Brookhaven National Laboratory/CC BY-NC-ND 2.0

Two large science experiments head a wish list drawn up by US nuclear physicists for the next decade: a quest to uncover the nature of neutrinos and a particle collider to study the forces that bind quarks.

The big-ticket items, each of which would cost hundreds of millions of dollars, are among the top priorities highlighted by the Nuclear Science Advisory Committee (NSAC) on 15 October. Every 5–7 years, this panel of high-level nuclear physicists presents a long-term plan to the US Department of Energy and National Science Foundation, after consulting the US nuclear-physics community.

The agenda assumes that US funding for nuclear science will increase by 1.6% per year above inflation — a realistic scenario, says NSAC chair Donald Geesaman, a physicist at Argonne National Laboratory in Illinois. “We have exciting science to do, and we are not asking for large increases,” he says.

The neutrino experiment, construction of which could begin by the end of the decade, would search for a theorized rare form of radioactive decay in which two identical neutrinos annihilate one another — an event that would imply that neutrinos are their own anti-particles. It could provide a way to measure the tiny mass of neutrinos and help to explain why the Universe has lots of matter but almost no antimatter.

Experiments around the world using materials such as liquid xenon have failed to detect the event, known as neutrino­less double β decay. One of the largest is the Enriched Xenon Observatory-200 (EXO-200) experiment, which uses 200 kilograms of xenon as a detector deep below the desert outside Carlsbad, New Mexico. But the NSAC report says that an experiment using a tonne or more of material — about ten times more than any previous attempt — could either find or rule out the phenomenon. Confirming neutrino­less double β decay "would in one stroke add lots of stuff to our knowledge of the natural world," says Giorgio Gratta, a physicist at Stanford University in California, and a former spokesperson for EXO-200.

Another priority, on which Nature reported in May, is a particle accelerator that would collide electrons with protons or heavy ions to investigate gluons, which carry the force that binds quarks. But construction would have to wait until the 2020s because NSAC’s top priority is to complete and maintain existing facilities, such as the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Upton, New York. RHIC faced closure two years ago, but an improved budgetary position means it can now be sustained into the next decade.

Although a more powerful heavy-ion experiment is housed in the Large Hadron Collider at CERN, Europe’s particle-physics laboratory in Geneva, Switzerland, researchers say that RHIC still has important science to do. Both experiments melt atomic nuclei together into a liquid-like soup of quarks and gluons, but each looks at different energy scales. “Collisions at lower energy create a very different kind of matter,” says Jamie Nagle, a physicist at the University of Colorado at Boulder who works on RHIC.

NSAC also wants to continue support for two other leading US facilities: the Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson National Laboratory in Newport News, Virginia, and the planned US$700 million construction of the Facility for Rare Isotope Beams (FRIB) at Michigan State University in East Lansing. Its final priority is to increase funding for small- and medium-scale projects through mechanisms such as the National Science Foundation’s Major Research Instrumentation programme, which supplies scientific equipment to universities. “All recommendations and priorities were agreed on by consensus,” says Geesaman.