US nuclear scientists hash out priorities for their field

An electron–ion collider (EIC) is likely to be the next major facility on the wish list of the US nuclear-physics community. Other big-ticket items on that list are finishing the Facility for Rare Isotope Beams (FRIB) at Michigan State University (MSU), turning on the upgraded 12-GeV electron beam at the Thomas Jefferson National Accelerator Facility (JLab) in Newport News, Virginia, and building a ton-scale neutrinoless double-beta decay detector. And scientists dream of a host of smaller experiments.

“Nuclear physics has never been in as good a position scientifically as it is now,” says Berndt Mueller of Brookhaven National Laboratory. “The US has by far the best program.” Now the US nuclear-physics community is grappling with how to best fit its ideas into a limited budget to maintain that edge.

Last spring the Department of Energy (DOE) and NSF tasked the Nuclear Science Advisory Committee (NSAC) with setting priorities for the decade 2016–25 in a long-range plan (LRP). The charge to NSAC is to “indicate what resources and funding levels would be required … to maintain a world-leadership position in nuclear physics” and what the priorities should be if the funding for the field is flat, with adjustment for inflation, at the level of the president’s fiscal year 2015 request. The FY 2015 request for nuclear physics at DOE is $593.6 million (the final appropriation in late December was $595.5 million), and NSF’s investment last year was about $47 million.

Hundreds of nuclear physicists attended a handful of town meetings across the country this past summer and fall. Organized by the American Physical Society’s division of nuclear physics, the meetings focused on nuclear structure and nuclear astrophysics; hadron and heavy-ion quantum chromodynamics (QCD); fundamental symmetries and neutrinos, neutrons, and the relevant nuclear astrophysics; and education and innovation. Additional ad hoc meetings were held in crosscutting areas such as high-performance computing and university facilities.

The meeting conveners are providing summaries to a writing group, which will meet in April “to fight it out,” as Michael Wiescher of the University of Notre Dame puts it. The writing group will submit recommendations for the field as a whole to NSAC, which is scheduled to report to the agencies in October. The main sticking point, says Wiescher, one of the group’s 58 members, can be summed up in one word: money. The process can be tense, he says, “but it usually works well. We know the budget situation, and we want to maintain a balanced field.”

Major facilities

With the EIC, scientists would study dense gluonic matter, look at how quarks and gluons turn into strongly interacting particles such as pi mesons, and probe such questions as how nucleons get their mass and spin. Scientists at BNL and JLab each have a vision for an EIC at their own site. At BNL, the EIC would use the existing heavy-ion accelerator but need an electron beam; JLab would repurpose its electron beam and need an ion source.

“There was concern that the QCD community would have vigorous and contentious debate about the EIC,” says Mueller. “But the science case is so compelling that when the [JLab and BNL] communities met, there was unanimous support.” For the EIC to be included in the LRP, the rest of the nuclear-physics community will need to get behind it too. Also key, Mueller notes, is for the QCD community to show that it could build an EIC that fits within the LRP budget. If the EIC does make it into the LRP—as most people in the community expect it will—then there is still no guarantee it will go ahead, but, he says, “we will have set ourselves up for a decision on how to build it and where to build it by the end of this decade.”

Ground for FRIB was broken last March, and the facility is expected to start doing science by 2022 (see the story on page 23 ). A national rare-isotope user facility was first included in an LRP in 1996, but it had to be reduced in scope and cost before getting the go-ahead; it also survived a close look in 2012. At that time, the community was faced with lower budgets than anticipated and had to make tough choices. Ultimately, the community said that FRIB and the JLab upgrade should proceed and reluctantly gave BNL’s Relativistic Heavy Ion Collider (RHIC) the short straw (see Physics Today, March 2013, page 32 ). In the end, Congress ponied up for RHIC.

