Elusive neutrinos from distant blazar captured at the south pole

Nature holds the key to unsolved mysteries that could, once revealed, allow us to push the limits beyond our current understanding of science. Cosmic rays is one such phenomenon that has intrigued astrophysicists since ages. It is not yet known how or where such rays of highly energetic particles are produced, which then traverse the interstellar medium, being scattered by the magnetic field, and then bombard our outer atmosphere generating secondary particle showers as they interact with the medium and lose energy.

One such elusive particle is the neutrino. These are neutral nearly massless particles that are so weakly interacting, that they can easily pass through the earth without being detected. According to the Standard Model (SM) of particle physics, such neutrinos are generated as a result of "weak interactions" that may occur in the sun (producing solar neutrinos), explosions of massive stars at the end of their life cycles (supernova neutrinos) or when cosmic rays decay in the atmosphere, having different energies as they reach the earth. In order to capture these fleeting neutrinos, one needs detectors with large surface area to increase the probability of capture. One type of detectors, known as Cherenkov detectors, contain a large volume of clear medium such as mineral oil or water to capture the light emitted when charged particles move through the medium faster than light. Such as detector, in Kamiokande in Japan, detected the first neutrinos from the supernova SN 1987A. Similar technology was later applied to build other neutrino detectors across the globe such a Sudbury Neutrino Observatory (SNO), MiniBooNE etc.

The alternative idea was to construct detectors in the sea, such as ANTARES in the mediterranean. However the latest improvisation was the AMANDA and later the IceCube observatory in the Antarctic, where a kilometre-cube of ice was used as the detector medium.

​In September 2017, a single ultrahigh energy neutrino (with energy of 290 terra electron volts), was captured by IceCube. The direction of detection was determined to be the Orion constellation. With the simultaneous detection of increased activity from the same direction by the space-based Fermi satellite and other terrestrial observatories, the source of the neutrinos was argued to be cosmic rays emitted by a distant "blazar" 4 billion light years away.

Blazars are galaxies with a black hole in the centre that can strip energetic particles, accelerate them to ultra high energies as collimated cosmic rays that reach the earth. The exact mechanism of this complex process is still far from being understood. Hence the detection of the neutrinos in coincidence with the blazar activity is a compelling evidence of its source, but not a conclusive one.

Even more intriguing is the recent detection of particle events detected by a balloon-borne experiment in Antarctica called ANITA (Antarctic Impulsive Transient Antenna). Following detections in 2006 and 2014, two recent events observed by this facility produced a lot of excitement as they require theories Beyond Standard Model (BSM) in order to explain them.

Supersymmetric (SuSy) models, an extension of the SM, predicts the existence of partners of the SM counterparts, but with different masses. The "stau" sleptons, that have been claimed to be detected by ANITA, are predicted by supersymmetric models as next to light

est supersymmetric partner particles. This claim now has supporting evidence of their existence from IceCube extremely high energy tracks.

Further information:

https://icecube.wisc.edu/news/view/586

"The ANITA Anomalous Events as Signatures of a Beyond Standard Model Particle, and Supporting Observations from IceCube", D. B. Fox et al., arXiv:1809.09615