First exclusive measurement of deeply virtual Compton scattering off Helium-4: Toward the 3-D tomography of nuclei

VIC COMELLO

November 30, 2017

Graph showing leading-twist model-independent extraction of the imaginary part of the 4He Compton form factor containing 4He GPD information. — This shows the leading-twist model-independent extraction of the imaginary part of the 4He Compton form factor containing 4He GPD information. The full red curve is a calculation based on a convolution model. The black-dashed curve is a calculation from the same model using different GPDs for the nucleons. The blue long-dashed curve is from an off-shell model. (Image courtesy of Argonne National Laboratory, Thomas Jefferson National Accelerator Facility and collaborating institutions.)

Scientific achievement

For the first time, researchers exclusively measured the beam-spin asymmetry of coherent deeply virtual Compton scattering (DVCS) off the ⁴He nucleus. This allowed the first extraction of a nuclear generalized parton distribution (GPD) and the first model-independent extraction of a GPD of any kind; GPDs contain tomographic information about the momenta and spatial degrees of freedom of the quarks and gluons inside the nucleus.

Significance and impact

Such a study of the 3-D structure of the ⁴He nucleus shows the feasibility of the approach taken and may be seen as the first step toward accessing the 3-D partonic structures of nuclei, a major quest in modern nuclear physics.

Research details

Researchers built a radial time projection chamber calibrated specifically to detect recoiling ⁴He nuclei to ensure the exclusivity of the measurement.
To identify coherent DVCS, researchers selected events containing one electron, one recoiling ⁴He nucleus, and at least one photon in the final state.

Photo of the radial time projection chamber used to isolate the rare scattering events where the electron beam interacted with the entire Helium-4 nucleus. — This shows the radial time projection chamber used to isolate the rare scattering events where the electron beam interacted with the entire Helium-4 nucleus, rather than just a single proton or neutron (Image courtesy of Thomas Jefferson National Accelerator Facility.)

This research was conducted at the U.S. Department of Energy’s (DOE) Thomas Jefferson National Accelerator Facility. The work was funded by DOE’s Office of Nuclear Physics, along with several other research institutions worldwide. They include the Chilean Comisión Nacional de Investigación Científica y Tecnológica, the Italian Instituto Nazionale di Fisica Nucleare, the French Centre National de la Recherche Scientifique, the French Commissariat ‘a l’Energie Atomique, the United Kingdom Science and Technology Facilities Council, the Scottish Universities Physics Alliance (SUPA), the National Research Foundation of Korea, the Office of Research and Economic Development at Mississippi State University and the Consulat G´en´eral de France ‘a J´erusalem.

M. Hattawy et al. (CLAS Collaboration), Phys. Rev. Lett. 119, 202004 (2017). DOI: https://doi.org/10.1103/PhysRevLett.119.202004

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