The $338 million upgrade at JLab is nearly complete. Some new experiments and the doubling of the lab’s electron accelerator to 12 GeV are ready, and electron–hadron and photon–proton collisions are set to start in the next few months. “With the upgrade, we have high-luminosity beams and high polarization,” says Robert McKeown, the lab’s deputy director for science. “We can explore nucleon structure and new aspects of QCD. We can do three-dimensional imaging. This will be a major extension of what was previously possible.” But, he says, 12 GeV “constrains us to the valence quark region. To get a complete picture of the nucleon requires higher energy, which can only be reached with an electron–ion collider.”

A different beast

“Fundamental symmetries and neutrinos were always part of nuclear physics, but they were considered parasitic,” says Michael Ramsey-Musolf, a theorist at the University of Massachusetts Amherst, but “the field has gone through a shift.” Two recommendations came out of the town meeting, which he helped convene. The first is that this subfield “needs to be fed and nurtured appropriately to capitalize on the advances of the last seven years,” he says. The second is for the “US to take leadership in developing and launching a ton-scale neutrinoless double-beta decay experiment.”

Such an experiment (see Physics Today, January 2010, page 20 ) could determine if neutrinos are their own antiparticles and whether lepton number is conserved. A discovery of lepton-number violation could be key to explaining the excess of matter over antimatter in our universe. Several detector types are being explored, and over the next two to three years a separate subcommittee of NSAC will work toward recommendations for a path to a ton-scale experiment.

A detector could cost up to a few hundred million dollars, but its ongoing operating costs will be low. “This is a big experiment, not a facility,” says Mueller. “It’s a different beast, but it’s still a challenge to see how it can be done.”

“There is a tension between the need for construction money, the need to operate the facilities, and the need to do research, the equipment to do research,” says Argonne National Laboratory’s Donald Geesaman, who chairs the LRP writing group. “The relative balance of resources in the field is an issue.” DOE spending on nuclear-physics research covers mostly grants to independent investigators and a couple of university facilities. In the past seven years, that spending has slipped from about 38% to 30% of the total budget for nuclear physics.

Weighing demands

Tim Hallman, associate director for nuclear physics in DOE’s Office of Science, notes that research funding covers core efforts in universities, and having less of it can have long-term consequences. “University PIs can’t hire that next postdoc or take on that next student,” he says. “It’s not by accident that we are at this juncture. The field is trying to accomplish a lot, but it’s time to start building up research so scientists can take advantage of the tools they have.”

Tied into finding a good balance among construction, operations, and research is outfitting the major facilities to best exploit them, and pursuing other, smaller, experiments. In recent years, the number of experiments with building costs in the $10 million to $50 million range has dropped. “There were four in some stage of execution when I came [to DOE in 2009],” says Hallman. “The number now is zero.” The newest is the heavy flavor tracker for RHIC, which came on line early last year. “These kinds of projects take an investment of three to four million dollars a year for several years,” says Hallman. “It’s not sustainable that we don’t have any of these projects in the queue.”

Not surprisingly, there is no shortage of potential queue joiners. Examples include an underground accelerator for nuclear astrophysics and an experiment at Oak Ridge National Laboratory to measure the neutron’s electric dipole moment. That would be a “powerful search for CP violation beyond the standard model and another key to explaining the cosmic matter–antimatter asymmetry,” says Ramsey-Musolf.

JLab users hope for two detectors for their upgraded accelerator. The Møller experiment would measure parity-violating asymmetry in elastic electron–electron scattering. “It would search for new interactions at the TeV scale, complementary to work at the Large Hadron Collider,” says McKeown. And the Solenoidal Large Intensity Device would be used to study deep inelastic scattering. That would be “further down the line, but we hope to get strong support from NSAC for proceeding,” McKeown says.

Similarly, a slew of equipment for exploiting FRIB got thumbs up at the town meeting for low-energy nuclear physics and astrophysics. Among the priority items is a magnetic mass separator that could be used to measure thermonuclear capture rates involved in supernovae, x-ray bursts, and other astrophysical phenomena. Also high on the list is GRETA, an instrument to measure gamma-ray decay of rare isotopes, and a high-rigidity spectrometer to identify reaction products. The HRS and GRETA will likely “form the centerpiece of FRIB equipment,” says Brad Sherrill, the lab’s associate director for users.

