SEATTLE—After a year of seemingly endless Zoom meetings, Slack chats and e-mails, nearly 800 particle physicists descended on the University of Washington to share their scientific dreams and nightmares in person. For 10 days at the end of July, whether masked inside conference rooms or sipping coffee beneath unusually sunny Seattle skies, they attempted to build a unified vision of their field’s future.
The story of 20th century particle physics is chronicled in the pantheon of elementary particles dubbed the Standard Model: quarks bound tight by gluons to make atomic nuclei; negatively charged electrons and their heavier counterparts, muons and taus; photons, the particles of light; heavy W and Z bosons, with their subtle influence; and evasive, lightweight neutrinos. Particles in the Standard Model are divided into fermions, the building blocks of matter, and bosons, forces that organize the matter. Perhaps ironically, searching at the smallest of scales has required experiments of increasing complexity and size. To find new particles, physicists have sifted for needles in haystacks of data produced by slamming known types of particles together at higher and higher energies. In 2012 the discovery of the Higgs boson at the Large Hadron Collider (LHC) at CERN near Geneva was accomplished by more than 5,000 scientists analyzing petabytes of data from detectors weighing thousands of tons at the biggest machine in the world.
Yet the triumph of the Higgs discovery—arguably the crowning achievement of the Standard Model—has been shadowed by worries that particle physicists are now stuck in a “nightmare scenario” with no clear path forward. Physicists have long believed the Standard Model’s pantheon should be bigger to account for phenomena such as dark matter and gravity. Many theories proposed these new particles would be within the LHC’s reach, but so far searches have come up empty—a nightmare for particle physicists.
While the phrase “nightmare scenario” often causes physicists to bristle and scoff, at the conference, a panel discussion simply entitled “Where Are We Going?” faced the question head-on. Tao Han, a theorist at the University of Pittsburgh, argued that the lack of new particles was actually a success of falsification—gaining knowledge by proving what isn’t rather than what is. “The nightmare scenario is not a failure,” he declared. Other panelists were less keen on that reframing, insisting particle physics was not in a nightmare scenario at all or that the nightmare was here but short-lived.
Some of this dissonance and discord is intentional. Roughly once a decade, hundreds of particle physicists participate in the Snowmass process (named for Snowmass, Colo., where initial meetings took place) to decide what to ask nature and which tools they need for answers. The preceding Snowmass in 2013 led to the identification of a few high-priority tasks, including characterizing properties of the newly discovered Higgs boson, measuring neutrino masses and determining the true nature of dark matter and dark energy.
The puzzles remain unsolved—a disconcerting lack of progress consistent with a nightmare scenario—but much of the field’s terrain has shifted for the better. New computational methods are allowing experiments to cut through noise and find signals previously assumed to be inaccessible. Possibilities for next-generation facilities such as a muon collider have invigorated the community. The search for dark matter, once constrained to a small number of candidate particles and types of detectors, has blossomed to encompass a wider range of possibilities.
A popular fantasy of science is that these puzzles will be solved by a lone thinker isolated in a lab, or scribbling on a chalkboard. Today, science is a communal endeavor, and the work of a career scientist is not always so different from the work of a politician or businessperson. At a plenary to kick off the conference, Hitoshi Murayama, a widely admired theorist at the University of California, Berkeley, gave a talk where he emphasized that particle physicists needed to do more than argue for their own projects. “We need to make a case for the entire field,” he said.
Getting particle physicists to agree on a unified vision is, in their jargon, “nontrivial.” Each subfield believes in its preeminence: neutrino researchers place their work first, while dark matter experts maintain that their search is more important. The debates are an essential part of a process that particle physicists know must end with common ground. On the first day of the latest Snowmass, Department of Energy representative Harriet Kung delivered a familiar warning: “Bickering scientists,” she intoned with a pause, “get nothing.”
Getting with the Program
U.S. particle physics subsists on a little more than $1 billion per year, primarily from the National Science Foundation and the DOE’s Office of Science.
Two projects draw the lion’s share of funds and attention: the LHC and the Deep Underground Neutrino Experiment (DUNE), which is under construction. Although the LHC is a pan-European project at CERN, roughly 30 percent of researchers working on LHC experiments are at U.S. institutions. DUNE’s 1,400 collaborators are also geographically diverse. Neither project is viable without international support, as Fermilab director Lia Merminga forcefully reminded the community at Snowmass: “Particle physics is global!”
But perhaps no project looms as largely as the one that was canceled. In 1993, after $2 billion had been spent and miles of tunnel dug under Waxahachie, Tex., near Dallas, the Superconducting Supercollider (SSC) was scrapped by Congress. Had it been completed, the SSC would have been the most powerful particle accelerator in the world. Its demise was a heavy blow for particle physics around the world, but the impacts on American physics verged on catastrophic. Suddenly, U.S.-based researchers found themselves without a collider to call home. Some migrated to other projects, while many simply left the field entirely.
