Scientists endorse billion-dollar collider to look inside protons and neutrons

The next dream machine for U.S. nuclear physicists got an important boost today in a report from the National Academies of Sciences, Engineering, and Medicine. The report committee glowingly approved of the science that could be done with the proposed Electron-Ion Collider (EIC), a billion-dollar accelerator that would probe the innards of protons and neutrons. The endorsement should help the Department of Energy (DOE) justify building the EIC at one of two national laboratories competing to host it, although the project probably won’t get the go-ahead for several years.

“We’re basically saying, ‘You’ve really got to do this,’” says Ani Aprahamian, a nuclear physicist at the University of Notre Dame in South Bend, Indiana, and co-chair of the report committee.

The inner structure of the proton and the neutron remains mysterious. Crudely, a proton consists of three subatomic particles called quarks, bound by the strong nuclear force. In actuality, a proton is far more complex. Because of the uncertainties inherent in quantum mechanics, its interior roils with quark-antiquark pairs popping in and out of virtual existence. It also teems with gluons, the quantum particles that convey the strong force. The mess is so complex that even basic properties of the proton remain unexplained. For example, its three quarks account for less than 5% of its mass, the rest arising somehow from energy of the virtual quarks and gluons.

By blasting a beam of electrons into a beam of protons or ions, the EIC would help solve this mystery and a parallel one: how the proton gets its spin. Just as in the case of mass, the proton’s spin is not simply the sum of the spins of the three quarks; it also has unknown contributions from gluons and from the quarks orbiting around one another. Finally, the EIC could probe the gluons’ behavior for so-called emergent properties. For example, some theories predict that a proton’s gluons crowd into a single quantum wave a bit like laser light. The EIC would be better for such studies than a machine that smashes protons into protons, such as the Large Hadron Collider (LHC) near Geneva, Switzerland. That’s because the electron is an infinitesimally small particle and produces cleaner, easier to interpret collisions, Aprahamian says.

The EIC would help maintain U.S. expertise in colliding beams, says Gordon Baym, a theorist at the University of Illinois in Urbana and co-chair of the report committee. “It’s the only collider in the U.S. that’s being considered for the next 50 years or so,” he says. Now, the United States only has the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Upton, New York, which was completed in 1999. Europe has the newer LHC and Japan runs the newly upgraded SuperKEKB electron-positron collider in Tsukuba.

Physicists at two DOE labs propose to assemble the EIC in different ways. At Brookhaven, RHIC smashes together heavy nuclei such as gold to melt them and create a soup called a quark-gluon plasma, the stuff that filled the universe just after the big bang. Brookhaven physicists eventually want to add an electron accelerator to convert RHIC into the EIC.

However, the Thomas Jefferson National Accelerator Facility (Jefferson Lab) in Newport News, Virginia, has recently upgraded its Continuous Electron Beam Accelerator Facility (CEBAF), which fires electrons into stationary targets to study protons, neutrons, and nuclei. Jefferson Lab researchers hope to add an ion accelerator to it to make the EIC. Jefferson Lab is a smaller facility that’s more narrowly focused on nuclear physics than Brookhaven, and its long-term survival could depend on landing the EIC.

The report avoids comparing the two labs’ proposals, but offers DOE officials a scientific case for the EIC. However, on top of running RHIC and CEBAF, DOE’s $684 million office of nuclear physics is also building the $730 million Facility for Rare Isotope Beams at Michigan State University in East Lansing, which will generate exotic nuclei upon completion in 2020. Given the costs of operating those facilities, DOE probably can’t afford the EIC any time soon, notes Donald Geesaman, a nuclear physicist at Argonne National Laboratory in Lemont, Illinois, and former chair of DOE’s Nuclear Science Advisory Committee.

More time may be welcomed, anyway, as the Brookhaven and Jefferson Lab proposals cannot yet meet key technical requirements. From 1992 to 2007, physicists in Germany ran the Hadron-Electron Ring Accelerator (HERA), which also collided electrons and protons and revealed the gluon. The EIC will run at lower energy than HERA did, but to achieve its goals, it will have to generate collisions at a rate 100 to 1000 times higher with highly polarized electron and proton beams.

For now, Brookhaven and Jefferson Lab scientists are collaborating on the R&D rather than competing. “It’s been very exciting to see the community self-assemble,” says Jefferson Lab Director Stuart Henderson. Brookhaven Director Doon Gibbs says, “The immediate objective is to keep walking down the road together with both labs in lockstep.” If all goes well, however, the two labs’ ambitions will eventually collide.