π p-elastic Scattering in the Resonance Region

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1 New Results on Spin Rotation Parameter A in the π p-elastic Scattering in the Resonance Region I.G. Alekseev Λ, P.E. Budkovsky Λ, V.P. Kanavets Λ, L.I. Koroleva Λ, B.V. Morozov Λ, V.M. Nesterov Λ, V.V. Ryltsov Λ, D.N. Svirida Λ, A.D. Sulimov Λ, V.V. Zhurkin Λ, Yu.A. Beloglazov ΛΛ, A.I. Kovalev ΛΛ, S.P. Kruglov ΛΛ, D.V. Novinsky ΛΛ, V.A. Shchedrov ΛΛ, V.V. Sumachev ΛΛ, V.Yu. Trautman ΛΛ, N.A. Bazhanov and E.I. Bunyatova Λ Institute for Theoretical and Experimental Physics, 25 B. Cheremushkinskaya, Moscow, , Russia Dmitry.Svirida@itep.ru ΛΛ Petersburg Nuclear Physics Institute, Gatchina, Leningrad district, , Russia Joint Institute for Nuclear Research, Dubna, Moscow district, , Russia Abstract. The paper presents new experimental data on the spin rotation parameter A obtained recently by ITEP-PNPI collaboration at the ITEP accelerator. The set of measurements was performed in carefully chosen critical points with precision sufficient for choosing the correct branches of partial wave analyses. The data for both π + and π -scattering at 1.0, 1.43 and 1.62 GeV/c is included. INTRODUCTION Partial wave analyses (PWA) of the pion-nucleon scattering are the main source of the information about the spectrum and properties of non-strange baryon resonances. Yet in the absence of the spin rotation parameter measurements they possess a principal ambiguity, though of the discrete type (Barrelet). Before the series of measurements presented in this paper there were no experimental data on spin rotation parameters above 0.75 GeV/c in the resonance region. Current state of the baryon spectroscopy is mainly based on the results of the two partial wave analyses KH80 [1] and CMB [2], carried out in early eighties. Later solutions by former VPI group SM90 SM99 FA02 [3] are believed to be missing too many resonant states. This experiment definitely shows that in the area of the measurements wrong solution branch was chosen by KH80 and CMB analyses. MOTIVATION OF THE KINEMATIC REGION A-parameter measurements require proton spin analysis in the final state, thus secondary (analyzing) scattering is necessary, leading to much smaller event rates compared to single spin experiments. This means that such measurements cannot be fulfilled on a

2 regular basis in a large number of kinematic points, making the choice of the kinematic region extremely important. Based on the careful analysis of the PWA ambiguities and discrepancies, the following areas were selected for the A-measurements: π + p at 1.43 GeV/c (120 o 140 o ) and π p at 1.00 GeV/c (157 o -171 o ) and at 1.43 GeV/c (155 o 172 o ) to resolve ambiguities of the PWA solutions and choose correct solution branch; π + p at 1.62 GeV/c (118 o 140 o ) to repeat and confirm our first measurement at 1.43 GeV/c out of the resonance region with a new and completely different polarimeter; π p at 1.62 GeV/c (118 o 140 o ) and π + p at 1.00 GeV/c (157 o -171 o ) to test PWA predictions and provide data for the direct amplitude reconstruction. EXPERIMENTAL SETUP The SPIN-LM experimental setup is located at the secondary pion beam of the ITEP proton synchrotron and is a joint effort of the PNPI and ITEP groups. It is based on the evaporation type cryo polarized proton target with super-conductive solenoid, several sets of wire chambers for tracking of all particles involved and a thick filter carbon polarimeter. More detailed description can be found in [4] DATA PROCESSING For each event the complete kinematic reconstruction was performed based on the tracking of all particles in the magnetic field of the polarized target. Unified χ 2 criterion was used for the elastic event selection and background (mainly quasielastic) determination: χ = ϕ=σϕ + θ=σθ where ϕ and θ are the deviations from the elastic kinematics in the azimuthal and polar angles, while σ ϕ and σ θ are the RMS of the corresponding distributions from Monte-Carlo simulations. Typical result is presented in fig. 1 for π + p at 1.62 GeV/c. The selection χ 2 -criterion was chosen to take 6 8% of the background and 85 95% of good events (see fig. 1c) for various momenta and pion sign. For every event a 3 3 matrix was calculated, describing recoiled proton spin rotation in the magnetic field of the setup along its trajectory from the vertex of the first scattering to the point of the rescattering on the carbon nucleus. Single track events in the polarimeter were selected with the polar angle of the second scattering > 3 o. Of them only those were taken for which all the azimuthal angles are allowed by the chambers geometry. Several thousand events ((4 16) 10 3 ) in various kinematic ranges and pion signs were selected for the treatment with the method of maximum likelihood to get the polarization parameters. The probability density was built only as a function of the

