We've long known that nuclei are made of protons and neutrons, which are in turn composed of quarks and gluons, the building blocks of matter. While we've mapped out the spatial and momentum distributions of the quarks, our understanding of the gluon distributions and the interaction of quarks and gluons in matter is still very limited. Recently, scientists demonstrated that certain reactions probe the correlations between the momentum and spatial distributions of quarks and gluons within an entire helium nucleus, rather than its component protons and neutrons, allowing new insight into their complex interactions. The team used the interactions to provide new insight into the structure of the helium nucleus, one of the most common elements in the universe.
The probing reactions involve studying "generalized" parton distributions (GPDs). An important technique that is used is known as deeply virtual Compton scattering (DVCS). The DVCS reaction is the preferred probe of GPDs and yielded the first 3-D partonic images of the proton. New groundbreaking measurements of exclusive DVCS from the helium-4 nucleus are a critical step towards providing similar 3-D pictures of the quark structure of nuclei. Further, the measurements provide a new approach to understanding the modification of protons and neutrons within the dense environment of a nucleus.
The researchers' new measurements provided the first beam-spin asymmetry data on exclusive DVCS from helium-4 using a highly polarized electron beam from the Continuous Electron Beam Facility (CEBAF) at Jefferson Lab. Scattering from a pressurized helium-4 target was measured in the CEBAF Large Acceptance Spectrometer (CLAS) in Hall B. Isolating exclusive scattering—where the nucleus is not broken up in the high-energy collision—requires detection of the recoil helium-4 nucleus. The recoil nucleus was detected using a new Radial Time Projection Chamber, built by Argonne National Laboratory, to ensure that this rare scattering process was cleanly identified. From the azimuthal dependence of the beam-spin asymmetry the real and imaginary parts of the helium-4 Compton form factor, which directly sample the nuclear GPD, were extracted in a model-independent way. This data, while limited in statistics, proved the experimental feasibility of measuring such nuclear exclusive reactions and led the way to the approval of the next generation of nuclear measurements. These future measurements will be carried out using the new CLAS12 spectrometer and the upgraded CEBAF 12-GeV electron beam. In addition to DVCS on helium-4, deeply virtual meson production of theta mesons will be measured, giving researchers the chance to compare directly the quark and the gluon 3-D tomography of the helium-4 nucleus. Another important aspect of this program is the study of the partonic structure of bound nucleons via the incoherent DVCS channels of the helium-4 nucleus, providing a new way to see if the dense nuclear environment and strong interactions between nucleons bound together change the internal structure of the protons and neutrons. Because helium-4 is a small but tightly bound nucleus, these measurements can be compared to the most sophisticated calculations available, making it the ideal nucleus for such studies.
Physics Division Director, Argonne National Laboratory
(630) 252-4012; Kawtar@anl.gov
This work was supported in part by the Chilean Comisión Nacional de Investigación Científica y Tecnológica (CONICYT), the Italian Instituto Nazionale di Fisica Nucleare, the French Centre National de la Recherche Scientifique, the French Commissariat à l'Energie Atomique, the Department of Energy Office of Science's Office of Nuclear Physics, the UK Science and Technology Facilities Council, the Scottish Universities Physics Alliance, the National Research Foundation of Korea, and the Office of Research and Economic Development at Mississippi State University. M. Hattawy also acknowledges the support of the Consulat Général de France á Jérusalem. The Southeastern Universities Research Association operates the Thomas Jefferson National Accelerator Facility for the Department of Energy.
M. Hattawy, et al. (CLAS Collaboration), "First exclusive measurement of deeply virtual Compton scattering off 4He: Toward the 3D tomography of nuclei." Physical Review Letters 119, 202004 (2017). [DOI: 10.1103/PhysRevLett.119.202004]
Argonne National Laboratory science highlight: First exclusive measurement of deeply virtual Compton scattering off Helium-4: Toward the 3-D tomography of nuclei
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