America’s Particle Physics Plan Spans the Globe — and the Cosmos

RALEIGH, N.C. — Particle physicist Hitoshi Murayama admits that he used to worry about being known as the “most hated man” in his field of science. But the good news is that now he can joke about it.

Last year, the Berkeley professor chaired the Particle Physics Project Prioritization Panel, or P5, which drew up a list of multimillion-dollar physics experiments that should move ahead. The list focused on phenomena ranging from subatomic smash-ups to cosmic inflation. At the same time, the panel also had to decide which projects would have to be left behind for budgetary reasons, which could have turned Murayama into the Dr. No of physics.

Although Murayama has some regrets about the projects that were put off, he’s satisfied with how the process turned out. Now he’s just hoping that the federal government will follow through on the P5’s top priorities.

“There are five actually exciting projects we think we can do within the budget program,” Murayama said this week during a presentation at the ScienceWriters 2024 conference in Raleigh. Not all of the projects recommended for U.S. funding are totally new — and not all of them are based in the U.S. Here’s a quick rundown:

• Looking for dark matter: About 85% of all the matter in the universe is thought to exist in an invisible form that so far has been detectable only through its gravitational effect. For years, an experiment being conducted in a converted South Dakota gold mine has been looking for traces of dark matter’s interactions with a huge reservoir of liquid xenon. The search hasn’t yet found anything, but the P5 report calls for boosting the size of the reservoir from seven to 70 tons.

• Following up on the Higgs boson: The discovery of the Higgs boson in 2012 provided the last missing piece in the Standard Model of particle physics, one of science’s most successful theories. But physicists don’t have a good grip on how the Higgs works. “You’d like to mass-produce this Higgs boson and study its properties in great detail, so we know how it got stuck and frozen into space, so that we can stay in one place,” Murayama said. That would require building a bigger particle collider, capable of smashing electrons and positrons — but the P5 panel determined that such a machine couldn’t be built in the U.S. Instead, the panel recommends supporting an “offshore Higgs factory” like the FCC-ee facility that CERN is considering, or the International Linear Collider that’s been proposed for construction in Japan.

• Studying the nature of neutrinos: The Big Bang is thought to have created equal amounts of matter and antimatter, which would theoretically annihilate each other. Fortunately for us, matter won out rather than being totally annihilated. How did it happen? “The only candidate elementary particle we know who might have done this is actually neutrinos,” Murayama said. “How do we know if that’s really the case? One thing we try to do is to look at the behavior of neutrinos by creating them in Illinois and shooting them to a location in South Dakota, because neutrinos can pass through the dirt without any problems.” The Deep Underground Neutrino Experiment is under construction, and excavation of the Long-Baseline Neutrino Facility was recently completed in South Dakota. The P5 report proposes upgrading DUNE’s capabilities.

• Getting a neutrino view of the cosmos: The P5 panel also called for a dramatic expansion of the IceCube Neutrino Observatory in Antarctica. “They managed to peer into the supermassive black hole in a nearby galaxy, and for the first time, they even took a picture of a galactic disk using neutrinos as well,” Murayama said. “So this is finally becoming a true tool to observe the universe in a different way from what we do with older telescopes.”

• Seeking signs of cosmic inflation: A widely held theory asserts that in the instant after the Big Bang, the universe inflated at a prodigious rate to “lock in” the slight perturbations that scientists see in the cosmic microwave background radiation. In 2014, astronomers claimed that an experiment at the South Pole had picked up evidence of that primordial cosmic inflation, but months later, they had to back away from those claims. The Antarctic studies are continuing, however, and the P5 panel supported an experiment known as CMB-S4 that would widen the search for evidence. “For that, we need two sites, one in Chile, another at the South Pole,” Murayama said.

In addition to the top five projects, the panel endorsed a longer-term effort to develop an advanced particle accelerator that would produce collisions between subatomic particles known as muons. Such a machine would increase the chances of finding new frontiers in physics in the 2030s, Murayama said.

“We call this a ‘muon shot,’ like a moonshot,” he said. “We don’t know quite well if we can really get there, but as you work toward it, that would end up producing so many interesting things on the way, more science and more technologies.”

Will the P5’s priorities prevail? Success isn’t guaranteed: For example, the National Science Foundation put the CMB-S4 experiment on hold in May to focus instead on upgrading aging infrastructure at NSF’s Antarctic facilities.

Looking ahead, it’s not yet clear how particle physics will fare when Donald Trump returns to the White House. For what it’s worth, the price tags for four of the projects add up to roughly $2.5 billion over the course of several years. The offshore Higgs factory is certain to cost billions more.

Murayama called attention to an issue that could affect IceCube, CMB-S4 and other Antarctic research in the nearer term. “There is a fleet of cargo airplanes that is owned by the U.S. Air Force that actually served us well over many decades,” he said. “But they were built back in the ’70s, and they’re about to retire, and right now there are no plans to replace them. Then we will lose access.”

Senate Majority Leader Chuck Schumer, D-N.Y., managed to get a $229 million appropriation for new planes into the Senate’s version of the defense budget bill for the current fiscal year, but the House still has to take action. That sets up a bit of a congressional cliffhanger for the weeks and months ahead.

“I don’t get a good sense of the priority,” Murayama confessed. “But this is supposed to be part of the defense budget, which is way bigger than the science budget — so in that part, it’s peanuts. Hopefully, it just can get in and get funded.”

For a critical perspective on the P5 wish list, check out physicist Sabine Hossenfelder’s YouTube video:

Alan Boyle is a volunteer board member for the Council for the Advancement of Science Writing, which was one of the organizers of the ScienceWriters 2024 conference.

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