Physicists have been left scratching their heads following the observation of two unusual radio signals by the balloon-borne Antarctic Impulsive Transient Antenna (ANITA). While many other signals spotted by ANITA are produced by cosmic rays crashing down through the atmosphere, the two anomalous events seem to be caused by particles travelling up through the Earth’s crust. Those particles may be neutrinos but their properties seem at odds with the Standard Model of particle physics.
ANITA, developed by a US-led collaboration and funded by NASA, contains 96 radio antennas suspended from a helium balloon. Flying at an altitude of nearly 40 km for several weeks at a time, it detects radio waves emanating from a 1.3 million square kilometre swathe of Antarctica. Its primary purpose is to pick up the signals produced by cosmic neutrinos travelling from deep space to try and pinpoint the origin of ultra-high energy cosmic rays – which are believed to be produced in the same places as cosmic neutrinos.
The neutrinos of interest pass through the Earth and interact with atomic nuclei in the Antarctic ice sheets, producing a shower of charged particles that then emit radio waves. ANITA also picks up signals from cosmic rays as they travel downwards and collide with the atmosphere. This also generates radio waves, which bounce off the ice and into ANITA’s antennas.
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