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Measurements were made of the cross section of the reactions π − p → ν ′(958)n, η ′ → 2 γ 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)°.
DATA AT 20, 25 AND 30 GEV/C SUMMED AND NORMALIZED TO THE CROSS SECTION AT 25 GEV/C.
DATA AT 20, 25 AND 30 GEV/C SUMMED AND NORMALIZED TO THE CROSS SECTION AT 25 GEV/C.
A high-statistics measurement of the reaction π − p→ η n; η →2 γ 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 A 2 effective trajectory deviates markedly for | t | ≳ 1 GeV/ c ) 2 from the linear behaviour valid for smaller | t |.
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New results on a high statistics measurement of pion-nucleon charge exchange scattering at 40 GeV/ c , extending in momentum transfer up to − t = 1.8 (GeV/ c ) 2 , are reported and compared with an optical impact parameter model, together with previous data for the reaction π − p → ηn at the same energy. The imaginary part of the pole trajectory b 0 ( s ) is determined from the slope of the tangent to the maxima of (−t) 1 2 d σ d t . The linear increase of Im b 0 ( s ) with log s , which has been observed at low energies, continues up to 40 GeV/ c .
AT SMALL -T, THE BINNING TENDS TO SMEAR OUT THE FORWARD DIP.
The reaction π − p→ π 0 ηn ↳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 4 γ seen are used for the analysis. A method is applied, which allows the determination of the number of π 0 η events for each mass, cos θ GJ and t bin. Mass spectra, t distributions and decay angular distributions for the π 0 η system are presented. The cross section for the production of A 2 0 is found to be 2.7 ± 1.1 μ b at 40 GeV/ c beam momentum. No indication of a resonant 1 − state in the π 0 η system is observed, in spite of the fact that this state is allowed for the π 0 η system on the same footing as the observed 0 + and 2 + resonances.
ERROR INCLUDES UNCERTAINTIES IN SEVERAL CORRECTIONS AND IN BACKGROUND SUBTRACTION.
The invariant mass spectrum of neutral meson states from π − p interactions at 40 GeV/ c incident momentum has been investigated in a high statistics experiment performed at the 70 GeV IHEP accelerator. To detect the high energy photons coming from the produced neutral states, a hodoscope spectrometer with a computer on-line was used. A clear structure on the mass spectrum of dipions produced in the reaction π − p→π°π°n is observed at 2 GeV. The decay angular distributions show in this mass region the variation with mass typical of a state with a spin J = 4. The mass of the observed meson is found to be M = (2020±30)MeV and the estimate of the full width is (180±60) MeV.
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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/ c 2 . The cross section of the reaction π − p→X(2.85)+n, times the branching ratio of the X→2 γ decay, is measured to be σ × BR ⋍ 2 × 10 −34 cm 2 .
MEAN VALUE OF -T = 0.28 +- 0.04 GEV**2.
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A high statistics measurement of the reaction π − p → π 0 n 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. 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. Also the behaviour of the differential cross section for small and medium | t | agrees with the prediction of a geometrical s -channel model which describes binary reactions in terms of a complex pole b 0 ( s ). The imaginary part of this universal pole, Im b 0 ( s ), has been determined and found to be growing logarithmically with s .
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The πpi-system produced in the charge exchange π−p-reaction at 100 GeV/c has been studied. The experiment was performed at the CERN SPS accelerator with the multiphoton hodoscope spectrometer GAMS-400
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η′η pair production in the charge exchange reaction π−p→η′ηn has been observed and studied in a search for glueballs with the 38 GeV/c pion beam of the 70 GeV IHEP accelerator. The γ-rays from the decay η′η→4γ were detected with the hodoscope spectrometer GAMS-2000. The η′η events in the mass range from threshold up to 1.8 GeV are decays of the scalar G(1590)-meson which has been already observed in its ηη decay mode. The measured ratio of the partial widths of the G→η′η and the G → ηη decay channels is 2.7±0.8 in agreement with the value expected for glueballs.
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A non q q ,J PC = 1 −+ , I G =1 − meson has been observed in the study of the exclusive reaction π − p→π 0 ηn at 100 GeV. Its mass is (1406±20) MeV. Its width is (180±30) MeV.
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A structure has been observed at 2220 MeV in the mass spectrum of ηη ′ systems produced by 38GeV/ c and 100 GeV/ c ′ negative pions on protons. The angular distribution of the decay products shows that this structure is presumably a spin J ⩾ 2 meson.
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In a high-statistics experiment (200 events/nanobarn) performed at the 70 GeV IHEP accelerator, a new dipion resonance has been observed in π−p collisions at 38 GeV/c momentum. The gamma-rays in reaction π−p → M0n, M0 → π0π0 → 4γ have been detected in the hodoscope spectrometer GAMS-2000. The new meson, r(2510), has a mass (2510±30) MeV/c2, a width (240±60) MeV/c2 and spin-parityJPC=6++. The production cross-section of the f, h and r mesons decreases exponentially with spin like exp [−0.9J].
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Significant production of G(1590), a scalar glueball candidate, is observed in a study of η pairs produced in π−N central collisions at 300 GeV/ c .
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Evidence for a new meson decaying into ηη′ has been found in 38 GeV π − p charge-exchange interactions. The mass and the width of this meson are (1917 ± 15) MeV and (90 −50 +35 ) MeV, respectively. Possible spin and parity assignments are J PC = 0 ++ , 1 −+ or 2 ++ , isospin and G -parity are I G = 0 + . The t -dependence of the differential cross section follows an exponential behaviour exp( bt with a small slope: b = (2±1) (GeV/ c ) −2 .Upper limits on its decay into ππ, ηη and K K exotic interpretation of this meson.
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The π − p→ ωω n reaction has studied for the first time at the 70 GeV IHEP accelerator. The six photons from the decay of the two ω mesons were detected in GAMS-2000, a hodoscope multi-photon spectometer. Two resonances decaying into ωω,X (1640) and X(1960), have been observed. Their quantum numbers are J PC =2 ++ , I G =0 + . The reaction is dominated by the one-pion exchange mechanism. The production cross sections σ. BR(X→ ωω ) are 0.65±0.15 μ b and 1.0±0.2 μ b, respectively.
X(1640) IS A JPC - 2++, IG - 0+ MESON WITH MASS 1643+-7 MEV.
X(1960) IS A JPC - 2++, IG - 0+ MESON WITH MASS 1956+-20 MEV.
Data on the 4π 0 decay mode of mesons have been obtained in a search for glueballs performed at the CERN SPS with the GAMS-4000 multiphoton spectrometer. A new tensor glueball candidate X(1810) appears as a clear peak in the 4π 0 mass spectrum. The 4π 0 decay of G(1590) is also observed. The measured value of BR(G→4 π 0 )/BR(G→ ηη )=0.8±0.3 further supports the glueball assignment of this scalar particle. The rate decay of f(1270) into 4π 0 has been measured: BR(f→4 π 0 ) = (3±1)×10 −3 .
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A narrow meson which decays into ηη is observed in 38 GeV/ c π − p collisions at large momentum transfer. The experiment has been performed at the Serpukhov accelerator with the GAMS-2000 spectrometer. The mass of this new meson is (1755 ± 8) MeV, its width is less than 50 MeV. Its spin-parity is J PC =0 ++ or less probably 2 ++ .
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