With to-do list checked off, U.S. physicists ask, ‘What’s next?’ | Science

Physicists at the Stanford Underground Research Facility assemble the heart of the Lux-Zeplin dark matter detector, which will hold 7 tons of liquid xenon.

Matthew Kapust/Sanford Underground Research Facility

As U.S. particle physicists contemplate their future, they find themselves victims of their own surprising success. Seven years ago, the often fractious community hammered out its current research road map and rallied around it. Thanks to that unity—and generous budgets—the Department of Energy (DOE), the field’s main U.S. sponsor, has already started on almost every project on the list.

So next week, as U.S. particle physicists start to drum up new ideas for the next decade in a yearlong Snowmass process—named for the Colorado ski resort where such planning exercises once took place—they have no single big project to push for (or against). And in some subfields, the next steps seem far less obvious than they were 10 years ago. “We have to be much more open minded about what particle physics and fundamental physics are,” says Young-Kee Kim of the University of Chicago and chair of the American Physical Society’s division of particles and fields, which is sponsoring the planning exercise.

Ten years ago, the U.S. particle physics community was in disarray. The high-energy frontier had passed to CERN, the European particle physics laboratory near Geneva where, in 2012, the world’s biggest atom smasher, the Large Hadron Collider (LHC), blasted out the long-sought Higgs boson, the last piece in particle physicists’ standard model. Some physicists wanted the United States to build a huge experiment to fire elusive particles called neutrinos long distances through Earth to study how they “oscillate”—morph from one of their three types to another—as they zip along. Others wanted the United States to help push for the next big collider.

Those tensions came to a head during the last Snowmass effort in 2013, and the subsequent deliberations of the particle physics project prioritization panel (P5), which wrote the road map. U.S. researchers agreed to build the neutrino experiment, but make it bigger and better by inviting international partners. They also decided to continue to participate fully in the LHC, and to pursue a variety of smaller projects at home. The next collider would have to wait. Most important, DOE officials warned, the squabbling and backstabbing had to stop. In fact, physicists recall, the 2013 process had an informal motto: “Bickering scientists get nothing.”

Physicists have just started to build the current plan’s centerpiece. The Long-Baseline Neutrino Facility (LBNF) at Fermi National Accelerator Laboratory (Fermilab) in Illinois will shoot the particles through 1300 kilometers of rock to the Deep Underground Neutrino Experiment (DUNE) in South Dakota, a detector filled with 40,000 tons of frigid liquid argon. LBNF/DUNE, which should come on in 2026, aims to be the definitive study of neutrino oscillations and whether they differ between neutrinos and antineutrinos, which could help explain how the universe generated more matter than antimatter.

“The angst in the neutrino community is a lot lower than it was last time around,” says Kate Scholberg, a neutrino physicist at Duke University. “The DUNE program will be going on at least into the 2030s.” However, researchers are already thinking of upgrades to the $2.6 billion experiment, she notes.

Missions accomplished

Major projects prioritized by the U.S. particle physics community in 2014 are all approved, under construction, or up and running.

Project Purpose Status
Long-Baseline Neutrino Facility/Deep Underground Neutrino Experiment Study how neutrinos change type as they fly from Fermi National Accelerator Laboratory (Fermilab) in Illinois to South Dakota. Civil construction begun
High-Luminosity Large Hadron Collider U.S. contributions to upgrades at the LHC Design work continuing
Camera for Vera C. Rubin Observatory Survey entire hemisphere of sky every 3 days. Completed
Second-/third-generation dark matter detectors Reach ton-scale detectors for dark matter particles. Running/under construction
Next-generation cosmic microwave background experiment Network of telescopes to study big bang afterglow Design work continuing
Dark Energy Spectroscopic Instrument Telescope to study distribution of galaxies and probe space-stretching dark energy Running
Short-baseline neutrino experiments Study properties of neutrinos in experiments at Fermilab. Under construction
Proton Improvement Plan II New linear accelerator to increase power of Fermilab complex Design work continuing

Particle Physics Project Prioritization Panel report (2014)

In other areas, the future looks less certain. The last time around, for example, scientists had a pretty clear path forward in their search for particles of dark matter—the so-far-unidentified stuff that appears to pervade the galaxies and bind them with its gravity. Researchers had built small underground detectors that searched for the signal of weakly interacting massive particles (WIMPs), the leading dark matter candidate, bumping into atomic nuclei. The obvious plan was to expand the detectors to the ton scale.

Now, two multiton WIMP detectors are under construction. But so far WIMPs haven’t shown up, and scaling up that technology further “is probably not going to work very well anymore,” says Marcelle Soares-Santos, a physicist at the University of Michigan, Ann Arbor. “So we need to think a little bit more out of the box.” Researchers are now contemplating a hunt for other types of dark matter particles, using new detectors that exploit quantum mechanical effects to achieve exquisite levels of sensitivity.

A perennial question for the field is what the next great particle collider will be. The obvious need is for one that fires electrons into positrons to crank out copious Higgs bosons and study their properties in detail, says Meenakshi Narain, a physicist at Brown University. But possible designs vary. Physicists in Japan are discussing such a Higgs factory in the form of a 30-kilometer-long linear electron-positron collider. Meanwhile, CERN has begun a study of an 80- to 100-kilometer circular collider. China has plans for a similar circular collider.

However, Vladimir Shiltsev, an accelerator physicist at Fermilab, says those aren’t the only potential options. “The real picture is much murkier.” Snowmass organizers have received at least 16 different proposals for colliders, including one that would smash together muons—heavier, unstable cousins of electrons—and another that would use photons. Snowmass participants should consider all options, Shiltsev says.

Joe Lykken, Fermilab’s deputy director for research, suggests physicists could even push for DOE to support a massive experiment that has nothing to do with particles: a next-generation detector of gravitational waves, ripples in spacetime set off when massive objects such as black holes spiral into each other. Their discovery in 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO) opened a new window on the universe.

LIGO consists of two L-shaped optical instruments with arms 4 kilometers long in Louisiana and Washington; it was built by the National Science Foundation. The next generation of ground-based detectors could be 10 times as big, and might better fit DOE, which specializes in scientific megaprojects, Lykken says. “It starts to sound like the kind of thing that the DOE would be interested in and maybe required for,” he says.

During the coming year, Snowmass participants will air the more than 2000 ideas researchers have already proffered in two-page summaries. Then, a new P5 will formulate a new plan. Whatever ideas scientists come up with, to execute their new plan they’ll have to maintain the harmony that in recent years has made their planning process an exemplar to other fields. “Being unified is the new norm for us,” quips Jim Siegrist, DOE’s associate director for high energy physics. “So we have to continue to keep a lid on divisiveness and that’ll be a challenge.”