Brookhaven facility to be transformed into electron–ion collider Free

Quantum chromodynamics (QCD), the theory of the strong force, describes the interactions between quarks and massless, force-carrying gluons in nuclei. The strong force increases as objects get farther apart, and it confines quarks in the nucleon; quarks and gluons are never found free outside a proton or neutron. “It’s a beautiful theory. It explains nearly all visible matter,” says Raju Venugopalan, a theorist at Brookhaven. “But there are many things we still do not understand in fundamental ways.”

Collisions between electrons and protons or heavier ions produce fewer particles than hadron–hadron collisions such as those at Brookhaven’s current flagship Relativistic Heavy Ion Collider (RHIC), and they can therefore yield more precise information about the structure and dynamics inside nucleons. The EIC will smash electrons up to 18 GeV into protons up to 275 GeV. The collision center-of-mass energy will range from 20 GeV to 140 GeV; that’s lower than HERA’s 318 GeV, but the EIC will have the advantage of greater tunability for exploring interactions as a function of energy. Both beams at the EIC will be polarized, and the luminosity will be 10^33–34 cm^–2s^–1, more than two orders of magnitude higher than at HERA. In addition to the wide energy range, polarized beams, and high luminosity, the EIC will be able to accelerate many different ion species. Those four characteristics combine to form “the ultimate QCD machine,” says Venugopalan. “We will try to understand the strongest fields at the smallest distances possible.”

An incident electron can scatter inelastically off individual quarks or gluons, or off clouds of gluons, or it can scatter elastically from the whole nucleus. The EIC will track and measure the electrons, photons, kaons, pions, protons, and other hadrons that emerge from collisions. It will identify and record particle mass, momentum, energy, and type.

The quark mass accounts for only 2% of the proton mass. And quarks account for only 30% of proton spin. One has to imagine a model in which the arrangement of quarks and gluons, their spins, and their orbital angular momenta contribute, says Carl Gagliardi of Texas A&M University. “Even antiquarks contribute to the spin of the proton. That means the ephemeral constituents play a real and visible role in the structures we are made of.”

With the EIC, scientists will create three-dimensional spatial and momentum space images of quarks and gluons inside protons. Previous generations of experiments have looked mainly in the direction parallel to the beams, says Richard Milner of MIT, who is on the steering committee of the EIC users group. The EIC will implement new types of measurements, pioneered at the Thomas Jefferson National Accelerator Facility in Virginia, to look in the transverse direction, he says.

In addition to the three valence quarks that make up a proton, quark–antiquark pairs constantly appear and disappear in nuclei. Elke-Caroline Aschenauer, a Brookhaven nuclear physicist, asks where those fleeting particles reside. “Do they form a rim? Are they scattered throughout the nucleus?” By studying the spatial and momentum distributions inside protons, she says, “we may get a better understanding of what confines gluons and quarks to the nucleus.”

If a quark is knocked out, the proton is destroyed and the quark binds with an antiquark or with other quarks to form a pion or other colorless hadron; jets of debris give clues to the quark distribution inside the original proton. Another possible outcome is deeply virtual Compton scattering, in which the energy is such that an electron–quark collision leaves the proton intact, and it, the scattered electron, and an ejected photon embody information about the quark’s motion. Polarization results will be used to probe spin and, with high energies and heavy nuclei, gluon density.

The EIC could provide access to a new state of gluonic matter. Theory predicts that gluons multiply, eventually reaching a maximum density, or saturated state, in the finite volume of a nucleus, says director of EIC science Abhay Deshpande. Gluonic matter has unique properties and should pervade all nuclei, he says. It would be observable at high energies, although it’s not clear how high. The trick with the EIC, Deshpande says, is to enhance the nuclear gluon density by accelerating heavy ions, like lead or gold. That will allow “a more effective search for saturated gluonic matter” than was possible with HERA’s higher-energy electron–proton collisions.

