Dynamics of η→3π0 decay

The η→3π⁰ decay was studied with the SND detector at the VEPP-2Me ⁺ e ^? collider. The parameter that describes deviation of the Dalitz plot of events from a uniform distribution was found to be α=?0.010±0.021±0.010.     

Measurement of η' → π+π−γ in γγ collisions

The production of the pseudoscalar meson η'(958) is observed in the reaction e⁺e−→ e⁺e⁻η′→e⁺e⁻π⁺π⁻γ with the ARGUS detector at DESY. We measure the product Γ_γγ(η')Br(η′ →ϱγ) to be 1.13±0.04±0.13 keV. Using the known branching ratio s, we calculate Γ_γγ(η′) to be 3.76±0.13±0.47 keV and Γ_η' to be 203±32 keV.     

Measurement of the matrix element for the decay η′ → ηπ 0 π 0 with the GAMS-4π spectrometer

The parameters of the Dalitz diagram for the decay η′ → ηπ ⁰ π ⁰ in a generalized and a linear representation were determined by using a statistical sample of 15 × 10³ events. A charge-exchange reaction at a momentum of 32 GeV/c was a source of monoenergetic η′ mesons. The experiment was performed with the aid of the GAMS-4π spectrometer.     

Measurement of the f1(1285) → π+π−π0 decay

The isospin symmetry-breaking (ISB) decay f ₁(1285) → π⁺π^?π⁰ has been studied at the VES facility. The branching ratio is measured: $\frac{{BR\left( {f_1 \to \pi ^ + \pi ^ - \pi ^0 } \right)}} {{BR\left( {f_1 \to \eta \pi ^ + \pi ^ - } \right)}} = 0.86 \pm 0.16_{ - 0.20}^{ + 0.70} \%Low-energy resonances in ³¹P(p, α ²⁸Si and ³⁵Cl(p, α)³²S were studied directly in order to gain a better understanding of reaction cycling in the Si-Ar region in novae. New resonance strengths at E _c.m. = 600 and 622 keV in ³¹P(p, α)²⁸Si were measured (ωγ _{p, α} = (2.2#x00B1;0.7) × 10^?2 eV and ωγ _{p, α} = (0.99±0.08) eV, respectively) as well as the E _c.m. = 610 keV resonance in ³⁵Cl(p, α)³²S [ωγ _{p, α} = (1.2±0.2) × 10^?2 eV], the lowest energy that any resonance in this reaction has been observed, directly or indirectly. The strengths of these resonances were found to be lower than previously determined, resulting in even weaker cycling in the Si-Ar region.     

To the problem of KL→π0γγ decay

It is shown that the contribution of diagrams with intermediate state vector mesons to the K_L ⁰ amplitude, intensively discussed in literature in recent years, is close to zero provided that theSU(3) group breaking is taken into account. At the same time, the contribution of scalar mesons in the intermediate state is essential. The obtained estimates for Br(K_L ⁰) conform with the recent experimental data Br(K_L ⁰ )=(2·1 ± 0·6)×10^–6.Dedicated to the memory of our friend Prof. M. Gmitro.We wish to thank S. B. Gerasimov and A. B. Govorkov for useful discussions.     

First observation of the KS→π0γγ decay

Using the NA48 detector at the CERN SPS, 31 K_S→π⁰γγ candidates with an estimated background of 13.7±3.2 events have been observed. This first observation leads to a branching ratio of BR(K_S→π⁰γγ)_z>0.2=(4.9±1.6_stat±0.9_syst)×10⁻⁸ in agreement with Chiral Perturbation Theory predictions.     

