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    • 2018-06-26 15:05:35
    • Article ID: 696548

    Tracking Down Helium-4's Quarks and Gluons

    Scientists obtain the first exclusive measurement of deeply virtual Compton scattering of electrons off helium-4, vital to obtaining an unambiguous 3-D view of quarks and gluons within nuclei.

    • Credit: Mohammad Hattawy and John Arrington

      A radial time projection chamber built at Argonne National Laboratory and used at Jefferson Lab isolated the rare scattering events where the electron beam interacted with an entire helium-4 nucleus, rather than just a single proton or neutron.

    The Science

    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 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 Impact

    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 toward providing similar 3-D pictures of the quark structure of nuclei. Furthermore, the measurements provide a new approach to understanding the modification of protons and neutrons within the dense environment of a nucleus. Studies of helium-4 are particularly important, as their partonic structure is encoded within a single GPD, simplifying their extraction and interpretation.


    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 Laboratory. 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.


    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]

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    Engineering professor receives Department of Energy grant

    New Mexico State University Department of Civil Engineering Assistant Professor Ehsan Dehghan Niri has received a United States Department of Energy grant. This is a three-year award for $400,000 and is a collaboration with Arizona State University.

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    Argonne to Advance High Performance Computing in Manufacturing

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    Missouri S&T researchers win multimillion dollar grant to build fast-charging stations for electric cars

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    Making batteries store more energy, last longer

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    Three Brookhaven Lab Scientists Named Fellows of American Physical Society

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    Southern Research first to win accreditation under ISO 14034

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    Kawtar Hafidi to head Physical Sciences and Engineering directorate at Argonne

    Physicist Kawtar Hafidi has been appointed Associate Laboratory Director, Physical Sciences and Engineering at the U.S. Department of Energy's (DOE) Argonne National Laboratory.

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    Cryocooler Cools an Accelerator Cavity

    Researchers demonstrated cryogen-free operation of a superconducting radio-frequency cavity that might ease barriers to its use in societal applications.

    Shining Light on the Separation of Rare Earth Metals

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    Breaking the Symmetry Between Fundamental Forces

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    Water Plays Unexpected Role in Forming Minerals

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    Discovered: Optimal Magnetic Fields Suppress Instabilities in Tokamak Plasmas

    U.S. and Korean scientists show how to find and use beneficial 3-D field perturbations to stabilize dangerous edge-localized modes in plasma.


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