Peering into Hadronic Matter: The Electron-Ion Collider
By Christine Aidala
The experimental technique of deep-inelastic scattering (DIS) of electromagnetic probes off of nucleons or nuclei has been a tool of the trade in studying the structure of hadronic matter since the 1960s. To date, DIS experiments studying the structure of nuclei as well as the spin structure of the nucleon have all utilized fixed targets. A DIS collider facility with capabilities for high-energy nuclear as well as polarized beams would offer outstanding opportunities to further explore the internal structure of protons and nuclei. A proposal is currently being developed for such a facility, the Electron-Ion Collider (EIC), as an expansion either of RHIC (eRHIC) or of the CEBAF electron machine at Jefferson Lab (ELIC).
The Role of Gluons in Matter
Despite the essential role they play in determining the properties of hadrons, a tremendous amount remains to be learned about gluons in matter. Using traditional theoretical tools within the framework of perturbative QCD, the gluon distribution in matter is predicted to rise sharply as one goes to lower and lower momentum fractions of the nucleon, eventually violating the unitarity of the theory and becoming unphysical. To deal with this, new tools have been developed which introduce saturation mechanisms, allowing gluons not only to shed other gluons carrying ever-smaller momentum fractions, but also for these soft gluons to recombine into a single gluon carrying greater momentum.
The onset of saturation is characterized by a dynamical scale which depends directly on both energy and nuclear size. The enhancement of the saturation scale with increasing nuclear size means that the saturation regime can be reached at significantly lower energies in heavy nuclei compared to the proton (see figure). With high energy nuclear beams and the straightforward access to kinematic variables offered by DIS, the EIC would be an ideal facility at which to study the saturation regime in QCD.
Precision Studies of Nucleon Spin Structure
Polarized DIS experiments, as nuclear DIS experiments, have thus far utilized fixed targets and therefore explored only a restricted kinematic region. In particular, knowledge of the gluon helicity distribution from polarized DIS experiments is limited by the narrow kinematic range of the present data. The EIC would be able to access the polarized gluon distribution down to significantly smaller momentum fractions through various inclusive and semi inclusive measurements, with uncertainties small enough to gain sensitivity to the functional form for the first time.
In recent years an increasing amount of attention has been focused on the transverse spin structure of the proton. Large transverse single-spin asymmetries have been observed over a broad range of center-of-mass energies and are believed to be due to spin orbit effects in the nucleon itself and/or in the process of fragmentation into hadrons. In order to study the orbital angular momentum of the partons within a hadron, one clearly needs a theoretical framework which takes into account not only the momentum carried by partons collinearly to the direction of the proton, but also in the transverse direction. An increasing amount of high-energy data relevant to transverse-momentum-dependent distribution and fragmentation functions has become available in the last few years, and the high energy DIS data sensitive to transverse-momentum-dependent distributions that the EIC could provide would help bring this field to maturity.
The proposed Electron-Ion Collider promises to be a formidable machine for the study of QCD matter. As a unique facility performing deep-inelastic scattering at high energies of electrons off of heavy nuclear as well as polarized proton and light ion beams, it will offer a wide range of physics opportunities, from exploring the nature of strong color fields in nuclei to precisely imaging the sea quarks and gluons within the nucleon. Efforts are ramping up as we prepare to make a clear physics case as input to the 2012 Nuclear Physics Long Range Planning process, with the goal of coming out of that process with a high-level recommendation for construction.
Further information on ongoing work toward the EIC can be found here.