Compton scattering on the charged pion and the process γγ→π0π0

The Compton scattering on a charged pion and the process γγ→π⁰π⁰ are studied using the dispersion relations. Unknown parameters of the S-wave ππ interaction and a sum and a difference of the π⁰ meson polarizabilities are found from a fit to the experimental data for the γγ→π⁰π⁰ process. The found parameters of the ππ interaction are used for the calculation of the cross section of the elastic γπ^± scattering. The analysis of the obtained results shows that the experimental data for the elastic γπ^± scattering in the energy region up to 1 GeV together with the data for the γγ→π⁰π⁰ process could be used both for a determination of the pion polarizability values and for study of the S-wave ππ interaction. Received: 20 November 1998 / Revised version: 22 January 1999     

Search for the radiative decay η→π0γγ in the SND experiment at VEPP-2M

The η→π⁰γγ decay was investigated by the SND detector at VEPP-2M e⁺e⁻ collider in the reaction e⁺e⁻→φ→ηγ. Here we present the results and some details of this study. We report an upper limit (90% c.l.) Br(η→π⁰γγ)<8.4×10⁻⁴ as our final result. Our upper limit does not contradict the earlier measurement by GAMS spectrometer. To facilitate future studies a rather detailed review of the problem is also given.     

Measurements of the γγ＊ → π^0 and γγ＊ → ηc transition form factors at BABAR

We present measurements of the γγ＊ → π^0 transition form factor for the momentum transfer range Q^2=4-40 GeV^2 and the γγ＊ → ηc transition form factor for the range Q^2=2-50 GeV^2. The current status of measurements of the meson-photon transition form factors for the η and η＇ mesons is discussed. The results of the measurement of the ηc mass, total and two-photon widths are also presented.     

Measurement of the cross section for the e + e ? → ωπ0 → π0π0γ process in the energy range of 1.1–1.9 GeV

The cross section for the e ⁺ e ^? ?? ???⁰ ?? ??⁰??⁰?? process has been measured in the energy range of 1.1?C1.9 GeV. The measurement has been made with the Spherical Neutral Detector at the VEPP-2000 e ⁺ e ^? collider. The data in the energy range of 1.1?C1.4 GeV are in agreement with the earlier measurements with the SND and CMD-2 detectors. The cross section has also been measured above 1.4 GeV.     

Upper limit on the KS→3π0 decay

The search for CP-violating K_S→3π⁰ decay was performed with SND detector at VEPP-2M e⁺e⁻ collider. The total amount of data corresponding to 7 million produced K_S mesons was analyzed. No candidate events were found, giving an upper limit of the decay branching ratio: Br(K_S→3π⁰)<1.4·10⁻⁵ at 90% confidence level.     

Observation of the decay Ks → 2γ and measurement of the decay rates KL → 2γ and KS → 2γ

The decay rates of K_L → 2γ and K_S → 2γ have been measured at the CERN SPS. The results are Γ(K_L→2γ)/χ(K_L→2π₀)=0.632±0.004±0.008 and Γ(K_S→2γ)/Γ(K_L→2γ)=2.3 ±1.0±0.4. This is the first observation of K_S→2γ decays.     

Upper limit for the Ξ0→Σ0γ radiative decay

We report here a new measurement of the upper limit for the branching ratio of the radiative weak decay Ξ⁰→Σ⁰γ of < 8.0×10⁻³ at the 90% confidence level. This is more than an order of magnitude less than previous limits and consistent with rates from similar 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.     

Theoretical analysis of the formation of η mesic nuclei in γ + A → N + ηA′ reactions

A model that describes the formation of η mesic nuclei in the γ+A → N+ηA′ reactions is formulated on the basis of the quasifree meson photoproduction on nuclei and the concept of a meson-nucleus optical potential. The amplitude of this reaction involving more than four nucleons is obtained in the general form within the nuclear shell model featuring intermediate coupling. The dependences of the cross sections for these reactions on the excitation levels of η mesic nuclei, on the discrete excitation levels of the nucleonic cores of these nuclei, on the type of emitted nucleon, and on the final-state interaction are analyzed for the incident-photon energies from the reaction threshold to 1 GeV.