On Tuesday, the National Academies of Sciences, Engineering, and Medicine released a report giving a full-throated endorsement to a new particle accelerator project called the Electron-Ion Collider (EIC). The sign off is a significant step toward getting the project off the ground and taking particle physics to the next level.
Unlike the Large Hadron Collider (LHC) in Europe, which smashes protons into protons at near light speed, the new machine would squirt high-energy beams of electrons into protons or heavier ions, Adrian Cho at Science reports. These collisions, Notre Dame nuclear physicist Ani Aprahamian, co-chair of the report, explains to Cho, produce cleaner and easier-to-interpret results compared to the LHC and is better for probing just how the quarks and gluons inside protons and atomic nuclei are arranged.
It turns out, while we have done a pretty good job at identifying new elementary particles, or the bits and pieces that make up protons, neutrons and atoms, we still have difficulty figuring out how they all fit together. We still don’t even know just what a proton—or a nucleus—really looks like.
MIT physicist Richard Milner tells Jennifer Chu at MIT News that the new accelerator would be able to answer three main questions: The first is to understand where the mass of a proton comes from, since it can be up to 100 times greater than the mass of the bits and pieces it contains. (The weight of the three quarks bound together by the strong nuclear force produced by gluons accounts for just 5 percent of a proton’s mass.) Second, researchers hope to better understand the concept of angular momentum or “spin.” Third, the EIC may help reveal how gluons, particles that hold other particles together, work. “Gluons in matter are a little like dark matter in the universe: unseen but playing a crucial role,” Milner says. “An electron-ion collider would potentially reveal new states resulting from the close packing of many gluons within nucleons and nuclei.”
Two national laboratories are already competing to host the accelerator, though it will likely be several years before the Department of Energy officially launches the project. It’s expected that the project would upgrade the existing ion collider at the Brookhaven National Laboratory in Long Island, or the electron beam at the Thomas Jefferson National Accelerator Laboratory in Newport News, Virginia. According to Edwin Cartlidge at Nature, a report by the Nuclear Science Advisory Committee three years ago suggested the project would cost at least $1 billion.
The timeline for the project is in question. The Department of Energy, which would build the facility, is currently in the middle of funding a $730 million facility for rare isotope beams at Michigan State University, and likely doesn’t have the capacity for managing another large-scale project until that is complete in 2020.
Cartlidge also points out that plans in China and at CERN in Europe are currently being batted around for electron-ion colliders as well, and that the three groups may need to consider collaborating to get the project off the ground anytime soon. And then there’s the United States fraught history with particle accelerators. In 1993, Congress cancelled the Superconducting Super Collider, a massive particle accelerator that would have led the discovery of the Higgs boson and other particles decades before it was discovered at the LHC in 2012. Which is to say, politics could get in the way of the EIC.
Cho reports that this wouldn’t be the first EIC to be constructed. A version of the project called the Hadron-Electron Ring Accelerator (HERA) was conducted in Germany between 1992 and 2007 and led to the discovery of the gluon. The U.S. version of the collider would run at lower energies, but would have 100 to 1,000 the number of collisions, producing exponentially more data.
Back in 2015, when the influential Nuclear Science Advisory Committee publicly endorsed the construction of the EIC, Donald Geesaman, a nuclear physicist at Argonne National Laboratory in Illinois, and the chair of NSAC, spoke to the scientific need for the creation of the EIC, declaring, “Until we have the EIC, there are huge areas of nuclear physics that we are not going to make progress in.”