3 FIGURE 1. a) χ 2 -distribution of the events from the polarized target (solid line) and from the carbon target (open dots). b) Real (solid line) and MC expected (triangles) distributions. c) Elastic event output and relative background vs χ 2 cut value. parameters A and P, while the absolute value of R was calculated using the equation P 2 + A 2 + R 2 = 1. RESULTS The results for the spin rotation parameter A are presented in fig. 2 compared to the predictions of several partial wave analyses. Only statistical errors are given, all the systematic errors such as false setup asymmetry, uncertainties in the target polarization, pc analyzing power, amount and polarization of the background are negligible compared to the statistical errors. ffl The results for π + p reaction at 1.43 GeV/c and 1.62 GeV/c does not contradict to the predictions given by the analyses SM90 and SM99 and is in strong disagreement with the predictions of KH80 and CMB. This remains true in a wide momentum range, confirming the conclusion that the difference between various PWA comes from the discrete ambiguity of Barrelet type [5] and from the choice of the branch of the transverse amplitude zero trajectory. ffl At 1.43 GeV/c in π p the new data definitely chooses SM99 solution and has strong discrepancy with CMB, suggesting some correction to KH80 and SM90. ffl In π p scattering at 1.62 GeV/c the parameter A from this experiment does not deviate much from PWA predictions, but looks to be more close to SM90 and SM99. ffl The A result for π p at 1.00 GeV/c confirms the PWA s KH80 and SM99 and is in contradiction with the predictions of CMB and SM90. ffl In π + p at this momentum slight correction to KH80 may be suggested.

4 a) π p at 1.00 GeV/c b) π + p at 1.00 GeV/c c) π p at 1.43 GeV/c d) π + p at 1.43 GeV/c e) π p at 1.62 GeV/c f) π + p at 1.62 GeV/c FIGURE 2. Spin rotation parameter A in elastic π p scattering (θ CM -dependence). Selected results for the normal polarization P are presented in fig. 3 in comparison with other experimental and PWA data. No contradiction can be seen within the errors to the results of other works and PWA predictions. Since a) the P-parameter is determined from the same statistical material as A and b) the outgoing normal proton spin component does not depend on the target polarization

5 a) π p at 1.62 GeV/c b) π + p at 1.62 GeV/c FIGURE 3. Normal polarization P in elastic π p scattering (θ CM -dependence). sign, the discrepancies in this parameter is a good measure of the false asymmetries in the setup. The systematic error in A caused by the false asymmetries is at least an order of magnitude smaller than that in P due to the regular target polarization sign reverse. At the same time, A and P measurements have the same scale uncertainty due to the pc analyzing power, and the reasonable agreement of our P result with the world data is an evidence of the good quality of the pc data used in the analysis. CONCLUSIONS The direct and one of the most important consequences of the A-parameter measurements is the observation of the discrete ambiguities in the PWA solutions in π + p scattering at 1.43 GeV/c and 1.62 GeV/c as well as in π p at 1.00 GeV/c and 1.43 GeV/c. The obtained results allow to make the distinct choice of the solution branch. From the other hand the satisfactory agreement of the results on the spin rotation parameters with the predictions of the partial wave analyses (except for the discrete ambiguities) gives an evidence of the relatively high accuracy of the amplitude reconstruction by the modern PWA s, and this is in spite of the fact that they are carried out without A and R measurements in large momentum range. REFERENCES 1. G. Höller, Handbook of Pion-Nucleon Scattering., Physics Data. No 12-1, Fachinformationzentrum, Karlsruhe, R.E. Cutcosky, et al., Phys. Rev. D20 (1979) R.A. Arndt et al., Phys. Rev. C52 (1995) 2120; R.A. Arndt et al., nucl-th/ ; 4. I.G. Alekseev et al., Phys. Atom. Nucl. 65, 220 (2002) [Yad. Fiz. 65, 244 (2002)]. 5. E. Barrelet, Nuovo Cim. A8 (1972) 331.