New Results on Spin Rotation Parameter A in the
π p-elastic Scattering in the Resonance Region
I.G. Alekseev , P.E. Budkovsky , V.P. Kanavets , L.I. Koroleva ,
B.V. Morozov , V.M. Nesterov , V.V. Ryltsov , D.N. Svirida † ,
A.D. Sulimov , V.V. Zhurkin , Yu.A. Beloglazov , A.I. Kovalev ,
S.P. Kruglov , D.V. Novinsky , V.A. Shchedrov , V.V. Sumachev ,
V.Yu. Trautman , N.A. Bazhanov‡ and E.I. Bunyatova‡
Institute for Theoretical and Experimental Physics,
25 B. Cheremushkinskaya, Moscow, 117259, Russia
†
E-mail: [email protected]
Petersburg Nuclear Physics Institute, Gatchina, Leningrad district, 188350, Russia
‡
Joint Institute for Nuclear Research, Dubna, Moscow district, 141980, 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
CP675, Spin 2002: 15th Int'l. Spin Physics Symposium and Workshop on Polarized Electron
Sources and Polarimeters, edited by Y. I. Makdisi, A. U. Luccio, and W. W. MacKay
© 2003 American Institute of Physics 0-7354-0136-5/03/$20.00
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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 (120o –140o ) and π p at 1.00 GeV/c (157o -171o ) and at
1.43 GeV/c (155o –172o ) — to resolve ambiguities of the PWA solutions and choose
correct solution branch;
• π p at 1.62 GeV/c (118o –140o ) 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 (118o –140o ) and π p at 1.00 GeV/c (157o -171o ) 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 2 ∆ϕ σϕ
2
∆θ σθ
2
where ∆ϕ and ∆θ are the deviations from the elastic kinematics in the azimuthal and
polar angles, while σϕ and σθ 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 ¡ 103 ) 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
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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
P2 A2 R2 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.
¯ 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.
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.
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.
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.
In π p at this momentum slight correction to KH80 may be suggested.
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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
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b) π p at 1.62 GeV/c
a) 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, 1979.
2. R.E. Cutcosky, et al., Phys. Rev. D20 (1979) 2839.
3. R.A. Arndt et al., Phys. Rev. C52 (1995) 2120;
R.A. Arndt et al., nucl-th/9807087;
http://gwdac.phys.gwu.edu/analysis/pin_analysis.html
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.
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