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Baryon spectroscopy with pion beams up to 2000 MeV.

Experiments at PNPI

The page opens in a separate window 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 33 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")

Experiments at ITEP

       To extend measurements of the spin rotation parameters A and R to the second resonance region of pion-nucleon scattering, similar experiments were carried out at the pion channel of the ITEP accelerator. Measurements of the spin rotation parameters A and R are the key experiments in the whole program of studying the elastic pion-proton scattering since only such measurements permit to remove discrete ambiguities arising in the course of a partial-wave analysis (PWA) if experimental data on the spin rotation parameters are absent. Planning of these experiments was made taking into account results of PWAs existing to the moment of starting experiments.
       To carry out the experiment on measurements of the spin rotation parameters A and R,
at PNPI a polarized proton target of special type with the vector of proton polarization lying in the horizontal plane was created and a proton polarimeter for measuring the asymmetry of the secondary scattering of the recoil protons by nuclei of a substance-analyzer was built. The spin rotation parameters A, R for elastic pion-proton scattering were measured at ITEP at three momenta of the incident pions: 1000, 1430 and 1620 MeV/c. Measurements were performed by PNPI scientists and engineers in collaboration with physicists of ITEP. One of the most interesting results is the fact that experimentally measured values of the parameter A agree well with the predictions of the PWA performed by the group of physicists from the George Washington University and contradict to the PWAs KH and CMU-LBL. Since all the characteristics of pion-nucleon resonances presented in Listings of the Review of Particle Physics are obtained just on the base of the analyses KH and CMU-LBL, which seem to be not entirely correct in the light of our last experimental data, it is necessary to revise and specify these fundamental constants by means of performing a new partial-wave analysis with inclusion our new experimental data on the spin rotation parameters.
       At the pion channel 322 of the ITEP synchrotron, which has high momentum resolution
(better than 0.1%), a new experiment EPECUR is underway. Its goal is to search for exotic baryonic resonance a non-strange member of the pentaquark antidecuplet P11 with expected mass about 1700 MeV.           fig.1 - new experiment EPECUR
          The experiment is carried out by scanning the invariant mass of the pion-proton system in the range 16201770 MeV. In 2008, tuning of the pion channel 322 was completed, the experimental setup was fully put into operation, and we started the first stage of experiment (fig.1), at which a search for the narrow exotic resonance is being performed in the reaction of elastic pion-proton scattering.
          To begin of 2012 year the statistics of 2.97 billion events was collected. The distribution of the collected triggers over runs and beam momentum intervals is at (fig.2.)
          Preliminary results on the differential cross section measurements as a function of the incident momentum for 5-degree the center of mass intervals are shown in fig. 3.
          These data points correspond to the analysis of about 15% of the statistics already collected. Some irregularity is manifested at beam pulse 10201040 MeV/c. The nature of this irregularity could be either a narrow resonance with the mass around 1690 MeV and width of 510 MeV, or a threshold effect, caused by opening of the channels of neutral K-meson production (mass 1690.2MeV) and positive charging K-meson production (mass 1691.1 MeV).
          (For more detail review see    "Experiments at ITEP ")

International Collaborations where the Meson Physics Laboratory of HEPD is involved


NRC "Kurchatov Institute"    Russian Academy of Sciences    Petersburg Nuclear Physics Institute
    High Energy Physics Division     Neutron Research Department     Molecular and Radiation Biophysics Department
    Theoretical Physics Division     Infrastructure Departments

Last update on:   by   Svetlana F. Udalova