Meson Physics Laboratory

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Meson Physics Laboratory
Introduction
(continuation - page2)
             The next stage of program being accomplished at the pion channel of the ITEP synchrotron the experiment on measuring the polarization parameter P. Already performed are measurements at the incident pions momenta of 800, 1780, 1940 and 2070 MeV/c in the angular range from 150o to 170o
(in the centre-of-mass system). No measurements were made till now at these angles because of extremely low values of the differential cross section.
             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. To realize this goal, the kinds of reactions induced by the interaction of the negatively charged pion with the proton are investigated the pion-ptoton elastic scattering and the pion-proton reaction with production of kaon and Lambda-hyperon. The experiment is carried out by scanning the invariant mass of the system pion-proton in the range from 1620 to 1896 MeV (corresponding interval of the incident pions momenta is from 900 to 1200 MeV/c). The scan is being done by changing the incident pions momentum by a step of approximately 1 MeV; an accuracy of measuring this momentum
0.08% (i. e. better than 1 MeV in terms of the invariant mass) was achieved with the help of multi-wire proportional chambers located in the first and second focuses of the pion channel. Secondary particles produced in a liquid hydrogen target are detected by a two-arm non-magnetic spectrometer made of arrays of drift chambers with hexagonal structure equipped with modern read-out electronics. All wire chambers were designed and manufactured at PNPI. 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, at which a search for the narrow exotic resonance is being performed in the reaction of pion-proton elastic scattering. In the first methodical run all systems of the experimental setup were tested, and 7x106 triggers were written when using the liquid hydrogen target.
             In 1994 the Meson Physics Laboratory entered the international collaboration performing the program Baryon and hyperon spectroscopy with the Crystal Ball. This collaboration was formed finally in 1995-1997, it included 14 institutions from 5 countries. Several experiments E913/914/953/958 were carried out during the period 1997-2002 in the C6 line of the Alternating Gradient Synchrotron at the Brookhaven National Laboratory (USA). Both pion and kaon beams were used in these experiments. Data obtained will allow to investigate numerous reactions with neutral particles in the final states as well as to study rare and forbidden decays of eta mesons. Now extensive data processing is underway in USA and in Gatchina; MPL scientist A.B. Starostin has defended at PNPI the Candidate Dissertation "Measurement of production of pairs of neutral pions in the nuclear medium".
             Since 1998, physicists of the Meson Physics Laboratory are involved in the collaboration based on the ELSA accelerator in Bonn. This electron accelerator has intense beams of tagged photons with energy up to 3000 MeV and an excellent device - the photon spectrometer Crystal Barrel - for detecting multiphoton final states with a high efficiency. With the use of a liquid hydrogen target, in one experiment the photoproduction of different neutral mesons off the proton was studied in one experiment simultaneously. In several runs the target operated in the liquid-deuterium regime, and statistics acquired in the course of these runs has allowed to study also the photoproduction of neutral mesons off the neutron and the deuteron. Now the processing and analysis of a huge amount of data obtained is underway.
             But of the especial value are data obtained in experiments using both the polarized proton (deuteron) target and the polarized (linearly or circularly) photons. Just for realizing such opportunity, in 2006-2007 a serious upgrade of the experimental setup was made. The photon spectrometer Crystal Barrel, into the inner cavity of which the polarized target with frozen spin was put, was moved to another beam line that allowed to put near the Crystal Barrel cryogenic apparatus to supply the needed polarization value. In the course of upgrading the Crystal Barrel was equipped with additional forward detectors providing the higher angular resolution when detecting particles emitted at small angles. Also reconstructed was the tagging system installed at this beam line with the goal to widen the covered interval of photon energies and to improve the energy resolution. Experiments at the upgraded setup started at 2008, first physical results are obtained.


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