Concerns at Snowmass swirled around DUNE, which some worried could go the way of the SSC because its price has swelled from $1.8 billion to $3.1 billion. One of DUNE’s main goals is to determine charge parity (CP) violation—essentially how much nature prefers producing neutrinos over their antimatter twins, antineutrinos. Hyper-Kamiokande (Hyper-K), a Japanese neutrino detector scheduled to begin operations in 2028, could also make such measurements. In postsession questions, critics prodded: Would DUNE be redundant? Supporters rebutted them, noting that DUNE has far better sensitivity to CP violation than Hyper-K—if, as some critics noted, it receives an upgrade costing an additional $900 million.
Nevertheless, when Merminga also announced that DUNE had cleared the latest round of DOE reviews, she received relieved applause. Mindful of the SSC, even scientists uneasy about DUNE’s scientific goals want it to succeed because its failure would negatively impact the whole community. As more than one researcher put it at Snowmass, “If DUNE’s dead, we’re dead.”
Concerns aren’t just limited to DUNE—funding for the entire field has dwindled over the past decade. Now, with U.S. industrial policy on the rise, some particle physicists are hopeful they will see a slice of the pie. The $280-billion CHIPS and Science Act recently signed into law by President Joe Biden includes provisions seeking to boost quantum technology, which is key to some new dark matter experiments, as well as more funding for machine learning (ML), which is driving novel searches for particles at the LHC.
“ML is powerful because our discoveries about fundamental particles must be statistical,” said Daniel Whiteson, an experimental particle physicist at the University of California, Irvine,at Snowmass. There, he and others raised the idea of a “data physicist”—a new breed of researcher using data in novel ways. One radical example came from David Shih, a theorist at Rutgers University. “Here’s a crazy idea,” Shih said cheerfully during his remote presentation via Zoom. “We could replace the LHC with a generative model.” Just as powerful models have demonstrated an ability to produce compelling images or write prose, one could produce collisions explorable by physicists. More provocative than serious, the idea set off both laughter and concerned murmurs.
While DUNE’s failure—or a more general lack of new funds—would be bad news for the field, it’s already the case that about two thirds of particle physicists, who spend years working toward a Ph.D. or in a postdoctoral position, are forced to leave research because there simply aren’t enough jobs. The “pipeline” is rich with aspiring researchers—and impoverished of suitable positions for them to hold.
To reduce the stigma of leaving particle physics, Snowmass conveners held a mixer where more than a dozen former physicists now at local companies, from small tech studios to Microsoft and Google, advertised paths away from academia. But for early-career researchers in search of a job to actually do physics, the prospects were few and far between.
Like most sciences in the U.S., physics suffers from a lack of diversity: among those whose race and gender were known, nearly 70 percent of physics PhDs awarded between 2014 and 2019 went to white men. But not everyone can agree on community efforts to address diversity, equity and inclusion. “The younger generation isn’t really interested in that discussion about whether there’s a trade-off between excellence and diversity,” Fermilab research scientist Bryan Ramson tells Scientific American. “I think physics as a whole would be much better off if you assume that everybody’s good enough.”
Dreams and Nightmares
One of the great shared dreams of particle physicists is to double their particulate pantheon so that each boson has a fermion counterpart and each fermion has a boson twin. This is the core concept of supersymmetry (SUSY), a set of theories that have profoundly shaped successive generations of today’s researchers. For example, under SUSY’s rules, photons would be mirrored by “photinos” and electrons mirrored by “selectrons.” Appealingly, a symmetry between force-carrying bosons and fermionic particles of matter could tame the uncontrolled Higgs mass (which the Standard Model otherwise predicts should be astronomically larger) and even act as dark matter.
Not only is there as yet no evidence for supersymmetry, but the LHC experiments ATLAS and CMS have successfully ruled out the most likely places its particles could have been hiding. Despite this, SUSY holds a pride of place among theories. And at Snowmass, many particle physicists—particularly those of an older vintage—still spoke of it in the present tense as an old friend.
Theories are hard to kill, and SUSY is not dead, but many younger researchers are beginning to move on. Inspired by a new vista of possibilities, they are looking for dark matter anywhere they can find it, not just for the weakly interacting massive particles (WIMPs) predicted by SUSY. They’re also trying not to throw the baby out with the bathwater. At an early-morning session, Nathaniel Craig, a theorist at the University of California, Santa Barbara, made the case that, irrespective of SUSY, the principle of naturalness should be salvaged.
Reductively put, naturalness is the idea that the universe should not be absurdly lucky. Over coffee, Craig gave an analogy: Suppose every pencil could be easily balanced on its tip. Should we expect the universe to be this lucky, or should we look for some hidden phenomenon that is secretly stabilizing the pencils?
While critics have sometimes derided naturalness as a mere aesthetic preference, Craig pointed to its historical success—naturalness stems from the theoretical physicist Victor Weisskopf’s 1939 work showing how the positron stabilizes the electron, and in 1974, led theorists Ben Lee and Mary Gaillard to predict the charm quark’s mass. “Naturalness is not a theory but a strategy to help us focus in the infinite places we could look,” Craig explained. Instead of abandoning naturalness because of SUSY’s dim prospects, he argued, physicists should consider nearly two dozen other theories inspired by naturalness.
