An Assessment of U.S.-Based Electron-Ion Collider Science (2018) / Chapter Skim
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2 The Scientific Case for an Electron-Ion Collider
2 The Scientific Case for an Electron-Ion Collider
Pages 23-44
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From page 23... ...
In relativistic theories, mass is given by m = E/c2, and the energy of the quark and gluon fields contributes to the mass of the nucleon. Quantum fields are richer than classical fields, because the vacuum of the theory is not empty, but filled with quantum fluctuations of particles and antiparticles.
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From page 24... ...
In this sense, the source of visible mass in the universe is not the Higgs field, but the gluon field. Conducting a thought experiment allows an exploration of the mass of the proton by ionizing it, removing sea quarks and gluons from the valence quark core, and carefully monitoring the binding energies in the process.
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From page 25... ...
In real meson production, the final state consists of the target nucleon as well as a quark-antiquark bound state, such as a vector meson. The virtual photon is characterized by its resolution and energy, as in DIS, but there is an additional kinematic observable, the momentum transfer between the initial and final state proton.
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From page 26... ...
images of the proton stacked along the Bjorken x direction. Starting at large x, in the domain of valence quarks, and proceeding toward lower x, the regime of sea quarks and gluons, these images will reveal where quarks and gluons are located in the transverse plane.
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From page 27... ...
In this case, orbital motion leads to correlations between spin and transverse momentum kT, and tomographic images of the kT distribution are fully 2+1 dimensional, as seen in Figure 2.7. profiles are also obtained independently through varying the probe resolution Q2 in real photon production.
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From page 28... ...
In the near future, JLab, with its 12 GeV-energy high-luminosity upgrade, will provide high-precision images of the valence quark region. An EIC would dramatically improve on these measurements, via detailed im ages of gluonic profiles and would also offer a path to determine the orbital con tribution of sea quarks and gluons to the nucleon spin.
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From page 29... ...
This regime is dominated by gluons and sea quarks. High energy also provides large kinematic coverage, which is crucial in extracting gluon distributions.
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From page 30... ...
such as an intact nucleon combined with a final state photon or vector meson, that occur in only a small fraction of all reactions. Parton imaging also requires an ac curate determination of not only total interaction rates, but of the dependence of these rates on the deflection angles of all scattered particles, for which large lumi nosity is also needed.
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From page 31... ...
Transverse momentum imaging therefore constrains the possible evolution of color fluctuations with Bjorken x, going from the valence sector at large x to the sea quark and gluon regime at small x. In the small x regime, the results provide important information about the limit of high gluon density, discussed in the last section of this chapter.
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From page 32... ...
Nucleons are bound states of quarks and gluons with total spin ½; the total angular momentum of a nucleon is the sum of the spin and orbital angular momenta of the quarks and gluons they contain. Similarly, the total angular momentum of nuclei is the sum of the spin and orbital motion of nucleons, and in atoms it is the combination of nuclear angular momentum with the spin and orbital motion of electrons.
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From page 33... ...
Exploratory measurements of the quark orbital angular momentum in the valence quark regime are an important part of the physics program at the 12 GeV upgrade of JLab. Polarized proton- proton collisions at the Relativistic Heavy Ion Collider (RHIC)
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From page 34... ...
deter mined in these measurements offer the possibility of isolating the contribution to the nucleon spin of the orbital angular momentum of gluons. Transverse Motion in Polarized Nucleons The dynamics of spin-orbit correlations in QCD can be studied using the trans verse momentum distribution of partons in a transversely polarized proton, one with its spin direction orthogonal to its direction of motion.
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From page 35... ...
For exam le, EIC measurements of transverse momentum dependent parton distribution func p tions will directly probe observables sensitive to the color phases generated by the proton's constituents. There is an interesting cross-check for this interpretation based on comparing spin a symmetries in DIS with analogous asymmetries in the Drell-Yan process, which involves quark-antiquark annihilation in proton-proton scattering.
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From page 36... ...
quarks in a proton. The figure shows three slices, ranging from the valence quark region at large Bjorken x to the sea quark regime at low x.
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From page 37... ...
These measurements would not only constrain the distribution of gluons, but also test theories of the interaction of virtual photons with dense gluon matter, in particular the dipole picture discussed below. An EIC would significantly extend measurements of nuclear parton distribution functions in the low x regime.
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From page 38... ...
With such capability, an EIC would allow imaging of nuclei by measuring both quark and gluon density profiles. The nucleus is also a laboratory for understanding the dynamics of confinement, the process by which a high-energy parton created by the interaction of a virtual photon with the nucleus is color neutralized and evolves into a hadron.
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From page 39... ...
With electrons as the probe, one can select the energy of the virtual photon, thus controlling the momentum transfer to the quark, and obtain clean measurements of medium induced energy loss by choosing high-photon energies, which lead to hadronization outside of the nucleus (see Figure 2.3.1, left)
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From page 40... ...
Producing dense, saturated gluon matter requires high energy and small x. Estimates of the saturation scale at HERA, which collided protons and electrons at a center-of-mass energy of 318 GeV, give a value around 1 GeV, which is not much larger than typical hadronic energy scales.
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From page 41... ...
5 There is some uncertainty in the value of the saturation scale, and there is no definitive theoreti cal prediction for how large Qs has to be for the full simplicity of the saturation picture to manifest itself. However, much of the experimental program, measuring nuclear effects in the gluon distribution function, studying diffractive scattering in the regime of high gluon density, and mapping the gluon distribution in the transverse plane, does not depend on any particular picture of QCD in the regime of high gluon density.
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From page 42... ...
. In the low x regime, the target is dense gluonic matter and the probability for absorbing the quark-antiquark dipole will be large, and may approach unity.
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From page 43... ...
The picture of DIS based on the dipole picture -- that the virtual photon turns into a quark-antiquark color dipole -- predicts the energy and nuclear mass dependence of diffractive DIS. The diffractive cross section rises steeply with energy at low energy, but becomes an approximately constant fraction of the total cross section in the regime that an EIC would explore.
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From page 44... ...
Schenke, 2016, Evidence of strong proton shape fluctuations from incoherent diffraction, Phys.
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