In the US, Brookhaven National Laboratory has a declared design for an EIC scheduled to be built in 2030. In Europe, CERN has plans for the LHeC. There are also Chinese and Russian plans for an electron ion collider.
eRHIC
Brookhaven National Laboratory's conceptual design, eRHIC, proposes upgrading the existing Relativistic Heavy Ion Collider, which collides beams light to heavy ions including polarized protons, with a polarized electron facility. On January 9th, 2020, It was announced by Paul Dabbar, undersecretary of the US Department of EnergyOffice of Science, that the BNL eRHIC design was selected over the conceptual design put forward by Thomas Jefferson National Accelerator Facility as the design of a future EIC in the United States. In addition to the site selection, it was announced that the BNL EIC had acquired CD-0 from the Department of Energy.
LHeC
The LHeC would make use of the existing LHC accelerator and add an electron accelerator to collide electrons with the hadrons.
Technical challenges
Polarization
In order to allow understanding of spin dependence of the electron nucleon collisions, both the ion beam and the electron beam must be polarized. Achieving and maintaining high levels of polarization is challenging. Nucleons and electrons pose different issues. Electron polarization is affected by synchrotron radiation. This gives rise to both self polarization via the Sokolov Ternov effect and depolarization due to the effects of quantum fluctuations. Ignoring the effects of synchrotron radiation, the motion of the spin follows the Thomas BMT equation.
High Luminosity Achievement
The luminosity determines the rates of interactions between electrons and nucleons. The weaker a mode of interaction is, the higher luminosity is required to reach an adequate measurement of the process. The luminosity is inversely proportional to the product of the beam sizes of the two colliding species, which implies that the smaller the emittances of the beams, the larger the luminosity. Whereas the electron beam emittance is determined by an equilibrium between damping and diffusion from synchrotrotron radiation, the emittance for the ion beam is determined by the initially injected value. The ion beam emittance may be decreased via various methods of beam cooling, such as electron cooling or stochastic cooling. In addition, one must consider the effect of intrabeam scattering, which is largely a heating effect.
Scientific purpose
An electron ion collider allows probing of the substructure of protons and neutrons via a high energy electron. Protons and neutrons are composed ofquarks, interacting via the strong interaction mediated by gluons. The general domain encompassing the study of these fundamental phenomena is nuclear physics, with the low level generally accepted framework being Quantum Chromodynamics, the 'chromo' resulting from the fact that quarks are described as having three different possible values for color charge. Some of the remaining mysteries associated with atomic nuclei include how nuclear properties such as spin and mass emerge from the lower level constituent dynamics of quarks and gluons. Formulations of these mysteries, encompassing research projects, include the proton spin crisis and the proton radius puzzle.
Collaboration
Electron Ion Collider user group:
Previous electron ion colliders
One electron ion collider in the past was HERA in Hamburg, Germany. Hera ran from 1992 to 2007 and collided electrons and protons at a center of mass energy of 318 GeV.
