PNPI Group Leader:    V.V. Sumachev

Realization of the experimental program "Baryon spectroscopy with pion beams up to 2000 MeV" is underway at the Meson Physics Laboratory (MPL). In the framework of this program, at the pion channel of the PNPI synchrocyclotron a series of experiments have been carried out in which precision measurements of the differential cross sections, the polarization parameters P, A, R for elastic pion-proton scattering were made at many momenta of the incident pions in the range of low-lying pion-nucleon resonances P₃₃(1232), P₁₁(1440), D₁₃(1520), S₁₁(1535). In total, more than 500 new experimental points were obtained during these experiments. Using these experimental data, we have performed a new partial-wave analysis PNPI-94. One of mostly fundamental results of this analysis is an observation of charge splitting of the Р₃₃ resonance that has given a possibility to determine separately masses and widths of different charge states of this resonance.
       A logical continuation of the above mentioned experiments is the study of pion-proton reactions with production of neutral mesons in the final state. For performing these investigations, at the meson Physics laboratory a noel device was designed and built – the neutral meson spectrometer; it consists of two total absorption electromagnetic calorimeter each made of 24 CsI(Na) crystals. Using the neutral meson spectrometer we have measured the cross sections of charge exchange pion-proton scattering at angles close to 0° (i.e. for forward scattering). Measurements were performed in that range of the incident pions momenta (from 417 to 710 MeV/c), in which published earlier results of other experiments were scarce, contradictory and inaccurate. Our measurements were made with a high accuracy at ten values of the incident pions momemum in the range mentioned above.
       At the present time, a series of measurements of the differential and total cross sections of the eta-production process is underway at the PNPI pion channel using the neutral meson spectrometer. Being performed at momenta of the incident pions near the threshold of this process (685 MeV/c). These measurements are aimed at studying the mechanism of this process the characteristic feature of which is that its threshold is very close to the position of the S₁₁(1535) resonance.
          (For more detail review see    "Experiments at PNPI")