Asymmetry Measurements in the Elastic Pion-Proton Scattering at 1.94 and 2.07 GeV/c. 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. Institute for

1
Asymmetry Measurements in the Elastic
Pion-Proton Scattering at 1.94 and 2.07 GeV/c.
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.Institute for Theoretical and
Experimental Physics, B. Cheremushkinskaya 25,
Moscow, 117218, Russia.Tel [phone],
Fax [phone], E-mail Vadim.Kanavets_at_itep
.ruYu.A. Beloglazov, E.A. Filimonov, A.I.
Kovalev, S.P. Kruglov, D.V. Novinsky, V.A.
Shchedrov, V.V. Sumachev, V.Yu. Trautman.
Petersburg Nuclear Physics Institute, Gatchina,
Leningrad district, 188350, Russia.N.A.
Bazhanov, E.I. Bunyatova, L.S. Zolin Joint
Institute for Nuclear Research, Dubna, Moscow
district, 141980, Russia.
2
Preface

• New data on polarization (asymmetry) in ? p
• Backward angles, 1.94 2.07 GeV/c
• The latest experiment in the series of the
  polarization parameter measurements by ITEP-PNPI
  collaboration in the last decade
• (A? at 1.00, 1.43 and 1.62 GeV/c, P? at 1.43,
  1.78 and 2.07 GeV/c)
• Available momentum range (0.8-2.1) GeV/c
• ? 65 of light quark resonances in 3 clusters
• MAIN GOAL
• Data for the UNAMBIGUOUS RECONSTRUCTION of the
  elastic ?p scattering amplitudes by PWA
• PHYSICS INTEREST to the LIGHT BARYON SPECTROSCOPY
  is enhanced by the observation of
• resonance clusters with masses 1.7 and 1.9
  GeV/c2
• resonances with negative parity in 1.9 GeV/c2
  peak
• parity doublets in these clusters
• missing resonances near 2 GeV/c2
• which do not fit well the constituent quark model

3
Background

• Light baryon resonances in the latest PDG are
  based mainlyon PWAs KH80 and CMB80, both
  performed more than two decades ago.
• More recent analyses by VPI/GWU group did not
  revealed 4 of 13 resonances in clusters
• Latest data on polarization in ??p and spin
  rotation parameters do not confirm KH80 and CMB80
  in the backward scattering region

Experiment

• PRESENT MEASUREMENTS
• ? p, ?cm(148-176)o, P1.94 and 2.07 GeV/c
• No or very poor experimental information up to
  now
• Main difficulty extremely small cross section
  in the explored domain
• Significant background
• Its careful subtraction and systematic error
  estimate

4
Experimental Setup

• MAIN ELEMENTS
• Vertically polarized proton target PT
• 4 sets of wire chambers for tracking
• Trigger scintillation counters
• Aerogel Cherenkov counter for ? tagging

Beam Arm
Proton Arm
Aerogel Counter
Pion Arm I
Pion Arm II

• TARGET
• Cylinder ?30mm, h30mm, propanediole C3H8O2,
  free proton density ?10
• Magnetic field 2.5 T
• 3He evaporation type cryostat at 0.5 K
• Dynamic nuclear orientation
• Polarization (70-80) , measurement accuracy 2
• Sign reverse once per day

5
Data Processing 1

• MAIN DIFFICULTY
• Very low cross section
• ELASTIC EVENT SELECTION
• Coplanarity and kinematic correlation
• ?? , ?? deviations from the elastic
  kinematics
• Additional background suppression by momentum
  analysis ? factor 2 in S/B ratio
• NORMALIZATION
• Using unpolarized event counts

1.94 GeV/c
2.07 GeV/c
Measurement region
6
Data Processing 2

• BACKGROUND SUBTRACTION
• 2-dim cubic polynomial fit for the background
• SYSTEMATIC ERROR ESTIMATES
• Various cuts on the locus region

7
Results Asymmetry P at 1.94 GeV/c in ? p

• Best correspondence to KH80 and the oldest
  VPI/GWU analysis
• Data suggests even deeper minimum at 165o
• Significant deviation from CMB80 and latest
  VPI/GWU solution

8
Results Asymmetry P at 2.07 GeV/c in ? p

• Best resemblance to CMB80
• Angular dependence looks contradictory to all
  other analyses
• Obvious contradiction to the latest VPI/GWU
  solution
• No disagreement with existing data

9
Conclusion

• GWU group significantly improved their analysis
  in the recent years
• New polarization parameter data require NEW
  GLOBAL TYPE PWA
• We extremely welcome the recent efforts of the
  Helsinki group on resurrection of KH80 analysis

Acknowledgements

• Our thanks to Professor G.Hoehler for very
  interesting and fruitful discussion
• We are grateful to the ITEP accelerator team for
  providing us with the excellent beam conditions
  and to ITEP cryogenic laboratory for supply with
  liquid helium and nitrogen.
• This work was partially supported by the Russian
  Fund for Basic Research grant 02-02-16111 and
  Russian State Scientific Program "Fundamental
  Nuclear Physics".
• GREAT THANKS to the organizers of this
  interesting conference ! ! !