Synopsis

ANITA Spots Another Inverted Cosmic-Ray-Like Event

Physics 11, s121
A fountain of high-energy particles that resembles an upside-down cosmic-ray shower is detected for the second time by the Antarctic Impulsive Transient Antenna.  
C. Miki/University of Hawaii

High-energy cosmic-ray particles constantly bombard Earth. When one of these particles collides with molecules in our atmosphere, it triggers a cascade of secondary particles, collectively known as a cosmic-ray air shower. But that isn’t the only way high-energy particles interact with Earth. On 28 December 2006, the balloon-borne Antarctic Impulsive Transient Antenna (ANITA) detected an “upward” air shower—a fountain of high-energy particles erupting from Antarctica’s icy ground. The team now reports the observation of a second such event, which occurred on 12 December 2014.

ANITA was designed to detect cosmic-ray air showers via the radio-wave signals they emit. ANITA detects the radio waves either on their way down or, more frequently, on their way back up after bouncing off the ice. This bounce inverts the phase of the radio waves, allowing the two sets of waves to be distinguished by way of their direction and phase.

But in 2006 and 2014, ANITA detected an unexpected signal—radio waves coming from the ice with an uninverted phase. The detections suggest that the signals came from upward-moving particles that tunneled through Earth before erupting from the ice. But cosmic rays aren’t predicted to do that in large numbers, leaving the source of these signals a mystery. One possibility is that cosmic rays from a bright supernova blasted all the way through Earth—but only the 2014 detection was coincident with such an event. Another possibility is that ANITA detected radio waves emitted from a particle not accounted for in the standard model. The team says that further detections of these weird signals are needed before making any definitive conclusions on their origin.

This research is published in Physical Review Letters.

–Katherine Wright

Katherine Wright is a Senior Editor of Physics.


Subject Areas

Particles and FieldsAstrophysics

Related Articles

Sterile Neutrino Down but Not Completely Out
Particles and Fields

Sterile Neutrino Down but Not Completely Out

Neutrino experiments place the most stringent limits to date on a hypothetical fourth neutrino, but the possibility that such a particle exists remains open. Read More »

Powdering Up for Neutrinos
Particles and Fields

Powdering Up for Neutrinos

The search for neutrinos from past supernovae is getting an upgrade as Japan’s Super-Kamiokande experiment begins adding gadolinium powder to its giant water-based detector. Read More »

Could Planet Nine Be a Black Hole?
Astrophysics

Could Planet Nine Be a Black Hole?

While astronomers look for a ninth planet to explain orbits in the outer solar system, a duo explores the possibility that the mystery object is a primordial black hole. Read More »

More Articles