We have analyzed the reaction ..pi../sup -/p ..-->.. ..pi../sup 0/etan(eta ..-->.. 2..gamma..) on the basis of the data from an experiment carried out in the 70-GeV accelerator at Serpukhov with use of the NICE 648-channel hodoscopic spectrometer for detection of the ..gamma.. rays. We used a method which permits determination of the number of ..pi../sup 0/eta events for each interval in mass, costheta/sub GJ/, and t. For the ..pi../sup 0/eta system we obtained mass spectra, t distributions, and distributions in the decay angle. The cross section for production of A/sup 0//sub 2/ mesons at momentum 40 GeV/c was found to be 2.7 +- 1.0 ..mu..b. No indications were obtained of a 1/sup -/ resonance in the ..pi../sup 0/eta system, although such a state is allowed for the ..pi../sup 0/eta system on the same basis as the observed 0/sup +/ and 2/sup +/ resonances.

The reaction ..pi../sup -/p..--> pi../sup 0/..pi../sup 0/n has been studied at momentum 25 GeV/c. The 648-channel hodoscopic spectrometer NICE was used for detection of the four ..gamma.. rays from decay of the ..pi../sup 0/ mesons. The D wave of the ..pi../sup 0/..pi../sup 0/ system agrees completely with the contribution of the f meson determined in ..pi../sup +/..pi../sup -/ experiments. On the basis of fit to the angular distributions, the intensity of the S wave of the ..pi../sup 0/..pi../sup 0/ system and the cosine of its phase shift relative to the known D wave have been determined. Argand diagrams are given for the S-wave amplitude with isospin I = 0 in the mass region 1000--1500 MeV. Two solutions are obtained. One of them exceeds the unitary limit for masses of the ..pi../sup 0/..pi../sup 0/ system above 1200 MeV. The other solution remains within the limits of unitarity and is almost elastic up to 1450 MeV. It indicates the existence of a resonance in the S/sub 0/ wave in the region of 1300 MeV (the Epsilon meson).

The reaction π−p→π0π0n has been measured with a 648 channel hodoscope spectrometer for the detection of the four γ's from the π0 decays. The π0π0 D-wave is fully compatible with the f0 contribution as it is determined in high-statistics π+π− experiments. The magnitude of the π0π0 S-wave and the cosinus of its phase angle (relative to the known D-wave) are determined from fits to the π0π0 angular distributions. Argand diagrams for the I = 0 amplitude S0 are given for the range 1000 to 1500 MeV/c2. Two solutions exist. One exceeds the unitarity limit above 1200 MeV/c2. The other remains within the unitarity limit and is nearly elastic up to 1450 MeV/c2. It indicates an S0 wave resonance around 1300 MeV/c2.

The reaction π−p→π0ηn↳2y↳2y has been analyzed using data of an experimental performed at the 70 GeV accelerator, with the NICE 648 channel hodoscope spectrometer for γ ray detection. Events with 4γ seen are used for the analysis. A method is applied, which allows the determination of the number of π0η events for each mass, cosθGJ and t bin. Mass spectra, t distributions and decay angular distributions for the π0η system are presented. The cross section for the production of A20 is found to be 2.7 ± 1.1 μb at 40 GeV/c beam momentum. No indication of a resonant 1− state in the π0η system is observed, in spite of the fact that this state is allowed for the π0η system on the same footing as the observed 0+ and 2+ resonances.

A high statistics measurement of the reaction π−p → π0n has been performed at the Serpukhov accelerator for 15, 20, 25, 30 and 40 GeV/c incident pion momentum using the NICE set-up with its associated 648-channel hodoscope spectrometer for γ-ray detection. More than 3 million charge-exchange events have been recorded in total.

A high-statistics measurement of the reaction π−p→ηn; η→2γ has been performed at the 70 GeV Serpukhov accelerator for 15, 20, 25, 30 and 40 GeV/c incident pion momentum using the NICE set-up with its associated 648-channel hodoscope spectrometer for γ-ray detection. It is found that the spin-flip and non-spin-flip amplitudes can be parametrized, for small |t|, as exponentials with the same slopes to within a few percent. For |t| ≳ 1 (GeV/c)2 there is a break in the differential cross section. In addition, the A2 effective trajectory deviates markedly for |t| ≳ 1 GeV/c)2 from the linear behaviour valid for smaller |t|.

Measurements were made of the cross section of the reactions π−p → ν′(958)n, η′ → 2γ at momenta at 15, 20, 25, 30 and 40 GeV/c. The experiment was carried out on the IHEP 70 GeV accelerator using the 648 channel hodoscope spectrometer NICE for γ-ray detection. A total of 6000 η′ mesons were recorded. A sharp drop is seen in the differential cross section for t → 0. The dependences of the differential cross sections for the π−p → η′n and π−p → η n on t are identical. On the basis of the ratio of the cross sections for these reactions at t = 0, i.e. R(η′n)t=0 = 0.55 ± 0.06, the singlet-octet mixing angle for pseudoscalar mesons was determined to be β = −(18.2 ± 1.4)°.

The invariant-mass spectrum of neutral states produced in charge-exchange scattering of 40-GeV/c ..pi../sup -/ mesons on protons is studied to search for heavy particles which decay into two ..gamma.. rays. The ..gamma.. rays were detected by a hodoscope spectrometer on-line with a computer. The mass spectrum shows a peak which is identified with chi (2.85). The reaction cross section times the branching ratio of the decay chi..-->..2..gamma.. is found to be 2 x 10/sup -34/ cm/sup 2/.

The invariant mass spectrum of neutral final states produced in π−p charge-exchange scattering at 40 GeV/c has been studied, searching for heavy particles decaying in 2γ. A peak is observed around 2.85 GeV/c2. The cross section of the reaction π−p→X(2.85)+n, times the branching ratio of the X→2γ decay, is measured to be σ × BR ⋍ 2 × 10−34cm2.

The invariant mass spectrum of neutral final states produced in π−p charge-exchange scattering at 40 GeV/c has been studied, searching for heavy particles decaying in 2γ. A peak is observed around 2.85 GeV/c2. The cross section of the reaction π−p→X(2.85)+n, times the branching ratio of the X→2γ decay, is measured to be σ × BR ⋍ 2 × 10−34cm2.