An Assessment of U.S.-Based Electron-Ion Collider Science (2018) / Chapter Skim
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1 Introduction
1 Introduction
Pages 6-22
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From page 6... ...
discovered that protons and neutrons, the building blocks of nuclei, are themselves made of smaller constituents -- "quarks." This remarkable structure was revealed by scattering electrons on protons and other nuclei, and indeed the SLAC 2-mile-long electron accelerator became the world's most powerful "electron microscope," peering inside neutrons and protons (see Box 1.1)
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From page 7... ...
Electron-scattering study of the structure of nuclei began with post-World War II experiments in Illinois and was continued in the 1950s by Robert Hofstadter at Stanford University, who used electrons to peer inside the nucleon itself. A scattered electron creates a virtual photon to see inside the nucleon; the photon energy (technically the square root of Q2, its total momentum squared)
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From page 8... ...
carrier is the massless "gluon." Quarks and gluons carry a color charge.2 The fundamental "strong force," which is also responsible for binding nucleons to gether in nuclei, is very different from the electromagnetic force holding atoms and molecules together, and is described theoretically by QCD, a remarkable but mathematically complicated generalization of ordinary electricity and magnetism. Discovering how the structure of nucleons arises from the dynamics of their quark and gluon constituents, and how interactions between protons and neutrons in nuclei arise from these dynamics, is a major goal of modern nuclear physics.
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From page 9... ...
However, the precise arrangement, or states, of gluons and sea quarks inside the nucleon is not known, and the mechanism by which mass is generated remains only partially understood. A second fundamental property of the nucleon is that it has internal angular momentum, or spin.
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From page 10... ...
The concept of an EIC -- which the 2015 Nuclear Science Advisory Committee (NSAC) Nuclear Physics Long Range Plan6 identified as the highest-priority project for new construction in nuclear physics -- builds upon a long heritage of electron scattering machines.
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However, two big puzzles remain in understanding atoms as the building blocks of the physical world. The first is the full extent of the periodic table of chemical elements and how many isotopes each element has.
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A new, related field arose, known as condensed matter (initially called solid-state) physics, and is concerned with the various phases of matter that exist, from conductors and insulators to crystalline states of matter to even more exotic phases of matter including superconductors and other macroscopic quantum states of matter.
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From page 13... ...
In other words, how do the con stituents of the nucleon, the valence quarks, the sea quarks, and the gluons, and importantly their interactions, lead to a mass some 100 times larger than the sum of the three constituent quarks alone? Physicists are used to the mass of a bound system -- a nucleus made of neutrons and protons, an atom made of a nucleus and electrons or even two black holes bound together by gravity -- having a mass less than the sum of its parts.
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From page 14... ...
Just as the van der Waals force holds neutral atoms together in molecules, a residual aspect of the color force holds colorless neutrons and protons together in the nucleus. And within the nucleus, neutrons and protons appear gen erally to retain their individual identities just as atoms do within a molecule (see Figure 1.2.1)
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From page 15... ...
The color force binds quarks into neutrons and protons, and neutrons and protons into nuclei. The electromagnetic force binds electrons and nuclei into atoms, and atoms into molecules.
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From page 16... ...
How, one may ask, can a colorless virtual photon probe colored gluons? The key is to observe reactions that are dominated by a two-tep process in which the virtual photon splits into a s quark-antiquark pair that interacts with the color field of the target nucleon (see Figure 2.2 in Chapter 2)
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From page 17... ...
The current point of view, shown on the right, is that the proton contains quarks, as well as dynamically generated sea quark-antiquark pairs and gluons; the total spin is composed of that of the elementary spins (colored arrows) and orbital motion, as indicated by the light blue arrow.
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From page 18... ...
Tomography provides a series of images of the proton in the transverse plane, labeled by the longitudinal momentum fraction of the parton. Scanning through these pictures, starting from the valence quark regime, will enable the determination of where and how gluons and sea quarks appear and whether the gluon distribution has a compact core, smaller than the electric charge radius of the proton, or whether the gluon distribution is extended.
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From page 19... ...
The orbital angular momentum of quarks and gluons can be extracted using the transverse position information contained in the tomographic measurements, discussed at length in Chapter 2. Measurements of the gluon-spin contribution to the spin of the proton are based on the idea that the gluon can transfer its polarization to a quarkantiquark pair, which can be probed using polarized electrons.
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From page 20... ...
An EIC would be able to reach unprecedented gluon densities by using the concentrated gluon fields of large nuclei. Relativistic length contraction implies that the number of gluons per transverse area is proportional to the radius of the nucleus, which is itself proportional to the one-third power of the nuclear mass number A
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From page 21... ...
Not only would development of an EIC advance accelerator science and technology in nuclear science, it would benefit other fields of accelerator-based science and society. The accelerator physics and technology advances required for an EIC will, importantly, have the potential to extend the capabilities of many particle accelerators built for other purposes, from medicine through materials science to elementary particle physics.
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From page 22... ...
JLab, conversely, has an electron accelerator but would need to add the hadron injectors and storage ring and an electron storage ring. This has resulted in two somewhat different designs for an EIC, both of which push the limits of present technology.
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