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Consisting of an electron bound to a positron, positronium is at once both the most exotic and simplest of atoms.

C. Bickel/Science

Tantalizing oddity found in exotic atom—but physicists aren’t hyping it

A property of positronium—an exotic atom consisting of an electron bound to its antimatter partner, a positron—differs significantly from theoretical predictions, a team of physicists reports. That difference could be a sign of new particles or phenomena beyond physicists’ standard model of fundamental forces and particles—the kind of result typically accompanied by a feverish press release. Yet, not only did the researchers issue no press release, their paper makes no mention of new physics.

“It’s very exciting,” says Gregory Adkins, a theorist at Franklin & Marshall College who works on positronium. “To find a discrepancy signifies that this is an interesting system to pursue.”

But physics is littered with tantalizing claims that fizzled, sometimes spectacularly: a discrepancy in the size of the proton; signs of a cosmic growth spurt called inflation; and evidence that particles called neutrinos travel faster than light, an experimental artifact eventually traced to a loose electrical cable. So David Cassidy, a physicist at University College London who led the new positronium work, claims only that the measurement is sound. “I’m convinced that it’s not some stupid, faster-than-light neutrino thing where we’ve done something that’s just not right.”

Because a positronium atom contains no nuclear matter, physicists can study it without worrying about the complexities of the weak and strong nuclear forces, which can alter an atom’s properties in subtle but significant ways. To predict positronium’s properties, physicists need use only the relatively simple quantum theory of electric and magnetic forces, or quantum electrodynamics (QED), perhaps the most precise and best-tested theory in physics. That makes positronium an ideal tool with which to look for new physics, Cassidy says.

Just like an ordinary atom, a positronium atom can absorb and emit light and other electromagnetic radiation only at specific frequencies as the electron and positron within it jump from one distinct quantum state to another. Theorists can calculate those wavelengths precisely with QED, and any discrepancies between those predictions and the experimentally measured frequencies could then signal new physics.

Making those comparisons is easier said than done. Theorists labor for years on improvements to the incredibly demanding calculations. And positronium atoms don’t exist in nature, so experimenters must make them by firing positrons into a target where some of them capture electrons. The resulting atoms don’t stick around very long, either. Within a fraction of a microsecond, the electron and the positron collide and annihilate each other in a flash of gamma rays.

Nevertheless, through painstaking efforts, Cassidy and colleagues were able to capture fleeting puffs of roughly 100,000 positronium atoms and expose them to microwaves that would drive a transition between a particular pair of quantum states. Tuning the microwaves’ frequency to maximize the rate, they found that the transition occurred at 18.50102 gigahertz, which clashes with the QED prediction of 18.49825 gigahertz. The two values differ by 4.5 times the combined experimental and theoretical errors, just shy of the five-times standard for declaring a discovery, the team reported on 12 August in Physical Review Letters.

Caution is in order for several reasons, physicists say. Other discrepancies between positronium’s predicted and observed properties have come and gone. For example, in the 1990s physicists thought, essentially, that positronium didn’t live as long as predicted. But better measurements resolved that difference in 2003. Researchers may also struggle to explain away the discrepancy with new theories, such as some new quantum particle that flits between the electron and positron. “It could be a little hard to modify theory to fit this experiment but not cause problems with other experiments,” Adkins says.

Perhaps the biggest reason for caution is because the new result suggests something is lacking in QED. The theory rests on just a few bedrock assumptions: that particles like electrons exist, that Albert Einstein’s theory of special relativity holds, and that the whole theory must have a particular mathematical symmetry called gauge invariance that immediately gives rise to everything else—the existence of electric and magnetic fields and all their properties. So questioning QED’s ability to explain positronium would be bold, indeed. “You don’t want to be disagreeing with QED in public too much,” Cassidy says. “It’s like a fight with Mike Tyson, you’re not going to win.”

Ultimately, QED alone probably can explain the discrepancy, Cassidy predicts. The microwaves might drive a positronium atom not only directly from the initial quantum state to the final one, but also to it through an intermediary quantum state. That additional way of getting from the initial to the final state could then interfere quantum mechanically with the direct pathway, affecting the measured frequency of the transition. Sorting out whether such a subtle, but conventional effect is at play could take 10 years, Cassidy says.

Meanwhile, physicists are under increasing pressure, even from funding agencies, to highlight the more provocative interpretations of their work, instead of letting results speak for themselves, Cassidy says. “It’s how you get money these days.”