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Exploring the Structure of the Bound Proton with Deeply Virtual Compton Scattering

M. Hattawy et al. (CLAS Collaboration)
Phys. Rev. Lett. 123, 032502 – Published 17 July 2019
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    Abstract

    In the past two decades, deeply virtual Compton scattering of electrons has been successfully used to advance our knowledge of the partonic structure of the free proton and investigate correlations between the transverse position and the longitudinal momentum of quarks inside the nucleon. Meanwhile, the structure of bound nucleons in nuclei has been studied in inclusive deep-inelastic lepton scattering experiments off nuclear targets, showing a significant difference in longitudinal momentum distribution of quarks inside the bound nucleon, known as the EMC effect. In this Letter, we report the first beam spin asymmetry (BSA) measurement of exclusive deeply virtual Compton scattering off a proton bound in He4. The data used here were accumulated using a 6 GeV longitudinally polarized electron beam incident on a pressurized He4 gaseous target placed within the CLAS spectrometer in Hall-B at the Thomas Jefferson National Accelerator Facility. The azimuthal angle (ϕ) dependence of the BSA was studied in a wide range of virtual photon and scattered proton kinematics. The Q2, xB, and t dependencies of the BSA on the bound proton are compared with those on the free proton. In the whole kinematical region of our measurements, the BSA on the bound proton is smaller by 20% to 40%, indicating possible medium modification of its partonic structure.

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    • Received 19 December 2018
    • Revised 12 March 2019

    DOI:https://doi.org/10.1103/PhysRevLett.123.032502

    © 2019 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Electron-ion collisionsForm factorsNuclear structure & decaysNucleon distributionQuark model
    Nuclear Physics

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    Vol. 123, Iss. 3 — 19 July 2019

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    Images

    • Figure 1
      Figure 1

      Representation of the leading-order, twist-2, handbag diagram of the incoherent DVCS process off He4, where the four vectors of the electrons, photons, and protons are denoted by k/k′, q/q′, and p/p′, respectively. x+ξ is the nucleon longitudinal momentum fraction carried by the struck quark, −2ξ is the longitudinal momentum fraction of the momentum transfer Δ (=q−q′), and t (=Δ2) is the squared momentum transfer between the initial and the final state nucleon.

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    • Figure 2
      Figure 2

      The distributions from left to right and from top to bottom are Δϕ, missing energy, missing mass squared, and the cone angle (θ) between the measured and the calculated photons in the e′p′ final-state system. The incoherent DVCS exclusivity cuts are represented by the vertical red-dashed lines. The black distributions represent the incoherent DVCS event candidates before the exclusivity cuts. The shaded distributions represent the incoherent DVCS events that passed all of these cuts except the quantity plotted.

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    • Figure 3
      Figure 3

      The incoherent ALU as a function of ϕ for different t bins. The error bars represent the statistical uncertainties. The gray bands represent the systematic uncertainties, including the normalization uncertainties. The black curves are the results of our fits with the form [a0sin(ϕ)/1+a1cos(ϕ)].

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    • Figure 4
      Figure 4

      The Q2 (left), xB (middle), and t dependencies (right) of the fitted ALU at ϕ=90° (black squares). The error bars represent the statistical uncertainties, while the gray bands represent the systematic uncertainties. On the middle plot, the curves are theoretical calculations from [31]. On the right plot, the solid (empty) green circles are the HERMES−ALU (a positron beam was used) inclusive measurements for the incoherent (coherent) enriched region [35]; the curves represent theoretical calculations from [31].

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    • Figure 5
      Figure 5

      The ALU ratio of the bound to the free proton at ϕ=90° as a function of t. The black squares are from this work, the green circle is the HERMES measurement [35]. The error bars represent the statistical uncertainties, while the gray band represents the systematic uncertainties. The blue and red curves are results of off-shell calculations [31]. The solid and dashed black curves are from on-shell calculations [33].

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