Moon used as giant particle detector
Researchers hunting for the elusive neutrino typically trek to Antarctica, the Mediterranean, and Lake Baikal. But a growing number of projects are looking for the most energetic neutrinos by aiming radio telescopes at the moon.
If the efforts are successful, they might reveal the universe's most powerful particle accelerators or even evidence of exotic new physics.
Neutrinos are fundamental particles that pass easily through matter, only occasionally colliding with atomic nuclei. Until now, the only extraterrestrial neutrinos that have been found were forged in the sun and in one nearby supernova called 1987A.
But astronomers suspect the universe is full of even higher energy neutrinos, produced by cosmic accelerators that whipped charged particles to energies about 100 million times as high as those generated in the most powerful particle accelerators on Earth.
Because neutrinos interact so rarely with matter, large expanses of material are needed to catch as many of them as possible. Detectors are designed to look for flashes of light that are produced when speeding neutrinos slam into atoms, creating a shower of particles that generate light as they zip through the medium.
Giant detector Lakes, oceans, and ice sheets are good materials to use for neutrino searches because light can pass relatively unimpeded through them to detectors. But the moon's relatively uniform, dense soil, or regolith, may also be a good neutrino target.
The high-energy neutrinos that collide with regolith atoms should produce nanoseconds-long bursts of radio waves that can move through the lunar surface for tens or hundreds of metres.
A radio telescope aimed at the edge of the moon could potentially find these brief bursts of energy. But identifying these signals will not be easy. Neutrino collisions at ultra-high energies are rare; astronomers might expect to see just a handful in a month. Radio telescopes are also bombarded by other signals, including a host of man-made interference, that must be excluded.
But the moon's sheer size may make up for such limitations. "Size matters in this game. You have to catch those rare beasts," says Heino Falcke of Radboud University in the Netherlands.
All eyes on the moon Falcke is a member of the NuMoon team, which finished a pilot project to search for neutrino signals on the moon in early 2009 with the Westerbork Synthesis Radio Telescope, an array of 14 dishes in the Netherlands. The team hopes to begin more sensitive measurements using the supercomputer-bolstered LOFAR telescope in late 2009 or early 2010.
NuMoon is not the only neutrino hunt aimed at the moon. A project called RESUN using the Very Large Array in New Mexico recently completed a 50-hour observation run. Another project called LUNASKA is using the Parkes 64-metre radio telescope to search for signals. And a team at the Green Bank Observatory in West Virginia hopes to revive two retired 25-metre telescopes at the observatory to create a three-telescope array to search for the particles.
What will they see? It's not clear. Astronomers suspect ultra-high-energy neutrinos exist because they have found extraordinarily speedy charged particles called ultra-high-energy cosmic rays (UHECRs). These cosmic rays are thought to slam into photons left over from the big bang, creating neutrinos in the process.
Exotic sources?
The current lunar searches can only detect neutrinos at extremely high energies, at least 10 to 1000 times higher than the highest-energy cosmic rays that have been observed.
"Whether they see a signal or not will depend on whether there are neutrinos of such energies," says theorist Subir Sarkar of Oxford University. "If they do see something, it might indicate there are exotic sources of ultra-high-energy neutrinos."
Neutrinos at such high energies may have a number of sources. They may be byproducts of collisions between ultra-high-energy cosmic rays and big bang photons, or they might be produced directly by supernovae or colossal black holes inside galaxies.
Alternatively, exotic phenomena like super-heavy dark matter particles created just after the big bang or topological defects in the fabric of space-time have been proposed as ways to accelerate particles to enormous energies.
Cosmic ray mystery
Energetic neutrinos could help solve the mystery of what produces ultra-high-energy cosmic rays. Astronomers suspect UHECRs may be accelerated to high energies in gamma-ray bursts or in the shock waves created by jets of matter speeding away from supermassive black holes.
UHECRs lose energy quickly when they crash into big bang photons. As a result, cosmic rays more than about 150 million light years away fizzle before they reach Earth.
Neutrinos, which suffer few collisions, could reveal how cosmic rays are created beyond that limit. And because they are not charged, neutrinos are not deflected by magnetic fields that weave through space and bend the paths of charged cosmic rays. This property could allow astronomers to track neutrinos back to their source.
"They will tell us what the highest-energy accelerators in the universe have done throughout history back to the big bang," says Peter Gorham of the University of Hawaii at Manoa, who ran an early lunar neutrino experiment called GLUE.
Antarctic balloon
For now, Gorham has set his sights on the Earth's largest particle target the Antarctic ice sheet with a balloon-borne experiment called ANITA that circles the South Pole.
ANITA, which also searches for the radio signals made by neutrino collisions, may have some advantages in the ultra-high energy neutrino hunt. Radio waves can travel farther in ice than in lunar regolith, producing stronger signals when they emerge. That telescope should be sensitive to neutrinos that are more or less guaranteed to be there, ones with energies comparable to the highest-energy cosmic rays we see.
But ANITA, which completed its second run in 2009, stays aloft for only weeks at a time, and radio telescopes can easily watch the moon for years.
And even if present moon experiments do not find neutrinos, researchers hope the techniques they develop will pave the way for searches with larger, more sensitive radio arrays, like the proposed Square Kilometer Array, which could also see neutrinos ANITA is hunting for.
"It's a bit of a race to see these ultra-high energy neutrinos right now," Gorham told New Scientist. "We don't really know what we're going to see when we first start glimpsing these particles. It will certainly begin a new era of astronomy."
