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Beam-recoil transferred polarization in K+Y electroproduction in the nucleon resonance region with CLAS12

D. S. Carman et al. (CLAS Collaboration)
Phys. Rev. C 105, 065201 – Published 1 June 2022
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  • INTRODUCTION
  • FORMALISM
  • HYPERON POLARIZATION EXTRACTION APPROACH
  • EXPERIMENTAL DETAILS
  • DATA ANALYSIS
  • SYSTEMATIC UNCERTAINTIES
  • DATA RESULTS
  • SUMMARY AND CONCLUSIONS
  • ACKNOWLEDGMENTS
    • References

    Abstract

    Beam-recoil transferred polarizations for the exclusive electroproduction of K+Λ and K+Σ0 final states from an unpolarized proton target have been measured using the CLAS12 spectrometer at Jefferson Laboratory. The measurements at beam energies of 6.535 and 7.546 GeV span the range of four-momentum transfer Q2 from 0.3 to 4.5 GeV2 and invariant energy W from 1.6 to 2.4 GeV, while covering the full center-of-mass angular range of the K+. These new data extend the existing hyperon polarization data from CLAS in a similar kinematic range but from a significantly larger dataset. They represent an important addition to the world data, allowing for better exploration of the reaction mechanism in strangeness production processes, for further understanding of the spectrum and structure of excited nucleon states, and for improved insight into the strong interaction in the regime of nonperturbative dynamics.

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    • Received 7 February 2022
    • Accepted 19 May 2022

    DOI:https://doi.org/10.1103/PhysRevC.105.065201

    ©2022 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Electromagnetic interactions
    1. Physical Systems
    Hyperons
    1. Properties
    Polarization
    Nuclear Physics

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    Vol. 105, Iss. 6 — June 2022

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    Images

    • Figure 1
      Figure 1

      Kinematics for K+Y electroproduction defining the c.m. angles and coordinate systems used to express the formalism and to present the polarization components extracted in the analysis.

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    • Figure 2
      Figure 2

      The e′K+ missing mass distribution after all particle identification and exclusivity cuts described in Sec. 5 for the 6.535 GeV dataset summed over all kinematics. The vertical lines about the Λ and Σ0 hyperon peaks identify the analysis ranges used to select the event samples.

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    • Figure 3
      Figure 3

      Model of the CLAS12 spectrometer in Hall B at Jefferson Laboratory. The electron beam is incident from the left side of this figure. The CLAS12 detector is roughly 20 m in scale along the beam axis.

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    • Figure 4
      Figure 4

      Kinematic coverage of the electron from the 6.535 GeV (left) and 7.546 GeV (right) datasets in terms of Q2 vs W (units GeV2/GeV). The overlaid rectangular boxes highlight the analysis region in this work.

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    • Figure 5
      Figure 5

      Kinematic coverage at 6.535 GeV of the electroproduced K+ in terms of cosθKc.m. vs Φ (deg), where Φ is the angle between the lepton scattering plane and the hadronic reaction plane. The left plot is for the K+ detected in the Forward Detector (FD) and the right plot is for the K+ detected in the Central Detector (CD).

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    • Figure 6
      Figure 6

      The kinematic phase space in terms of momentum vs laboratory polar angle θ of the reconstructed K+ (left) and p (right) in CLAS12 for the 6.535 GeV dataset, combining events reconstructed in the Forward Detector and the Central Detector. The acceptance gap between the two CLAS12 detector systems occurs at about 35∘.

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    • Figure 7
      Figure 7

      Left: M(e′K+) distribution requiring detection of a proton in the final state. Middle: M2(e′K+p) vs M(e′K+) phase space showing the cut employed on the M2(e′K+p) distribution to improve selection of the ground state hyperons. Right: M(e′K+) distribution shown in the left plot but with the additional cut on M2(e′K+p). Data are shown from the 6.535 GeV dataset.

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    • Figure 8
      Figure 8

      Representative M(e′K+) fit results using hyperon templates derived from Monte Carlo (Λ: green curve; Σ0: red curve) and a background template based on beam data (magenta curve). The blue curve shows the full fit result. The fits shown are from the 1D analysis binned in Q2 with the left plot for Q2 from 0.6–0.7 GeV2 and the right plot for Q2 from 2.8–3.1 GeV2 from the 6.535 GeV dataset.

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    • Figure 9
      Figure 9

      Hyperon yields from the 6.535 GeV (left) and 7.546 GeV (right) datasets vs Q2, W, and cosθKc.m. summed over the other two variables. The blue (red) data points are for the K+Λ (K+Σ0) events in the Λ (Σ0) mass region. Note that the abrupt shift at Q2=1.5 GeV2 occurs due to the change in Q2 bin size at this point.

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    • Figure 10
      Figure 10

      Yield ratios for the 6.535 GeV dataset showing the NΣ/NΛ and NB/NΛ ratios in the Λ mass region (left) and the NΛ/NΣ and NB/NΣ ratios in the Σ0 mass region (right).

