G. Sigurdsson

Reaction π−p→ηn in the 15 to 40 GeV/c momentum range

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. 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 b0(s). The imaginary part of this universal pole, Im b0(s), has been determined and found to be growing logarithmically with s.

Reaction π− p → η′n in the 15–40 GeV/c momentum range☆

Abstract 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)°.

Observation of a spin 4 neutral meson with 2 GeV mass decaying in π° π°

Abstract 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.

Analysis of the reaction π−p → π0ηn at 40 GeV/c beam momentum

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.

X0 → 2γ from π− p → neutral meson + neuron at 3.8 GeV/c

Abstract We report on a measurement for the branching-ratio X 0 → 2γ X 0 ar all. Our result is X 0 → 2γ X 0 → all = (2.9 ± 0.9)% .

Observation of a meson X→2γ, with mass 2.85 GeV/c2, produced in the charge-exchange reaction π−p → Xn at 40 GeV/c

Abstract 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 .

Results on ππ-interaction in the reaction π−p → πoπon at 8 GeV/c

Abstract We compare published ππ-phase shifts with results from an analysis of the reaction π − p → π o π o n. The ‘up’-solution as well as the ‘in-between’ -solution for the I = 0 s-wave ππ-scattering are ruled out. The only set of s-wave phase shifts consistent with our data is the ‘up-down’-solution with a rapid rise of σ 0 0 near the KK-threshold.

Measurement of π−p→π0π0n at 25 GeV/c

Abstract The reaction π − p→ π 0 π 0 n has been measured with a 648 channel hodoscope spectrometer for the detection of the four γs from the π 0 decays. The π 0 π 0 D-wave is fully compatible with the f 0 contribution as it is determined in high-statistics π + π − experiments. The magnitude of the π 0 π 0 S-wave and the cosinus of its phase angle (relative to the known D-wave) are determined from fits to the π 0 π 0 angular distributions. Argand diagrams for the I = 0 amplitude S 0 are given for the range 1000 to 1500 MeV/ c 2 . Two solutions exist. One exceeds the unitarity limit above 1200 MeV/ c 2 . The other remains within the unitarity limit and is nearly elastic up to 1450 MeV/ c 2 . It indicates an S 0 wave resonance around 1300 MeV/ c 2 .

Measurement of π−p → π0n at large momentum transfer☆

Abstract 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 .

Results on the production cross sections π− p → neutrals and π− p→ χ↳2γ0 n at 3.8, 6, 8 and 12 GeV/c☆

The ratio between the cross sections for the reactions π− p→ χ↳2γ0 n and π−p → η→2γ n has been measured to be (2.4 ± 0.9) × 10−2, (2.1 ± 0.6) × 10−2 and (2.8 ± 1.3) × 10−2 at 3.8,6,8 and 12 GeV/c incident momentum respectively. At the same momenta the cross section for πt- p → neutrals is (1.48 ± 0.09) mb, (0.86 ± 0.05) mb, (0.64 ± 0.04) mb and (0.42±0.03) mb.