The Emerging QCD Frontier
A cornerstone of the standard model of particle physics is Quantum Chromodynamics (QCD), the theory of strong interactions. The exchange of gluons between quarks mediates the force that provides the internal binding in all strongly interacting particles, just as the exchange of photons between electrically charged particles mediates the electromagnetic force. However, unlike photons, gluons are much more than mere force carriers. The gluons also have a color charge, and thus interact among themselves. The self-interactions of gluons determine all the unique features of QCD. Around 98% of the mass in the visible universe can be attributed to the strong interaction where gluons play the dominant role. Without gluons there would be no protons, no neutrons, and no atomic nuclei. Despite this dominance, the properties of gluons in matter remain largely unexplored.
Our current knowledge of the role of gluons in hadronic matter comes mainly from deep inelastic scattering experiments of electrons off protons most notably from HERA at DESY. Although electrons only interact with electrically charged particles, and gluons carry only color charge, a high-energy electron beam can still be used as an excellent gluon microscope. Since the electromagnetic interaction of electrons with quarks is understood to high precision, small changes in the resolution of quark distributions with respect to the transfer of energy and momentum can be used to infer the gluon distribution with good accuracy.
In order to open a new window into a kinematic regime that allows the systematic study of glue, an international group of scientists is proposing to construct the world's most versatile nuclear microscope, the Electron Ion Collider (EIC). With its wide range in energy, nuclear beams, high luminosity and clean collider environment, the EIC will offer an unprecedented opportunity for discovery and precision measurements. The EIC will allow us to study the momentum and space-time distribution of gluons and sea-quarks in nucleons and nuclei and gain insights into the role and nature of color neutral excitations of the QCD vacuum known as Pomerons. The addition of spin-polarized proton and light-ion beams to collide with polarized electrons and positrons would give the EIC unparalleled access to the spatial and spin structure of protons and neutrons in the gluon-dominated region. Two complementary concepts to realize EIC are under discussion: eRHIC, the construction of an electron beam to collide with the existing RHIC ion complex here at BNL and ELIC, the construction of an ion complex to collide with the upgraded CEBAF accelerator at JLAB.
Terra Incognita: The Structure of Nuclei at High Energies
A major discovery of the last decade is the dominant role of gluons in nucleons viewed by a high-energy probe. While these studies shed some light on the inner workings of QCD they also raise new questions. If an observer were (hypothetically!) to hop on to the electron probe and examine a small cross-sectional area of the oncoming proton, he/she would observe different things depending on the protons energy: At lower energies, the observer would, on occasion, see a quark but rarely a gluon. As the energy of the proton is boosted, the observer would see more and more gluons until eventually it would appear as if the proton were a tidal wave of onrushing gluons. This growth reflects a QCD cascade in which higher-momentum (harder) parent gluons successively split into two or more lower-momentum (softer) daughter gluons. But this explosive growth cannot continue unabated without eventually violating fundamental rules of physics. Physicists expect the growth to be tamed because at sufficiently high gluon densities softer gluons will again recombine into harder ones. The competition between splitting and recombination processes should lead to a saturation of gluon densities at small x.
In fact the stability of the vacuum in QCD requires that the intensity of the gluon fields be at most proportional to the inverse strength of the coupling between gluons. This absolute bound on gluon intensities ensures that the proton, as viewed by our observer, is transformed from a dilute "parton gas" of quarks and gluons into a novel regime of field intensities that are the maximum possible in nature. This state is the Color Glass Condensate (CGC). The transition from parton gas to CGC is represented by a ``saturation" momentum scale Qs. For a fixed boost value, known as the rapidity, the proton is a parton gas for resolving momenta greater than Qs, while it appears as a CGC for resolving momenta smaller than Qs, as depicted in the figure.
To reach CGC conditions, one needs a suitable combination of small gluon momentum fraction x, i.e. high beam energy, and large nuclei. The recombination sensitivity of gluons increases when the target nucleons are contained within a heavy nucleus; the many closely spaced nucleons along the beam path interact coherently with the probe, amplifying the gluon density. The predicted result of these multiple dependences is a saturation scale Qs2 ~ (A/x)1/3 that grows with increasing nuclear mass number A and with decreasing x, as illustrated in the figure. Using heavy nuclei as an amplifier of gluon densities therefore allows us to reach this regime with an order of magnitude less beam energy than would be needed in an electron-proton collider.
Tantalizing hints of this saturation have been extracted from measurements of electron-proton collisions at HERA and of deuteron-nucleus and nucleus-nucleus collisions at RHIC. Saturated gluon densities would have a profound influence on heavy-ion collisions at the LHC. But a definitive search for this universal gluonic matter, with a cleanly interpretable probe of its properties, requires an Electron-Ion Collider (EIC).
Searching for Spin in the Nucleon Sea
Few discoveries in nucleon structure physics have had a bigger impact than the surprising finding that quarks and anti-quarks together carry only about 30% of the nucleon's spin. Determining the source of the ``missing" spin has developed into a world-wide quest central to nuclear physics. The sum rule 1/2 = ∆S/2 + ∆G + Lq + Lg states that the proton spin is the sum of the quark and gluon intrinsic spin (∆S, ∆G) and orbital angular momenta (Lq, Lg) contributions. The EIC, with its unique high luminosity, highly polarized electron and nucleon capabilities, and its extensive range in center-of-mass energy, will allow access to quark and gluon spin contributions at substantially lower momentum fractions, x, than important current and forthcoming experiments at RHIC, DESY, CERN and JLab. Precise measurements of the spin contributions of individual sea quark flavors will further our intuition of the nucleon internal landscape, and provide a benchmark for models of nucleon structure. A key measurement at the EIC would be that of the spin distribution of the proton and its constituents, represented by the spin-dependent structure function g1(x,Q2), over a wide range in energy-scales, Q2, and down to small relative momentum fractions, x ~ 10-4. While data from polarized proton collisions at RHIC are now beginning to establish preferences among models at x > 0.03, the RHIC data will not be able to constrain the shape of the gluon spin distribution at lower x, where the density of gluons rapidly increases. The great power of the EIC in providing precise information in this high-gluon density region is self-evident.
Outlook
The Electron-Ion Collider embodies the vision of our field for reaching the next QCD frontier: the study of the glue, which binds all atomic nuclei. The EIC is the natural evolution of both the JLab 12 GeV upgrade, which will be focused on the role of valence quarks, and the RHIC II upgrade, which will complete the study of the hot, dense matter discovered at RHIC. Precision measurements at the EIC, directly interpretable within the framework of QCD, will open a new window into the non-linear regime of universal strong color fields; extracting the properties of this regime can radically transform our understanding of key features of the strong interactions.
To develop the most compelling case for the EIC in a timely way, it is clear that over the next five years significant progress must be achieved in the design of the accelerator and an optimized suite of detectors. The CAD division at BNL is working intensively on the eRHIC concept based on an Energy Recovery Linac, added to the existing RHIC complex (see figure).
