Giant space magnet may have trapped antihelium, raising idea of lingering pools of antimatter in the cosmos

By Joshua SokolApr. 19, 2017 , 3:45 PM

Sam Ting speaks softly and deliberately as he gets ready to deliver some juicy news to his audience. “You normally cannot hear me anyway,” jokes the physicist at the start of a talk this past December at CERN, the particle physics laboratory near Geneva, Switzerland, while a technician fiddles with his microphone.

Ting may be soft-spoken, but few would call him retiring. Two decades ago, Ting persuaded funders to spend $1.5 billion to build the Alpha Magnetic Spectrometer (AMS). In 2011, NASA launched the 8.5-metric-ton magnet on the penultimate space shuttle flight and attached it to the International Space Station (ISS). Now he is capturing attention again, with a hint—buried at the end of his talk—that the AMS is finally delivering on the promise of its original name, when “AM” stood for “antimatter.”

So far, the AMS has measured the masses and electric charges of some 90 billion particles that have passed through the magnet’s maw. Nearly all of those are protons and helium nuclei, along with a smattering of electrons and nuclei of carbon, oxygen, and iron. A precious few are antiprotons and positrons: the antimatter counterparts of protons and electrons. To Ting, those antiparticles may be clues to the unseen “dark matter” that weighs down galaxies with extra gravity, although many astrophysicists regard them as the byproduct of humdrum galactic events.

Those antiparticles are not Ting’s big news, however. At CERN, and again in a 16 February talk at the Massachusetts Institute of Technology (MIT) in Cambridge, where he has worked since 1969, Ting says that the AMS may have trapped a bigger and weirder form of antimatter. The AMS, he says, has seen a handful of candidate particles of antihelium-3, made of two antiprotons and an antineutron. In labs on Earth, physicists have made antihelium for a few fleeting instants, but no one has ever detected it in space.

“It was shocking,” says physicist Michael Salamon, AMS project manager at the Department of Energy (DOE) in Washington, D.C., who heard about the results when Ting called him during a vacation. It’s so unexpected that Ting says he has refrained from publishing the finding or even asking theorists what might be going on. “I want to make sure the signal is genuine,” he tells Science. Detecting antihelium in nature could shake up cosmology. A single confirmed detection could indicate the existence of islands of antimatter that have survived since the big bang, or point to particle interactions beyond the standard model of physics. And for Ting, who turned 81 in January, it would be a vindication, a final retort to his strident critics.

In 1976, Ting shared the Nobel Prize in Physics for discovering a subatomic particle called the J/Psi meson. It was 1994 when he first proposed using the AMS to take the particle hunt to space. He promptly drew flak. Some contended that the project, funded by international partners and DOE, had won support through savvy political maneuvering instead of a normal scientific review, and that it wouldn’t deliver big insights. Ting’s results so far amount to “physics by press conference,” says Greg Tarlé, an astrophysicist at the University of Michigan in Ann Arbor and a vocal critic of Ting and the AMS.

Ting’s marquee AMS result—that more high-energy positrons than expected are buzzing around the galaxy—has not impressed the doubters. That positron excess, which a European satellite found in the mid-2000s and the AMS confirmed, has sparked hundreds of theory papers connecting it to hypothetical dark matter particles. The mutual annihilation of those particles might create a half-and-half blend of electrons and positrons in a narrow energy range. The electrons would fade into a sea of electrons from other sources, but the rarer positrons might stand out. To Ting, the best explanation for the extra positrons is a dark matter particle with a mass of 1 teraelectronvolt—about as much energy as a flying mosquito.

Other researchers favor more familiar astrophysical sources. The proliferation of dark matter models, including those that Ting points to, “maybe has more to do with communities and how fast they write papers than it does with science,” says Tim Linden, a particle astrophysicist at The Ohio State University in Columbus. He and others note that the Milky Way is a messy laboratory, roiling with pulsars—the spinning, highly magnetic cores of collapsed stars—and supernovas, which accelerate protons to ultrahigh energies and send them slamming into cooler gas. Both phenomena could generate the antimatter that the AMS sees.

But the “four or five” antihelium candidates Ting says it has tallied over the past 5 years would be something else altogether. There are few conceivable ways for conventional astrophysical processes or dark matter particles to generate that much antihelium, says Kerstin Perez, a particle astrophysicist at MIT. She is co-leading a balloon experiment that could search for antihelium when it launches over Antarctica in 2020. “If it’s real,” she says of Ting’s claim, “it’s something fundamentally new.”

It also would validate Ting’s original proposal. When Ting sold NASA and DOE on the AMS, he said it might find runaway particles from oases of antimatter, helping solve a deep riddle. The big bang produced matter and antimatter in equal amounts. Soon after, they began colliding and annihilating each other in puffs of gamma rays. But somehow, matter came to dominate the observable universe. That could be because of some fundamental difference between the two—or maybe it was just a coin flip, where certain regions of space came to be ruled by one or the other. Ting’s idea to look for those regions galvanized his critics, who considered it outlandish because clumps of antimatter coexisting with normal galaxies would produce more gamma radiation than astronomers observe. Moreover, large antiparticles could not easily survive the journey to the AMS. But if antimatter were there, the AMS would sniff it out—or so the original pitch went.

The feeling both inside and outside of the AMS team, though, is that it’s still far too early to rule out a more mundane explanation: a problem in the detector. As charged particles pass through the doughnut-shaped magnet, its field bends their paths into signature curves that indicate their charge and momentum. The particles arc through nine cooled-down silicon detectors that track the curves. About a billion times a year the particle turns out to be a helium nucleus, with two positive charges. But each year has also brought one event or so that for all the world looks like it is curving with charge equal to minus two, Ting says—the expected signature of antihelium. The events could just be heliums bouncing unusually off an atom inside the experiment, leading to a misidentification. But the team has used computers to model all the possible paths a particle could take in the detector. “We still do not see any possible way this could come from any background,” Ting says. “Many people in the collaboration think we should publish it.”

That he hasn’t done so yet is typical of Ting, his supporters say. “That is kind of his trademark, so to speak , to be extremely sure when something comes out,” says Philip von Doetinchem of the University of Hawaii in Honolulu, who is a member of the AMS team but has not worked on the antihelium problem. But critics see it differently. “He knows that he has an instrumental problem,” Tarlé says. To Tarlé, Ting is strategically playing coy to drum up further support for the mission.

And Ting may need it. DOE, which will review the AMS in 2019, is impatient for breakthroughs—not routine astrophysics. “Understanding the spectra of particular species of cosmic rays is good to know, but it’s not as important, quite frankly,” Salamon says. A more pressing concern is that just one of four redundant pumps that cool its silicon trackers is working at full strength.

If the AMS can last until 2024, when the United States and other nations plan to stop funding the ISS, the magnet should be able to double its census of particles. Achieving that would not only help differentiate between exotic and mundane interpretations of its positron signals but also could give Ting more antihelium candidates. Ting won’t say whether having 10 or more will provide the statistical power required to call this a discovery, but he says each one helps.

Ting says he is planning to replace the broken pumps with a new system that astronauts would install during a spacewalk. A NASA spokesperson confirms that planning for the repair is underway, in case the last pump breaks. The agency has devoted $16 million to possible spacewalks between now and 2019. DOE and Ting’s international partners have already purchased the replacement parts, he says. “There’s no money issue,” Ting says.

If, after all these years, the AMS falls short of finding antihelium, it won’t be for lack of trying.

Source: Science Mag