“It’s a very exciting thing for the field of neutrino astronomy,” said UA associate professor of physics Dawn Williams.
Williams is among the co-authors of a paper in the Nov. 22 edition of the journal Science about the observation of 28 very high-energy particle events that researchers believe constitute the first evidence of high-energy neutrinos from beyond our solar system.
The observations came from data collected between May 2010 and May 2012 from the IceCube Neutrino Observatory, a sensor array of more than 5,000 digital optical modules suspended along 86 cables embedded in a cubic kilometer of ice beneath the South Pole.
The observatory was constructed with a National Science Foundation grant and some funding from participating international agencies.
Also among the co-authors are UA assistant professor of physics Pat Toale, former UA postdoctoral researcher Pavel Zarzhitsky, and UA graduate students Michael Larson, James Pepper and Donglian Xu.
The UA researchers are part of a team of researchers that includes 250 physicists and engineers from the U.S., Germany, Sweden, Belgium, Switzerland, Japan, Canada, New Zealand, Australia, the United Kingdom and Korea who are working with the observatory, which is designed to observe evidence of the collisions between neutrinos, nearly mass-less subatomic particles capable of passing through most matter unimpeded, and nuclei of the ice atoms.
Neutrinos are generated as a result of the decay of radioactive elements or nuclear reactions such as those that power the sun or power plant reactors. The particles are also generated when cosmic rays — radiation from outer space — collide with atoms in the atmosphere.
Neutrinos are also produced by high-energy events such as the births, collisions and deaths of stars, according to the IceCube site.
The neutrinos generated within our solar system and earth’s atmosphere are of a relatively low energy, Williams said. The low-energy neutrinos that bombard the earth form a background, a control, by which the researchers can look for exceptions.
“So what we are in effect doing is looking for something that deviates from that background,” Toale said.
High-energy neutrinos are valuable because of their ability to travel across the distance of space, for the most part, without interference.
“There are a variety of astronomical messengers that we use to study the universe. Light is our workhorse astronomical messenger,” Williams said. “But light can be absorbed and scattered by intervening media. Charged particles are bent in magnified fields. Neutrinos don’t scatter or absorb in the intervening media but for extremely high-energy events.”
The uncharged particles tend to travel straight from their sources and are capable of covering long distances, including from the far edges of the Milky Way or beyond.
“If you can observe neutrinos being produced there, that would allow us to look into the dark heart to what is going on in the objects,” Toale said. “There are things we can learn with neutrinos that we cannot learn just with light.”
The Antarctic ice makes an ideal medium for the observations, the researchers say.
The ice is a stable platform that is very transparent and free of radioactive interference. The size of the array — which stretches for more than a mile and a half beneath the ice — was necessary to be able to successfully capture the infrequent high-energy events.
“It is still extremely rare that a neutron reacts. Most of the time they pass right through,” Toale said.
The sensor array records information that helps the researchers determine the energy, direction and shape of the collisions between the neutrinos and ice atoms, Toale said.
“We look for events that deposit a great deal of light in the detectors,” Williams said.
The researchers are looking for light created by the collision between the neutrinos and the ice. How much light and which direction helps the researchers differentiate between the high-energy events and background noise.
The array encounters thousands of events per second, Williams said. Researchers use computer algorithms to sort the events and identify the interesting ones which indicate potential high-energy sources.
The researchers don’t have a clear idea about the source of the high-energy neutrinos — other than they likely come from outside of our solar system. The 28 events are spread out randomly across the sky without clear correlations to known astrophysical objects such as supernovas, Williams said.
The researchers hope, with five to 10 more years of data, patterns will begin to emerge relating to astrophysical objects.
“(Neutrinos) are opening up a new energy window in astronomy. History has shown when we open up a new energy window, we learn new things,” Williams said.