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Target and beam-target spin asymmetries in exclusive pion electroproduction for Q2>1GeV2. I. ep→eπ+n

P. E. Bosted et al. (CLAS Collaboration)
Phys. Rev. C 95, 035206 – Published 20 March 2017
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  • INTRODUCTION
  • FORMALISM
  • EXPERIMENT
  • ANALYSIS
  • ASYMMETRIES
  • RESULTS
  • SUMMARY
  • ACKNOWLEDGMENTS
    • Supplemental Material
    References

    Abstract

    Beam-target double-spin asymmetries and target single-spin asymmetries were measured for the exclusive π+ electroproduction reaction γ*p→nπ+. The results were obtained from scattering of 6-GeV longitudinally polarized electrons off longitudinally polarized protons using the CEBAF Large Acceptance Spectrometer at Jefferson Laboratory. The kinematic range covered is 1.1<W<3 GeV and 1<Q2<6GeV2. Results were obtained for about 6000 bins in W, Q2, cos(θ*), and ϕ*. Except at forward angles, very large target-spin asymmetries are observed over the entire W region. Reasonable agreement is found with phenomenological fits to previous data for W<1.6 GeV, but very large differences are seen at higher values of W. A generalized parton distributions (GPD)-based model is in poor agreement with the data. When combined with cross-sectional measurements, the present results provide powerful constraints on nucleon resonance amplitudes at moderate and large values of Q2, for resonances with masses as high as 2.4 GeV.

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    • Received 9 August 2016
    • Revised 10 October 2016

    DOI:https://doi.org/10.1103/PhysRevC.95.035206

    ©2017 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Lepton induced nuclear reactionsPolarization phenomenaResonance reactions
    Nuclear Physics

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    Target and beam-target spin asymmetries in exclusive pion electroproduction for Q2>1 GeV2. II. ep→eπ0p

    P. E. Bosted et al. (CLAS Collaboration)
    Phys. Rev. C 95, 035207 (2017)

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    Vol. 95, Iss. 3 — March 2017

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    Images

    • Figure 1
      Figure 1

      Sample electron-pion missing mass distribution for the topology ep→eπ+(n), averaged over the full kinematic range of the experiment. Counts from the ammonia target are shown as the solid circles and counts from the carbon target (scaled by the ratio of integrated luminosities on bound nucleons) are shown as the open circles. The vertical dashed lines indicate the cuts used in the analysis.

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    • Figure 2
      Figure 2

      Same as Fig. 1, but for the topology ep→eπ+n. Cuts on the neutron angle have been applied.

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    • Figure 3
      Figure 3

      Distribution of the in-plane angular difference in predicted and observed nucleon direction cosines for the topology ep→eπ+n. The black points are for the ammonia target, while the open circles are from the carbon target, scaled by integrated luminosity. The analysis cuts correspond to the edge of the histogram. All other relevant exclusivity cuts (i.e., on Mxeπ and δϕN) have been applied.

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    • Figure 4
      Figure 4

      Same as Fig. 3, except now for the out-of-plane angular difference (after application of cuts on Mxeπ and δθN). The vertical dashed lines indicate the cuts used in the analysis.

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    • Figure 5
      Figure 5

      Distribution in (W,Q2) of events for the ep→eπ+(n) topology passing all exclusivity cuts. The vertical dashed lines show the limits of the W bins used in the analysis, while the left-to-right curves show the bin limits in Q2, defined by fixed bins in θe of 15.5, 18, 21, 26, and 38 deg (from bottom to top).

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    • Figure 6
      Figure 6

      Dilution factors f as a function of W for the ep→eπ+n topology (dashed curves) and the ep→eπ+(n) topology (solid curves) for the four Q2 bins of this experiment and the middle bin in cos(θ*). The values of f decrease weakly with increasing Q2.

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    • Figure 7
      Figure 7

      Beam-target double-spin asymmetry ALL for the ep→eπ+n topology (red crosses) compared to the ep→eπ+(n) topology (blue circles), averaged over ϕ*, as a function of W, in the six cos(θ*) bins of this analysis and the four Q2 (θe) bins used.

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    • Figure 8
      Figure 8

      Beam-target double spin asymmetry ALL for the reaction ep→eπ+n as a function of ϕ* in seven bins in W (columns) and six cos(θ*) bins (rows). The results are from the two lower Q2 bins of this analysis. The error bars reflect statistical uncertainties only. The solid red curves are from the MAID 2007 fit [9], the blue long-dashed curves are from a JANR fit [25], and the green short-dashed curves are for the GPD-inspired model from Goloskokov and Kroll [12].

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    • Figure 9
      Figure 9

      Same as Fig. 8, except for the two larger Q2 bins of this analysis.

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    • Figure 10
      Figure 10

      Target single-spin asymmetry AUL for the reaction ep→eπ+n as a function of ϕ* in seven bins in W (columns) and six cos(θ*) bins (rows). The results are from the two lower Q2 bins of this analysis. The error bars reflect statistical uncertainties only. The solid red curves are from the MAID 2007 fit [9], the blue long-dashed curves are from a JANR fit [25], and the green short-dashed curves are for the GPD-inspired model from Goloskokov and Kroll [12].

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    • Figure 11
      Figure 11

      Same as Fig. 10, except for the two larger Q2 bins of this analysis.

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