Mixing parameters of η and η′ mesons

Mixing angles of η and η′ mesons are obtained in the scheme of octet-singlet mixing θ_η = −15.4°, θ′_η = −17.9°, and in the scheme connected with expansion in the quark basis $ \bar q $ \bar q q − $ \bar s $ \bar s s φ = 39.3°. The constants gV _ηγ, g _η′ V _γ of radiative decays η′ → V _γ and V → ηγ (V ≡ ρ, ω, φ) and the ratios of constants of weak decays of η and η′ mesons are calculated. The numerical values that were found are in agreement with the experimental data.     

Study of η and η＇ physics at BES-III

Decays of both η and η＇ provide very useful information in our understanding of low-energy QCD, and experimental signatures for these decays would be extremely helpful at BES-III. The rare decays of the η and η＇ mesons could serve as a low-energy test of the Standard Model and its beyond. The sensitivities of the measurements of η and η＇ decays are discussed at BES-III, in which the η and η＇ mesons are produced in the ψ decays.     

Study of η and η' physics at BES-Ⅲ

Decays of both η and η' provide very useful information in our understanding of low-energy QCD, and experimental signatures for these decays would be extremely helpful at BES-Ⅲ. The rare decays of the η and η' mesons could serve as a low-energy test of the Standard Model and its beyond. The sensitivities of the measurements of η and η' decays are discussed at BES-Ⅲ, in which the η and η' mesons are produced in the ψ decays.     

Radial excitation states of η and η′ in the chiral quark model

A chiral quark model is applied to calculate the spectra of pseudoscalar mesons η and η . By analyzing the obtained spectra, we find that the mesons η (2 1 S 0 ), η(4 1 S 0 ), η (3 1 S 0 ) and η (4 1 S 0 ) are the possible candidates of η(1760), X(1835), X(2120) and X(2370). The strong decay widths of these pseudoscalars to all the possible two-body decay channels are calculated within the framework of the 3 P 0 model. Although the total width of η (21S0 ) is compatible with the BES Collaboration’s experimental value for η(1760), the partial decay width to ωω is too small, which is not consistent with the BES result. If X(1835) is interpreted as η(4 1 S 0 ), the total decay width is compatible with the experimental data, and the main decay modes will be πa 0 (980) and πa 0 (1450), which needs to be checked experimentally. The assignment of X(2120) and X(2370) to η (31S0 ) and η (41S0 ) is disfavored in the present calculation because of the incompatibility of the decay widths.     

Investigation of near-threshold eta-meson production in the reaction π ? p→ ηn

Differential and total cross sections for eta-meson production in the reaction ?? ^? p ?? ??n were measured within the experimental program eta-meson physics implemented in the pion channel of the synchrocyclotron of the Petersburg Nuclear Physics Institute (PNPI, Gatchina). These measurements were performed at incident-pion momenta (700, 710, 720, and 730 MeV/c) in the vicinity of the threshold for the process under study by using the neutral-meson spectrometer designed and created at the Meson Physics Laboratory of PNPI. It is shown that, in the immediate vicinity of the threshold (685 MeV/c), the process of eta-meson production proceeds predominantly via S ₁₁(1535)-resonance formation followed by the decay S ₁₁(1535) ?? ??n (the respective branching fraction is Br ?? 60%), but that, as the momentum of incident pions increases, the role of the D wave becomes ever more important. A detailed analysis of this effect indicates that it is due to the increasing contribution of the D ₁₃(1520) resonance. Although the branching fraction of the decay of this resonance through the ??n channel is assumed to be very small (BR ?? 0.24%), the effect is enhanced owing to the interference between the D wave and the dominant resonance S ₁₁(1535).     

A measurement of the inclusive π0 and η production cross sections at high pT in p̄p and pp collisions at s = 24.3 GeV

Inclusive π⁰ and η production at large transverse momentum were studied in both p̄p and pp interactions at √s = 24.3 GeV. The experiment was performed using an internal molecular hydrogen gas jet target in the CERN SPS collider. No significant differences between production in p̄p and pp were observed in the transverse momentum range 2.5 < P_T < 5.1 GeV/c.     

π0 and η spectra fromp\bar p annihilations at rest

The low-energy part of theπ ⁰ spectrum associated with \(p\bar p\) annihilation at rest was measured in order to search for bound baryonium-like states. The upper limit for reaching such states via the emission of monochromaticπ ⁰'s was found to be 8% per annihilation in the mass region of 1650 MeV. The low-energy part of the η spectrum from \(p\bar p\) annihilations at rest was also observed.     

On production of η′ mesons in pp collisions close to threshold

The near threshold behaviour of the reaction cross section for pp→ppη^′, recently measured in experiments at COSY and SATURNE, is analyzed. The interaction in the pp as well as in the η^′ p final states is taken into account. The suppression of the total cross section for this process at excess energies Q < 3 MeV observed in these experiments is interpreted as an evidence for a strong repulsive η^′ p interaction. Received: 26 May 1999 / Revised version: 8 July 1999     

Constraints on distribution amplitudes of the η and η′ mesons in the light of new experimental results

Constraints on the distribution amplitudes of the SU _f (3) singlet η ₁ and octet η ₈ states are obtained from a comparison of the theoretical predictions for the η γ and η′γ electromagnetic transition form factors with experimental data of the CLEO and BaBar Collaborations. In calculations of the form factors F _η(η′)γ (Q ²) the power-suppressed corrections arising from the end-point integration regions x→0,1 are taken into account by employing the infrared renormalon approach. It is demonstrated that the power-suppressed corrections allow one to describe the data on the η γ and η′γ transitions within a framework of the SU _f (3) octet–singlet basis using one mixing angle for both the physical states and decay constants.     

