References - Bogoliubov Laboratory of Theoretical Physics - JINR

ASYMMETRY MEASUREMENTS IN THE ELASTIC PION-PROTON
SCATTERING IN THE RESONANCE ENERGY RANGE
I.G. Alekseev 1 , N.A. Bazhanov 2 , Yu.A. Beloglazov 3 ,P.E.Budkovsky 1 ,E.I.Bunyatova 2 ,
E.A. Filimonov 3 ,V.P.Kanavets 1 ,A.I.Kovalev 3 ,L.I.Koroleva 1 , B.V. Morozov 1 ,
V.M. Nesterov 1 , D.V. Novinsky 3 ,V.V.Ryltsov 1 , V.A. Shchedrov 3 , A.D. Sulimov 1 ,
V.V. Sumachev 3 ,D.N.Svirida 1 † , V.Yu. Trautman 3 and L.S. Zolin 2
(1) Institute for Theoretical and Experimental Physics, Moscow, Russia
(2) Joint Institute for Nuclear Research, Dubna, Russia
(3) Petersburg Nuclear Physics Institute, Gatchina, Russia
† E-mail: Dmitry.Svirida@itep.ru
Abstract
The asymmetry parameter P was measured for the elastic pion-proton scattering
in the very backward angular region of θcm ≈ 150 − 170 o at several pion beam
energies in the invariant mass range containing most of the pion-proton resonances.
The general goal of the experimental program was to provide new data for partial
wave analysis in order to allow the unambiguous light baryon spectrum reconstructions.
The experiment was performed at the ITEP U-10 proton synchrotron by the
ITEP-PNPI collaboration in the latest 5 years.
Current situation in light baryon spectroscopy is far from perfect both from theoretical
and experimental points of view. Most data on light baryons is taken from the partial
wave analyses of πN scattering KH80 [1] and CMB80 [2], both performed about 30 years
ago. Later solutions by GWU group [3, 4] do not reveal several resonant states seen
by [1, 2], though sufficient improvements were made to their analysis in the recent years.
This work was aimed at providing new experimental data to the partial wave analyses in
kinematic areas, where their behavior is most unstable and where they are most suspicious
for various kinds of ambiguities.
Polarization parameter P in the elastic scattering is determined by the direct measurement
of the azimuthal asymmetry of the reaction produced by pions on a proton target
polarized normally to the scattering plane.
The experimental setup SPIN-P02 is schematically shown in fig. 1. The polarized target
with vertical orientation of the polarization vector � PT was located at the focus of the
secondary pion beam line of the ITEP proton synchrotron. The incident and scattered pions
and the recoil proton were tracked with blocks of multi-wire chambers CH1-CH14 and
the full event reconstruction was performed allowing good selection of the elastic events.
In case of positive beam TOF technique was used to separate pions from protons at low
beam energies, while at beam momenta above 1.8 GeV/c the aerogel Cherenkov counter
AGCC was additionally introduced into the beam for the pion tagging. Further details on
the SPIN-P02 setup could be found in [5]. The asymmetry is determined by measuring
the normalized event counts for two opposite directions of the target polarization vector.
The procedure of the elastic event selection is illustrated by fig. 2. For each event
the deviation from the elastic kinematics was calculated in terms of two variables: Δθ
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