Nov. 9, 2010
UI physicist uses the moon in search for elusive ultra-high-energy neutrinos
The phrase "harvest moon" recently took on a new meaning as a University of Iowa astrophysicist and his UI colleagues used the moon in an attempt to harvest evidence of elusive cosmic particles called ultra high energy (UHE) neutrinos in the most sensitive such radio search ever attempted.
The results of the project -- conducted by professor Robert Mutel, associate professor Kenneth Gayley and National Research Council post-doctoral fellow Theodore Jaeger, all of the UI Department of Physics and Astronomy -- have just been published in the December 2010 issue of the journal Astroparticle Physics. What the researchers hoped to detect was the tell-tale signatures of neutrinos, elementary particles of neutral charge, in order to learn more about the fundamental building blocks of matter.
Neutrinos are nearly massless and have no charge, making them difficult to detect. This is because they usually pass through matter -- including humans and the entire Earth -- without any trace of their passage.
However, ultra high energy (UHE) neutrinos, thought to be generated by extremely energetic cosmic sources such as massive black holes, collide with matter readily. The interaction produces a shower of charged particles, followed by a very short-lived radio burst. Other, more exotic generation mechanisms include "topologic defects," a kind of tear in the space-time fabric of the universe, and decays of heavy particles called "Z bosons," which are thought to be left over from the Big Bang. Although astronomers have been searching for the elusive UHE neutrinos for over 15 years, so far none have been detected.
The project itself took place in the Fall of 2009, when Mutel and his colleagues used the Very Large Array, the world’s most powerful radio telescope array, to search for UHE neutrinos. They used the moon as a target, searching for the telltale radio bursts which would result from UHE neutrino impacts. They searched for a total of 200 hours, using a custom, high-speed data sampling system designed to detect pulses as short as one nanosecond (one-billionth of a second).
Although they didn't detect neutrino-generated pulses, they did succeed in setting new, lower limits on the cosmic UHE neutrino flux. These limits provide important constraints on models of neutrino generation from a variety of cosmic neutrino generation models. In particular, the lower limit eliminates some Z-burst models which had predicted UHE neutrinos originating from the halo of the Milky Way galaxy. In addition, the pulse detection scheme and a new analysis of the moon as a neutrino target developed by the UI team will guide future moon-based radio searches with the next generation of radio telescope arrays.
The UI team was not the first to use the moon as a kind of neutrino laboratory. Previous lunar-target radio searches have been conducted in Australia, California, the Netherlands and Russia. In addition, a large international team has flown balloons equipped with small radio telescope arrays over the Antarctic ice cap to look for UHE neutrino interactions in the ice.
"All of these experiments have not yielded any detection thus far, but this is indicative of the fundamental importance of the search," Mutel said. "The first detection of ultra-high energy neutrinos will be a major discovery in astrophysics. Neutrino astronomy will open a new window to the physics of cosmic processes, similar to the first observations of stars and galaxies at radio, ultraviolet and X-ray waves, all made in the past 75 years."
For more information, see the project website at: http://resun.physics.uiowa.edu/.
The project was funded by the National Science Foundation.
The Department of Physics and Astronomy is a part of the UI College of Liberal Arts and Sciences.
Astroparticle Physics is the leading journal for research in astrophysics related to particle physics.
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