Hints of weird particles from space may defy physicists’ standard model

Dangling from a balloon high above Antarctica, a particle detector has spotted something that standard physics is at a loss to explain.

Two unusual signals seen by the detector, known as the Antarctic Impulsive Transient Antenna, or ANITA, can’t be attributed to any known particles, a team of physicists at Penn State reports online September 25 at arXiv.org. The result hints at the possibility of new particles beyond those cataloged in the standard model, the theory that describes the various elementary particles that make up matter.

Like the old man in the Pixar movie Up, ANITA floats on a helium balloon, at an altitude of 37 kilometers for about a month at a time. It searches for the signals of high-energy particles from space, including lightweight, ghostly particles called neutrinos. Those neutrinos can interact within Antarctica’s ice, producing radio waves that are picked up by ANITA’s antennas.

The two puzzling signals appear to be from extremely energetic neutrinos shooting skyward from within the Earth. A neutrino coming up from below isn’t inherently surprising: Low-energy neutrinos interact with matter so weakly that they can zip through the entire planet. But high-energy neutrinos can’t pass through as much material as lower-energy neutrinos can. So although high-energy neutrinos can skim the edges of the planet, they won’t survive a pass straight through.

The steep angle of the particles’ paths suggests that the neutrinos traveled through several thousands of kilometers of Earth — too much for a high-energy neutrino to make it out the other side. That’s according to computer simulations in the new study, by researchers who are not members of the ANITA collaboration. ANITA researchers have been looking for a way to explain the signals with neutrinos, says Derek Fox, a coauthor of the study. But according to Fox and colleagues’ simulations, “those attempts must fail.”

A high-energy particle could make such a long trek through the Earth only if it were even more reticent to interact with matter than neutrinos are. A hypothetical heavy particle called a stau, proposed in a theory called supersymmetry (SN: 10/1/16, p. 12), could fit the bill, Fox and colleagues say. After being created on the other side of the planet by a high-energy neutrino slamming into the Earth, a stau could make it through unscathed before decaying into lighter particles that would eventually result in the signals detected by ANITA.

But don’t cancel your membership in the standard model fan club just yet. “It’s still possible that there is a very mundane reason that we are seeing these events in ANITA,” says ANITA physicist Stephanie Wissel of Cal Poly in San Luis Obispo, Calif. Other spacefaring particles called cosmic rays, which rain down from above, produce similar signatures in ANITA. A basic misunderstanding of the physics behind cosmic rays’ signatures could explain the observations, Wissel says.

Backing up their claim, Fox and colleagues also identify three events in another Antarctic neutrino detector, called IceCube, that they say have some similarly puzzling properties. But the leader of that experiment, physicist Francis Halzen of the University of Wisconsin–Madison, isn’t convinced. “These events are of course worth paying attention to,” he says, but he doesn’t see any evidence that they require a new explanation.

What’s needed is more data, physicists say. The ANITA team plans to send the detector up for another Antarctic balloon ride, says ANITA physicist Amy Connolly of Ohio State University in Columbus. “My view is that we should keep trying to find a mundane explanation for these events.”

The standard model has been confirmed time and again, so physicists are loathe to abandon it without overwhelming evidence. “The case that ANITA is seeing something weird is strong,” says astrophysicist John Beacom, also of Ohio State. But “I always bet for the standard model.”

Still, these events have such extreme energies that they are reaching into realms not accessible at particle colliders like the Large Hadron Collider near Geneva, Beacom says. “There’s a lot we just don’t know about how physics works at these high energies.”