Existence of cosmic neutrinos confirmed by IceCube Neutrino Observatory
Just days after Fermilab’s NOvA experiment detected neutrinos fired from 500 miles away, researchers confirmed existence of astrophysical neutrinos from our galaxy as well as cosmic neutrinos from sources outside the Milky Way by using the IceCube Neutrino Observatory.
Researchers confirmed the existence of cosmic neutrinos after sorting through the billions of subatomic particles that zip through IceCube Neutrino Observatory’s frozen cubic-kilometer-sized detector each year. The latest results provide an independent confirmation that support a similar finding back in 2013.
In the latest study, published in the journal Physical Review Letters by the IceCube Collaboration, researches detected 21 ultra high-energy muons, which are the secondary particles created on the very rare occasions when neutrinos interact with other particles.
The finding has been pegged as an “unequivocal signal” for astrophysical neutrinos as it proves the existence of ultra high-energy particles that have traversed space unimpeded by stars, planets, galaxies, magnetic fields or clouds of interstellar dust.
Neutrinos have no mass or no electric charge and because of these two properties, they are very hard to detect and can only be observed when in the rarest of cases they interact with other particles to create muons, which are the telltale secondary particles.
The latest observations were made by pointing the Ice Cube Observatory — composed of thousands of optical sensors sunk deep beneath the Antarctic ice at the South Pole — through the Earth to observe the Northern Hemisphere sky. The Earth serves as a filter to help weed out a confusing background of muons created when cosmic rays crash into the Earth’s atmosphere.
Over the course of two years – between May 2010 and May 2012 – IceCube recorded more than 35,000 neutrinos. However, only about 20 of those neutrino events were clocked at energy levels indicative of astrophysical or cosmic sources.
Neutrino collisions are very rare and a particularly large space is required to capture the signature of these collisions. The IceCube observatory, which is spread in cubic kilometer of deep Antarctic ice, is big enough to capture these collisions.
When such a collision does happen, it creates a muon, which, in turn, leaves a trail of Cherenkov light that faithfully mirrors the trajectory of the neutrino. The “optical sonic booms” created when neutrinos smash into another particle are sensed by the optical sensors that make up the IceCube detector array and, in theory, can be used to point back to a source.
“Looking for muon neutrinos reaching the detector through the Earth is the way IceCube was supposed to do neutrino astronomy and it has delivered,” explains Francis Halzen, a UW-Madison professor of physics and the principal investigator of IceCube. “This is as close to independent confirmation as one can get with a unique instrument.”
But while the new observations confirm the existence of astrophysical neutrinos and the means to detect them using the IceCube Observatory, actual point sources of high-energy neutrinos remain to be identified.
Albrecht Karle, a UW-Madison professor of physics and a senior author of the Physical Review Letters report, notes that while the neutrino-induced tracks recorded by the IceCube detector have a good pointing resolution, within less than a degree, the IceCube team has not observed a significant number of neutrinos emanating from any single source.
Researchers believe that these neutrinos may have originated beyond our galaxy because in previous observations of the plane of our galaxy, the IceCube Observatory didn’t detect any.
“The plane of the galaxy is where the stars are. It is where cosmic rays are accelerated, so you would expect to see more sources there. But the highest-energy neutrinos we’ve observed come from random directions,” says Karle, whose former graduate student, Chris Weaver, is the corresponding author of the new study. “It is sound confirmation that the discovery of cosmic neutrinos from beyond our galaxy is real.”
Researchers believe that the high-energy neutrinos are created in some of the most violent phenomena in space including black holes and once created they are accelerated to energy levels that exceed the record-setting earthbound accelerators such as the Large Hadron Collider (LHC) by a factor of more than a million.
Researchers have long sought to study these particles owing to the fact that they are one of the most pristine particles ever known – unchanged even after travelling million of light-years. The ability to study the highest-energy neutrinos promises insight into a host of problems in physics, including how nature builds powerful and efficient particle accelerators in the universe.