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Suspended from a balloon over Antarctica, a particle detector has discovered something that classical physics can not explain.
Two unusual signals can be attributed to the detector Antiterror Antiterror Transient Antenna (ANITA) to a known particle, a Penn State physics team reports online on September 25th at arXiv.org. The result suggests the possibility of new particles beyond those listed in the standard model, the theory that describes the different elementary particles that make up matter.
Like the old man in the Pixar movie To the topANITA floats on a helium balloon at 37 kilometers altitude for about a month at a time. It looks for signals of high energy particles in space, including light and ghostly particles called neutrinos. These neutrinos can interact in the Antarctic ice, producing radio waves picked up by ANITA antennas.
The two intriguing signals seem to come from extremely energetic neutrinos pulled skyward from Earth. A neutrino from below is not inherently surprising: low energy neutrinos interact with matter so weakly that they can traverse the entire planet. But high energy neutrinos can not cross as much material as low energy neutrinos. Thus, although high-energy neutrinos can fly over the edges of the planet, they do not survive a direct pass.
The steep angle of particle trajectories suggests that neutrinos have traveled several thousand kilometers of Earth – too much for a high-energy neutrino to get out of the other side. According to the computer simulations of the new study, carried out 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, co-author of the study. But according to simulations by Fox and his colleagues, "these attempts must fail."
A high-energy particle could not cross the Earth for so long if it was even more reluctant to interact with matter than neutrinos. A hypothetical heavy particle called stau, proposed in a theory called supersymmetry (SN: 10/1/16, p. 12), could match the bill, Fox and his colleagues say. After being created on the other side of the planet by a high-energy neutrino slamming into the Earth, a stau could manage without degrading into lighter particles that would eventually produce the signals detected by ANITA.
But do not cancel your membership in the standard fan club yet. "It is still possible that ANITA is facing a very common reason," says Stephanie Wissel, ANITA physicist, Cal Poly in San Luis Obispo, California. similar signatures in ANITA. According to Wissel, a poor understanding of the physics behind cosmic ray signatures may explain these observations.
Based on their claim, Fox and his colleagues also identified three events in another Antarctic neutrino detector called IceCube, which they say has equally amazing properties. But physicist Francis Halzen, of the University of Wisconsin – Madison, is not convinced. "Of course, these events deserve special attention," he says, but he sees no evidence that they need a new explanation.
What is needed is more data, say the physicists. The ANITA team plans to send the detector for another Antarctic tour, says Amy Connolly, a physicist at Ohio State University in Columbus. "I think we should continue to try to find a banal explanation of these events."
The standard model has been confirmed many times, so that physicists are reluctant to abandon it without overwhelming evidence. "The case where ANITA sees something weird is strong," says astrophysicist John Beacom, also of the Ohio State. But "I always bet for the standard model".
However, these events have energies so extreme that they reach areas inaccessible in particle colliders like the large hadron collider near Geneva, Beacom says. "We do not know much about how physics works at these high energies."
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