Probing the proton

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Science  23 Jan 2015:
Vol. 347, Issue 6220, pp. 363-365
DOI: 10.1126/science.347.6220.363

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Many decades after their discoveries, the basic building blocks of the atomic nucleus—the proton and the neutron—remain among the most mysterious of subatomic particles. In the cartoon view, the positively charged proton and the uncharged neutron both consist of trios of particles called up quarks and down quarks. In actuality, each is a pullulating mass of countless quarks, antiquarks, and gluons, massless particles that convey the strong nuclear force that holds quarks together. A nucleon is so messy that physicists can't say exactly how its most basic properties, such as its mass and spin, emerge from the tangle. But now physicists at Thomas Jefferson National Accelerator Facility here are finishing a $338 million upgrade to their particle accelerator, the Continuous Electron Beam Accelerator Facility, to double its energy and probe the innards of protons and neutrons with unprecedented precision. In the coming decade, a mosaic of measurements may finally give physicists a clearer view into the proton and the neutron.

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