An article in ScienceNews reports that data from the IceCube experiment under Antarctic ice have shown that the highest energy neutrinos they detect come from all directions, indicating that they are probably at cosmological distances (see Section 12.7c, pp. 324-325). The results were first announced at the American Physical Society’s meeting in April 2014.
Neutrinos open a window into the very distant and high-energy Universe that is extremely difficult to access by any other means. This is because neutrinos, unlike every other subatomic particle, are electrically neutral and rarely interact with matter. By detecting these particles and charting the directions they come from, scientists aim to identify the sources of neutrinos: star-forming galaxies, supermassive black holes or perhaps some as-yet unknown violent objects. These sources can accelerate neutrinos and other subatomic particles to energies far greater than any human-made machine could achieve.
IceCube was specifically built to aid in this quest. For three years, strings of sensors stretching as deep as 2.5 kilometers below the surface of an Antarctic glacier have detected subtle flashes of light created when neutrinos and other particles collide with atoms. Last year, IceCube researchers identified 28 high-energy neutrinos from all directions that are almost certainly from outside the Solar System. The researchers have since found nine more, including the highest energy neutrino ever detected.
To complement this painstaking search for the highest energy neutrinos, Christopher Weaver, an IceCube physicist at the University of Wisconsin-Madison, decided to cast a wider net for the larger population of slightly lower-energy astronomical neutrinos. His approach relied on selecting particles that fell from the skies of the Northern Hemisphere, whizzed through Earth’s interior and arrived at IceCube from below. Only neutrinos, and not other particles that often trigger IceCube’s sensors from above, can make it through Earth’s dense crust and core. He also limited his search to detections at a specific energy – about 100 trillion electron volts – so that the number of neutrinos from space wouldn’t be dwarfed by the amount of neutrinos produced in the atmosphere. (IceCube’s sensors can’t distinguish between the two.)
That left Weaver with about 35,000 neutrinos, at least some of which began their journeys beyond the Solar System. He tracked the directions they came from and found no evidence of clustering in any particular parts of the sky – a finding that confirmed previous analyses and suggests that no local source is primarily responsible for the population of neutrinos whizzing by Earth. As IceCube continues to collect more data, scientists hope these two independent neutrino search methods will converge on trends in the neutrinos’ direction of arrival. It’s an exciting time in neutrino astrophysics.
Links: ScienceNews article