Theorists aren’t the only ones moving on from SUSY. XENONnT and LUX-ZEPLIN (LZ)—two experiments using giant containers of liquid xenon to spot dark matter—recently reported results that, while null, still set impressive new limits on the plausible properties of WIMPs. Yet those results occupied only a small portion of the conversation at Snowmass. Freed from the need to fulfill SUSY, which predicts dark matter in a relatively narrow mass range, researchers are now looking for various candidate dark matter particles with masses ranging across some 30 orders of magnitude—about the difference between the mass of an ant and that of our sun. They are also figuring out ways to penetrate the once foreboding “neutrino floor,” the level at which noise from cosmic neutrinos would drown out any dark matter signal. The new approach is embodied by a motto workshopped at the conference: “Delve deep, search wide.”
Physicists working with colliders are also trying new methods. During the LHC’s third run, which is now underway, both ATLAS and CMS will be looking for long-lived particles. At Snowmass, researchers discussed how best to search for such particles, which can putter around before decaying, leading to unusual-looking events that might have been overlooked in the past.
Physicists are also reassessing particle flavor, a quantum property that defines the species of fermion: up quark, down quark, electron, muon, and so on. Flavor has often been taken for granted, but anomalies that indicate flavor-based behavioral differences between electrons and muons have reawakened interest in the subject. “Flavor is something that no one knows the answer to,” said Patrick Meade, a theorist at Stony Brook University, at Snowmass. If “any theorist tells you they know what the right model of flavor is, they’re lying to you.” As in so many other cases, physicists may simply have to wait for more data. If experiments such as Belle II confirm the flavor anomalies seen in the LHCb and Muon g-2 experiments, flavor could become a top-priority unknown.
Visions from the Frontier
If you wanted to conduct a quick, crude version of Snowmass, you might ask, “Which particle is the best to study?” Physicists disagree emphatically—some would choose mysterious neutrinos; others might point to whatever unknown particles constitute dark matter or even to better-known particles such as muons or bottom quarks, for their rare decays.
Among these choices, it is the drive to study the Higgs that may most shape the field. Though ATLAS and CMS have precisely measured its mass to one part per 1,000, much is still unknown about the Higgs. How it couples to lighter particles—if at all—remains unclear. Through an upgrade later this decade, the LHC will accumulate over 20 times more data than it has collected so far, allowing it to make more precise measurements of the Higgs. The particle is also fertile ground for new physics, and models with multiple types of Higgs—or where the Higgs interacts with dark matter particles—are easy to create. But regardless of what researchers learn about the particle, the effort to study it will shape the field.
Particle physicists are hungry for a new collider. They are, by and large, tired of smashing protons—essentially messy bundles of quarks—and would much prefer the more tidy collisions of electrons and positrons. With cleaner collisions, they could create a factory churning out Higgs bosons to subject to further, more intense scrutiny. The nearest-term possibility for such a Higgs factory is the International Linear Collider, which would be built in Japan. Though it is shovel-ready, the project has been delayed for years, and in February it was dealt another, possibly fatal blow when the Japanese government refused to allow it to go forward.
Then there is the Future Circular Collider (FCC), a proposed 90-kilometer-wide ring that would lie under a wide swathe of Swiss countryside. According to CERN director general Fabiola Gianotti, the FCC would probably begin operations circa 2050. Meanwhile, accelerator scientists in the U.S. are eager to host the next collider. In a white paper released late last October, a team of researchers introduced a new “cold copper” technology that could accelerate particles more rapidly without liquid helium cryogenics, allowing for a smaller, cheaper and more feasible collider.
But many researchers are unhappy with the idea of waiting 20 years or more for a mere Higgs factory. They want to explore high energies far out of the LHC’s reach and with unprecedented precision. Over the past two years, the idea of a muon collider has spread throughout the particle physics community. In the past, the Muon Accelerator Program drew little attention from theoretical physicists, few of whom mourned its demise. Experimentally, little has changed about a muon collider, which faces daunting technical obstacles. Socially, the community is invigorated—especially younger researchers, many of whom sported stylish muon collider T-shirts at Snowmass (a propaganda feat that was later mimicked by cold-copper-collider proponents who handed out chic buttons).
Ironically, where physicists’ ambitions are greatest is where Snowmass struggles the most as a format. In theory, it is Snowmass’s goal to outline a scientific vision without setting priorities, which is the job of the Particle Physics Project Prioritization Panel (P5). But a scientific vision cannot exist in a priority-free vacuum unless it impractically ignores all resources and constraints. The tortured logic meant that at the latest Snowmass, particle physicists could point to the promise of investigating Higgs parameters with a muon collider but not actually endorse a muon collider over any alternative.
As Snowmass ended, a coherent vision was not immediately clear. The task of refining 10 days and 500 white papers now falls to P5 and its newly announced chair Hitoshi Murayama.
Discussing the role of theorists with words that might also apply to his new role during Snowmass, Murayama said, “I hope we can provide guidance,” and then added puckishly, “although it’s sometimes misguidance.”