ considerably from previous NaI work. The previous discrepancy in radiative strengths for the 2.61 MeV resonance is explained by this data. The strengths of the first five resonances at E_α = 1.77, 1.99, 2.19, 2.37 and 2.42 MeV appear to be (39 ± 13)% lower than previously reported. Spin-parities of l^?, 2⁺ and 2⁺ have been assigned to the levels at 8.50, 8.69 and 8.86 MeV respectively. The radiative width of the E_p = 1.467 MeV, J^π = 3^? resonance in the ³¹P(p,γ)³²S reaction has also been measured.     

Study of the 19F(p, γ)20Ne radiative capture reaction from 0.2–1.2 MeV

The ¹⁹F(p, γ)²⁰Ne radiative capture reaction has been investigated for proton energies of 0.2–1.2 MeV. Excitation functions of γ-rays arising from capture reactions have been measured and the properties of known J^π = J^?J+1 resonances were studied. From the branching ratio and angular distribution measurements, the partial widths for gamma ray and proton emission of the 13.88 MeV level in ²⁰Ne were obtained, and J^π = 2⁺, T = 1 assignments for this resonance at E_P = 1091keV were made. Lower upper limit for the total width Γ = 0.8 keV has been obtained.     

A microscopic theory of giant electric isovector resonances

The electric multipole isovector (τ = 1) giant resonances for Δτ_z = 0, ± 1 are studied using the self-consistent HF-RPA theory.The distributions of strength, energies, the isospin compositions and other properties of the J = 0⁺, 1^?, 2⁺ resonances in the ⁴⁸Ca, ⁹⁰Zr, ¹²⁰Sn and ²⁰⁸Pb regions are calculated. Sum rules for the charge-exchange Δτ_z = ±1 excitations are derived.     

Spin determination of the 12C + 12C resonance at Ec.m. = 6.25 MeV

The J^π value of the ¹²C + ¹²C resonance at E_c.m. = 6.25 MeV has been determined to be 2⁺. This is in contrast to several theoretical predictions which give J^π = 0⁺. In addition, J^π values of several sub-Coulomb resonances of the ¹²C + ¹²C reaction have been determined for the first time.     

Coherent A2− production of nuclei observed in the K−KS0 decay channel at 17.2 GeV/c

We have studied A₂^? production by π^? on a nuclear target in the K^?K_S⁰ decay channel, where the A₂^? is observed above a small background at 17.2 GeV/c incident momentum. Direct confirmation of coherent A₂ (J^P = 2⁺) production has been found.     

Note on the two inequivalent spin 1/2 baryon field operators

There are two inequivalent spin 1/2 local baryon field operators that can be constructed from 3 quarks. A priori theJ ^P =1/2⁺ baryons can couple to any linear combination of these operators. We show however that the coupling of the 1/2⁺ baryons to these operators is determined by the value of theSU(3) ratio ofF toD type pseudoscalar-baryon couplings. The experimental value of this ratio implies, for example, that the proton couples strongly to (u ^T C _γ d)u and weakly to (u ^T C d)γ _s u. This is of interest in QCD sum rule applications.     

Configuration Mixing of Quark States in Baryons

Mixing angles are used to describe the SU(6) ? O(3) symmetry breaking in [70, 1^?] multiplet in the sector of the lowest mass nucleon resonances, which are investigated extensively in constituent quark models for baryon spectroscopy. The transition amplitudes for the meson photoproduction off nucleon can also be expressed in terms of the mixing angles to take into account the configuration mixing. Those amplitudes are derived as a function of the mixing angles between ${|N^2P_M{J}^-\rangle}$ and ${|N^4P_M{J}^-\rangle}$ states, with J = 1/2 and 3/2, for the processes γp → ηp, K ⁺ Λ, K ⁺ Σ ^°, K ^° Σ ⁺. The present status of our knowledge on the mixing angles between S ₁₁(1535) and S ₁₁(1650) (θ _S ), as well as between D ₁₃(1520) and D ₁₃(1700) (θ _D ) is reported. Since these resonances play very important role in the threshold region for both η and kaon production mechanisms, they are expected to provide crucial tests of different quark models for the baryon spectroscopy.     

Mixing of the lowest-lying qqq configurations with J~P=1/2~- in different hyperfine interaction models

We investigate mixing of the lowest-lying qqq configurations with J~P=1/2~-caused by the hyperfine interactions between quarks mediated by Goldstone Boson Exchange,One Gluon Exchange,and both Goldstone Boson and One Gluon exchange,respectively.The first orbitally excited nucleon,Σ,ΛandΞstates are considered.Contributions of both the contact term and tensor term are taken into account.Our numerical results show that mixing of the studied configurations in the two employed hyperfine interaction models are very different.Therefore,the present results,which should affect the strong and electromagnetic decays of baryon resonances,may be used to examine the present employed hyperfine interaction models.     

Laser-induced effects for overlapping autoinoizing rydberg states of xenon

The effect of a laser field on the cross section of photoionization of an atom by probe radiation in the region of closely lying autoionizing resonances is studied theoretically. The cross sections in the region of the overlapping autoionizing Rydberg states 5p ⁵(² P _1/2)6d′ J = 1 and 5p ⁵(² P _1/2)8s′ J = 1 resonantly coupled with the discrete state 5p ⁵(² P _3/2)7p[1/2]₁ in the xenon atom are calculated. The behavior of the shape and position of resonances in relation to the frequency and intensity of laser radiation is studied.     

Production and decay of the π−π−π+ system in the reaction π−d → dπ−π−π+ at 9 GeV/c

In this paper we present a study of the reaction π^?d → π^?π^?π⁺d at 9 GeV/c. The mass spectra are in fair agreement with the predictions of a reggeized pion-exchange Deck model. However, the s-channel azimuthal angular distribution indicates a ?-exchange Deck contribution. The results of the partial-wave analysis of the (3π)^? system are compared with those obtained in a hydrogen target: the J^P = 2⁺ contribution is very small (at most 1.5 ± 1.2)% in our data, which are dominated by J^P = 1⁺ for low mass, whereas the J^P = 0^?, 1⁺, 2^? have comparable importance in the higher mass region.     

Effects of pseudoscalar-baryon channels in the dynamically generated vector-baryon resonances

We study the interaction of vector mesons with the octet of stable baryons in the framework of the local hidden gauge formalism using a coupled-channels unitary approach, including also the pseudoscalar-baryon channels which couple to the same quantum numbers. We examine the scattering amplitudes and their poles, which can be associated to the known J ^P = 1/2⁻, 3/2⁻ baryon resonances, and determine the role of the pseudoscalar-baryon channels, changing the width and eventually the mass of the resonances generated with only the basis of vector-baryon states.     

SU(6)W and decays of baryon resonances

The decay rates of baryon resonances assigned to the {70, 1^?} and {56, 2⁺} presentations of SU(6)_W ? O(3) are subjected to a detailed comparison with an SU(6)_W model motivated by the recent work of Melosh. We conclude that SU(6)_W is remarkably successful as an approximate symmetry for decay vertices.