In contrast to the strategy developed in the high-energy physics community (see Physics Today, July 2014, page 18 ), the NSAC long-range planning process deals with major initiatives and thrusts in the field, but does not prioritize all the proposed projects. With nuclear physics, the funding agencies “are looking for guidance on prioritizing different categories within the field,” says Hallman. “Individual experiments need not be mentioned, but the categories should be. Typically, the community provides a short list of four or five major recommendations that we try to follow, and then a dozen or so other recommendations. We pay attention to all of it.” If the LRP can identify scientific questions and opportunities, says Allena Opper, NSF program director for experimental nuclear physics, “it will provide valuable context for our funding recommendations.”

Psst, call it development

Also tied into balancing funding is education. “In DOE, we—formally speaking—don’t support education, but we care about workforce development,” says Hallman. Not enough people are being trained to oversee the nuclear stockpile, tend nuclear detection devices, and operate reactors for power and medical isotope production. (Isotopes and their many applications are being considered in a separate NSAC planning exercise.)

In the LRP process, the Association for Research at University Nuclear Accelerators (ARUNA) is arguing for continued funding and higher visibility for the 10 university-based nuclear physics facilities. “If I bring a student to an experiment at a national laboratory, I have to make sure the experiment works on a tight schedule, so I won’t let the student touch anything,” says ARUNA secretary Ingo Wiedenhoever of Florida State University. “At local facilities, students have to set up experiments, take data, analyze the data, and publish. They have more freedom to make mistakes and learn from them.” Education and maintaining the smaller university facilities, he says, “should be part of the big strategic plan, not just a footnote.” ARUNA’s budget totals about $14.4 million.

In long-range plans, “education has always gotten ‘motherhood and apple pie’ statements,” says Sherry Yennello, a nuclear chemist and director of the cyclotron institute at Texas A&M University. “ARUNA labs are part of that. It would be better if it was said clearly they need to be supported.”

“The overarching theme from the town meeting on education and innovation,” says co-convener Michael Thoennessen of MSU, “is that basic research intimately involves the education of the next generation—teaching students, training and preparing them for careers. This is the responsibility of researchers, funding agencies, and the [nuclear-physics] community as a whole.” In addition, participants agreed that communicating research results to the public is an important part of doing nuclear-physics research.

Among the recommendations from the town meeting was to keep holding a nuclear-chemistry summer school. “The government was saying DOE shouldn’t fund it, basically because it has the word ‘school,’ “ says Thoennessen, “so that is why we stressed that these summer schools are important and should be continued.” Another recommendation, he says, was to plug a hole in the innovation pipeline. Funding for the first steps in developing an application is scarce, Thoennessen says, so “we recommended that the federal agencies provide and coordinate funding opportunities for innovative ideas with potential future applications.”

Nuclear theory

On the theoretical side, the past five years have seen many advances. Says Martin Savage of the University of Washington, “We are now in a position to perform computations of the nuclear forces in terms of quarks and gluons starting from QCD. We continue to develop quantum many-body techniques to calculate the properties of complex nuclei.”

Savage attended the NSAC LRP town meetings to rally support for boosting funding for high-performance computing to about $10 million annually. He notes that DOE is bringing pre-exascale machines online in the next few years (see Physics Today, January 2015, page 24 ), “and the field needs to be prepared. We need to get codes ready and to be able to fully exploit these new machines. It’s an exciting time. Once you get into the petascale, the landscape changes. You are moving toward being able to fully quantify the uncertainties in nuclear physics calculations and to systematically reduce them.” Overall, he says, “I see a vibrant community with good projects. We are spoiled for choice, and we have to prioritize.”

“We are listening,” says Geesaman. “We are trying to understand as best we can what the community wants. That is why the process takes as long as it does.”