Understanding the structure of nuclei has broader implications too. Andrew Strominger of Harvard University is keen to compare EIC data with measurements at facilities such as LIGO, the Laser Interferometer Gravitational-Wave Observatory. The mathematics, he says, implies analogies between the “color memory” produced by the motion of quarks and antiquarks in dense color gluon shock waves and the “gravitational memory” produced by gravitational waves emitted in a black hole merger or other violent cosmological event. In both cases “memory” is information from the event that is retained in the system.

Jürgen Berges of the University of Heidelberg in Germany says theory describes “remarkable similarities” between gluons at saturation and cold atoms in a Bose condensate. By comparing data about the energy and momentum distributions of gluons and excitations in cold quantum gases, he says, links between the two different quantum systems may be found on vastly different energy scales. “The EIC is very interesting for studying this probable universality.”

Brookhaven’s bid to host the EIC won out over a competing design by Jefferson Lab. DOE under secretary for science Paul Dabbar says the agency made the choice after the two proposals were reviewed by an international review panel, which considered six criteria: scientific merit, technical maturity, schedule, cost, project management, and the likely impact on other ongoing or planned DOE missions. “It was a standard DOE review process,” he says.

Observers note that Jefferson Lab, which hosts an electron accelerator for fixed-target collisions, would have needed to add an ion accelerator, which would likely cost more than the electron accelerator and storage ring Brookhaven will have to install in its 4 km RHIC tunnel to realize an electron–ion collider. They also note that Brookhaven was more in need of a new raison d’être to maintain a vibrant nuclear physics program; Jefferson Lab’s program extends further into the future than Brookhaven’s would without the EIC.

Of course, scientists at Jefferson Lab are “quite disappointed,” says Haiyan Gao, a Duke University physicist whose research is based at Jefferson Lab. But the lab is expected to be a major partner in the EIC and also has plans for new experiments, she says. “The bigger picture is that going ahead with the EIC is hugely positive news.”

The technology for realizing the EIC’s accelerators at Brookhaven already exists, says Robert Tribble, the lab’s deputy director for science and technology. “We have the ion beams we need. The polarization exists at close to the levels we need.” Minor modifications will be made to RHIC’s existing configuration, he says. And a 150 MeV electron beam will propagate with the ions to cool them and thus maximize the collider luminosity.

The detectors, however, present many technical difficulties. They need to cover a wide range of angles, including small ones close to the beam pipes. Probably the most challenging technology is particle identification, says Brookhaven scientist Thomas Ullrich. Because the particles produced in the collisions come out with a broad range of energies, from 200 MeV up to 50 GeV, “the challenge cannot be met with a single technology.” Overall, Cherenkov radiation detectors, gas and solid-state methods to detect and measure particle tracks and momentum, time-of-flight measurements, and calorimeters will be needed, he says.

So far, DOE has committed to just one detector site, although that could change. “We could cover both high energies and proton imaging at one [collision] site,” says Tribble. “The community needs to decide if that’s optimal.” The majority of users prefer two detectors, says Silvia Dalla Torre, a nuclear physicist at INFN (Italy’s National Institute for Nuclear Physics) in Trieste and spokeswoman for that country’s participants in the EIC users group. “Each technology has advantages and disadvantages, and despite being clever when you make choices, nature always has surprises, so two complementary approaches would ensure better results.” With two detectors, results could be confirmed, and each could be optimized for specific observations. That decision will be made in the coming months. It’s not just a matter of money, according to Dabbar, who says the project can go ahead with US money alone. International partners are welcome, he adds.

In the world context, notes Dalla Torre, the EIC “is the only such project that is becoming concrete.” Two other electron–ion colliders with different capabilities and scientific objectives are on the drawing boards: a higher-energy facility at CERN (see Physics Today, May 2017, page 29) and a lower-energy one in China. The green light for the EIC is good news for the international science community, she says. More than 40% of the EIC users group’s nearly 1000 members are based outside the US. “If a relevant nation like the US were to give up this kind of science, it would be noticed, and other countries might want to save money by cutting funding to this area. In going forward, the US sends a strong signal that this is strong science.”