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    • Figure 11
      Figure 11

      Measured polarizations determined for the multipion background in the 6.535 GeV dataset that underlies the hyperon peaks in the M(e′K+) distributions for the 1D analysis vs cosθKc.m., summing over Q2 and W for both the Λ (left) and Σ0 (right) mass regions defined in Sec. 3 for the primed system. The error bars include the statistical uncertainties only.

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    • Figure 12
      Figure 12

      Distributions of the transferred Λ (top) and Σ0 (bottom) polarization components relative to the y′ axis (left) and y axis (right) vs W from the 1D data analysis. The data shown are from the higher statistics 6.535 GeV dataset. The inner error bars on each data point represent the statistical uncertainties and the outer error bars represent the total uncertainties.

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    • Figure 13
      Figure 13

      Transferred Λ polarization components P′ with respect to the (x′,z′) and (x,z) axes vs Q2 for beam energies of 6.535 GeV (left) and 7.546 GeV (right). The data are limited to Q2 from 0.3 to 3.5 GeV2 (6.535 GeV) and from 0.4 to 4.5 GeV2 (7.546 GeV), and W from 1.625 to 2.4 GeV. In the text this is referred to as the 1D sort (see Sec. 5e for details). The inner error bars on the data points represent the statistical uncertainties and the outer error bars represent the total uncertainties. The curves are calculations from RPR [39] (solid green: full RPR-2011 model; dashed green: RPR-2011 model with resonance terms off), BS3 [40] (solid red), and Kaon-MAID [66, 67, 68] (solid blue).

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    • Figure 14
      Figure 14

      Transferred Λ polarization components P′ with respect to the (x′,z′) and (x,z) axes vs W for beam energies of 6.535 GeV (left) and 7.546 GeV (right). See the Fig. 13 caption for details.

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    • Figure 15
      Figure 15

      Transferred Λ polarization components P′ with respect to the (x′,z′) and (x,z) axes vs cosθKc.m. for beam energies of 6.535 GeV (left) and 7.546 GeV (right). See the Fig. 13 caption for details.

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    • Figure 16
      Figure 16

      Transferred Λ polarization components Px′′ and Px′ vs W for a beam energy of 6.535 GeV. The data are binned in Q2 from 0.3 to 0.9 GeV2 (left 2×4 plots) and Q2 from 0.9 to 3.5 GeV2 (right 2×4 plots) for four different bins in cosθKc.m.. In the text this is referred to as the 3D sort. The inner error bars on the data points represent the statistical uncertainties and the outer error bars represent the total uncertainties. See the Fig. 13 caption for a description of the model curves.

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    • Figure 17
      Figure 17

      Transferred Λ polarization components Pz′′ and Pz′ vs W for a beam energy of 6.535 GeV. See the Fig. 13 caption for a description of the model curves and the Fig. 16 caption for details on the data.

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    • Figure 18
      Figure 18

      Transferred Σ0 polarization components P′ with respect to the (x′,z′) and (x,y) axes vs Q2 for beam energies of 6.535 GeV (left) and 7.546 GeV (right). The data are limited to Q2 from 0.3 to 3.5 GeV2 (6.535 GeV) and from 0.4 to 4.5 GeV2 (7.546 GeV), and W from 1.625 to 2.4 GeV. In the text this is referred to as the 1D sort (see Sec. 5e for details). The inner error bars on the data points represent the statistical uncertainties and the outer error bars represent the total uncertainties. The curves are calculations from RPR [39] (solid green: full RPR-2007 model; dashed green: RPR-2007 model with resonance terms off), Kaon-MAID [66, 67, 68] (solid blue), and SL [71] (solid purple).

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    • Figure 19
      Figure 19

      Transferred Σ0 polarization components P′ with respect to the (x′,z′) and (x,y) axes vs W for beam energies of 6.535 GeV (left) and 7.546 GeV (right). See the Fig. 18 caption for a description of the model curves.

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    • Figure 20
      Figure 20

      Transferred Σ0 polarization components P′ with respect to the (x′,z′) and (x,y) axes vs cosθKc.m. for beam energies of 6.535 GeV (left) and 7.546 GeV (right). See the Fig. 18 caption for a description of the model curves.

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    • Figure 21
      Figure 21

      Transferred Σ0 polarization components Px′′ and Px′ vs. W for a beam energy of 6.535 GeV. The data are binned in Q2 from 0.3 to 0.9 GeV2 (left 2×4 plots) and Q2 from 0.9 to 3.5 GeV2 (right 2×4 plots) for four different bins in cosθKc.m.. In the text this is referred to as the 3D sort. The inner error bars on the data points represent the statistical uncertainties and the outer error bars represent the total uncertainties. See the Fig. 18 caption for a description of the model curves.

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    • Figure 22
      Figure 22

      Transferred Σ0 polarization components Pz′′ and Pz′ vs W for a beam energy of 6.535 GeV. See the Fig. 18 for a description of the model curves and the Fig. 21 caption for details on the data.

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