Radial excitation states of η and η' in the chiral quark model

A chiral quark model is applied to calculate the spectra of pseudoscalar mesons η and η'. By analyzing the obtained spectra, we find that the mesons η'(2¹S₀), η(4¹S₀), η'(3¹S₀) and η'(4¹S₀) are the possible candidates of η(1760), X(1835), X(2120) and X(2370). The strong decay widths of these pseudoscalars to all the possible two-body decay channels are calculated within the framework of the ³P₀ model. Although the total width of η'(2¹S₀) is compatible with the BES Collaboration's experimental value for η(1760), the partial decay width to ωω is too small, which is not consistent with the BES result. If X(1835) is interpreted as η(4¹S₀), the total decay width is compatible with the experimental data, and the main decay modes will be mπ a₀(980) and m π a₀(1450), which needs to be checked experimentally. The assignment of X(2120) and X(2370) to η'(3¹S₀) and η'(4¹S₀) is disfavored in the present calculation because of the incompatibility of the decay widths.     

Radial excitation states of η and η' in the chiral quark model

A chiral quark model is applied to calculate the spectra of pseudoscalar mesons η and η'. By analyzing the obtained spectra, we find that the mesons η'(2¹S₀), η(4¹S₀), η'(3¹S₀) and η'(4¹S₀) are the possible candidates of η(1760), X(1835), X(2120) and X(2370). The strong decay widths of these pseudoscalars to all the possible two-body decay channels are calculated within the framework of the ³P₀ model. Although the total width of η'(2¹S₀) is compatible with the BES Collaboration's experimental value for η(1760), the partial decay width to ωω is too small, which is not consistent with the BES result. If X(1835) is interpreted as η(4¹S₀), the total decay width is compatible with the experimental data, and the main decay modes will be mπ a₀(980) and m π a₀(1450), which needs to be checked experimentally. The assignment of X(2120) and X(2370) to η'(3¹S₀) and η'(4¹S₀) is disfavored in the present calculation because of the incompatibility of the decay widths.     

An overview of η and η' decays at BESIII

The world's largest sample of J/ψ events, 1.31 billion events accumulated at the BESIII detector, provides a unique opportunity to investigate η and η physics via two-body J/ψ radiative or hadronic decays. For many η decay channels the low background data samples are up to three orders of magnitude larger than collected in any previous experiment. Here we review the most significant results on η and η obtained at BESIII so far. The analyses range from detailed studies of common decay dynamics, observations of new radiative and Dalitz decays, and searches for rare/forbidden decays with sensitivity up to B～10-5. Finally, prospects of forthcoming runs at the J/ψ peak forη and η physics are discussed.     

Hadro- and photo-production of η′ in quasi-free proton and neutron processes

The η′ photoproduction process on quasi-free proton and neutron and the reaction NN→ η′NN are investigated within a relativistic effective Lagrangian approach to hadronic interactions. Resonances with spins 1/2 and 3/2 are considered together with the nucleonic and t-channel meson-exchange current contributions. In photoproduction processes, the S11 resonance is found to be responsible for the sharp rise of the cross sections near threshold. In pp → η′pp, it is found that the S11 resonance dominates the total cross section over the entire energy region considered. The spin observables, in particular the beam and target asymmetries, are shown to be very sensitive to the reaction mechanism and will help impose more stringent constraints on the model parameters.     

Near-threshold η production in pp collisions

We study near-threshold η meson production in pp collisions within an effective Lagrangian approach combined with the isobar model, by allowing for the various intermediate nucleon resonances due to the π, η, and ρ-meson exchanges. It is shown that the ρ-meson exchange is the dominant excitation mechanism for these resonances,and the contribution from the N*(1720) is dominant. The total cross section data can be reasonably reproduced,and the anisotropic angular distributions of the emitted η meson are consistent with experimental measurements.Besides, the invariant mass spectra of pp and pη explain the data well at excess energy of 15 Me V, and are basically consistent with the data at excess energy of 40 Me V. However, our model calculations cannot reasonably account for the two-peak structure in the pη distribution at excess energies of 57 and 72 Me V, which suggests that a more complicated mechanism is needed at higher energy region.     

Measurement of the relative probability of the decay φ → η′γ in the channel η′ → ηπ0π0, η → γγ

The process e ⁺ e ^? → η′γ, η′ → η π⁰π⁰, η → γγ is investigated by an SND detector in experiment on a VEPP-2M equipment with colliding electron-positron beams. The analysis of experimental data yields a value of B(? → η′γ)=(6.7 _?4.2 ^+5.0 ±1.5)×10^?5 for the decay probability. Taking into account a previous measurement of this quantity by an SND detector in the decay channel η′ → π⁺π^?η, one finally obtains B(?→η ′γ)=(6.7 _?2.4 ^+2.8 ±0.8)×10^?5.     

Radial excitations of scalar, η, and η′ mesons in a chiral quark model

First radial excitations of the scalar-meson nonet and of the pseudoscalar mesons η and η′ are described within a nonlocal chiral quark model of the Nambu-Jona-Lasinio type with ′t Hooft interaction. In this model, simple form factors are used, which allows us to describe first radial excitations of the mesons and to retain the gap equations describing spontaneous breakdown of chiral symmetry in the standard form. The external parameters of form factors are fixed by the masses of excited pseudoscalar mesons, and the same form factors are used to predict the masses of excited scalar mesons. The strong decays of excited scalar mesons and of η and η′ mesons are described in satisfactory agreement with experimental data.