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Measurement of deeply virtual Compton scattering off He4 with the CEBAF Large Acceptance Spectrometer at Jefferson Lab

R. Dupré et al. (CLAS Collaboration)
Phys. Rev. C 104, 025203 – Published 11 August 2021
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  • INTRODUCTION
  • THEORETICAL FRAMEWORK
  • PAST NUCLEAR DVCS MEASUREMENTS
  • THE CLAS NUCLEAR DVCS EXPERIMENTAL SETUP
  • DVCS EVENT SELECTION
  • RESULTS
  • SUMMARY
  • ACKNOWLEDGMENTS
  • APPENDICES
    • References

    Abstract

    We report on the measurement of the beam spin asymmetry in the deeply virtual Compton scattering off He4 using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab using a 6 GeV longitudinally polarized electron beam incident on a pressurized He4 gaseous target. We detail the method used to ensure the exclusivity of the measured reactions, in particular the upgrade of CLAS with a radial time projection chamber to detect the low-energy recoiling He4 nuclei and an inner calorimeter to extend the photon detection acceptance at forward angles. Our results confirm the theoretically predicted enhancement of the coherent (eHe4→e′He4′γ′) beam spin asymmetries compared to those observed on the free proton, while the incoherent (eHe4→e′p′γ′X′) asymmetries exhibit a 30% suppression. From the coherent data, we were able to extract, in a model-independent way, the real and imaginary parts of the only He4 Compton form factor, HA, leading the way toward 3D imaging of the partonic structure of nuclei.

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    • Received 19 February 2021
    • Accepted 6 July 2021

    DOI:https://doi.org/10.1103/PhysRevC.104.025203

    ©2021 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Form factorsLepton induced nuclear reactionsPerturbative QCDQCD in nuclear reactions
    Nuclear PhysicsParticles & Fields

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    Vol. 104, Iss. 2 — August 2021

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    Images

    • Figure 1
      Figure 1

      General representation for the GPDs of a nucleon represented by the triple lines and noted N. Single lines can represent quarks or antiquarks probed in the nucleon shown by the triple lines.

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    • Figure 2
      Figure 2

      Illustration of the scattering (or leptonic) and production (or hadronic) planes in the DVCS process.

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    • Figure 3
      Figure 3

      Diagram representing the coherent nuclear DVCS, where we indicate the limit between the hard and the soft components with the dot-dashed factorization line.

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    • Figure 4
      Figure 4

      Coefficients presented in Eqs. (7) to (10). Note the prescaling factors used for α0, α2, and α3.

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    • Figure 5
      Figure 5

      Diagram representing the incoherent nuclear DVCS.

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    • Figure 6
      Figure 6

      The sin(ϕ) moment of the BSA as a function of −t measured by HERMES for a series of nuclei [10]. The gray bands represent the systematic uncertainties.

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    • Figure 7
      Figure 7

      The sin(ϕ) moment of the BSA at low and high −t as a function of A measured by HERMES [10]. The inner error bars represent the statistical uncertainty, while the outer represent the quadratic sum of the statistical and systematic uncertainties.

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    • Figure 8
      Figure 8

      View of the CLAS detector setup.

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    • Figure 9
      Figure 9

      Representation of the inner calorimeter (IC) of CLAS. The crystals that compose the sensitive part of the detector are represented in purple.

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    • Figure 10
      Figure 10

      Representation of the center of CLAS with the beam background in red with and without the solenoid field activated, right and left, respectively.

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    • Figure 11
      Figure 11

      Cut view of the RTPC.

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    • Figure 12
      Figure 12

      Distributions of the coherent photon production events before (blue) and after (black line filled in gray) the exclusivity cuts used to select coherent DVCS represented by the red dashed lines. The histograms are shown as a function of the seven variables used for the exclusivity selection described in the text, plus the missing Px and Py components, in order left to right and top to bottom.

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    • Figure 13
      Figure 13

      Distributions of the incoherent photon production events before (blue) and after (black line filled in gray) the exclusivity cuts used to select coherent DVCS represented by the red dashed lines. The histograms are shown as a function of the seven variables used for the exclusivity selection described in the text, plus the missing Px and Py components, in order left to right and top to bottom.

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    • Figure 14
      Figure 14

      The measured (filled blue) and simulated (red) distributions of coherent exclusive π0 production as a function of x (a), Q2 (b), −t (c), and ϕ (d).

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    • Figure 15
      Figure 15

      The measured (filled blue) and simulated (red) distributions of incoherent exclusive π0 production as a function of x (a), Q2 (b), −t (c), and ϕ (d).

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    • Figure 16
      Figure 16

      The estimated coherent (left) and incoherent (right) π0 contamination fraction in the DVCS events as a function of the transferred momentum squared −t and integrated over the kinematic variables Q2, xB, and ϕ.

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    • Figure 17
      Figure 17

      The BSA at 90 degrees [ALUIncoh(90deg)] as a function of Q2 (a), xB (b), and −t (c), using the photon-based t definition (red) and the proton-based t′ definition (black).

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    • Figure 18
      Figure 18

      The BSA in the coherent exclusive photoproduction off He4 as a function of ϕ and Q2 (top panels), x (middle panels), and −t (lower panels). The error bars are statistical and the gray bands represent the systematic uncertainties. The full red lines show the fit of the data with the form of Eq. (6).

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    • Figure 19
      Figure 19

      The BSA at 90 degrees as a function of Q2 (a), x (b), and −t (c). Our results are shown with black squares, HERMES results with green circles [10]. The theoretical prediction by Liuti et al. [30, 31] is shown by the full blue line, while the calculation by Fucini et al. [32] is shown with the magenta dashed line.

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    • Figure 20
      Figure 20

      The imaginary part of the He4 CFF HA is shown as a function of Q2 (a), x (b), and −t (c). The real part of the He4 CFF HA is shown as a function of Q2 (d), x (e), and −t (f). The red full line is the theoretical calculation by Guzey et al. [18, 38], the black dashed line is the same calculation using the VGG model as input [39, 40], and the blue long-dashed line shows the predictions by Liuti et al. [30, 31].

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    • Figure 21
      Figure 21

      The BSA in the incoherent exclusive photoproduction off a proton bound in He4 as a function of ϕ and Q2 (top panels), x (middle panels), and −t (lower panels). The error bars are statistical and the gray bands represent the systematic uncertainties. The data are fitted with the form αsin(ϕ)1+βcos(ϕ); the results of the fits are drawn with black full lines.

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    • Figure 22
      Figure 22

      The BSA at 90 degrees as a function of Q2 (a), −t (b), and xB (c). Our measurement is represented with black squares and the HERMES measurement [10] with green circles. The theoretical prediction by Liuti et al. [30, 31] is shown by the full blue line, while the calculation by Fucini et al. [41] is shown with the magenta dashed line.

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    • Figure 23
      Figure 23

      DVCS BSA ratio of the bound proton to the free proton as a function of Q2 (a), −t (b), and xB (c). The present measurement is represented with black squares and the HERMES measurement [10] with green circles. The theoretical prediction by Liuti et al. [30, 31] is shown by the full blue line, the calculation by Fucini et al. [41] is shown with the magenta dashed line, and the black dot-dashed line is the calculation by Guzey et al. [38].

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