Wednesday, June 02, 2010

Neutrino news

As reported yesterday in the WSJ, astronomers in Antarctica are building a telescope out of ice on the Earth’s southernmost continent. The project is being headed by the University of Wisconsin and is expected to cost a cool $270-million, with NSF covering the vast majority of funding. Appropriately called Ice Cube, the project will use one square kilometer of ice to create a detector that can glimpse the rare instances where water-ice atoms and neutrinos collide. The astronomers can then use their observations to do something that has never been done before-create a neutrino map of the sky.

The mass of a neutrino hasn’t been precisely determined yet (more on that in a minute), but they are strikingly small. The subatomic particles can pass through solid matter at nearly the speed of light without having a collision. Neutrinos are more common than even protons and neutrons and these high-energy neutrinos frequently emanate from cosmic explosions, such as gamma-ray bursts.

In the rare event a neutrino does collide with an ice atom, the interaction creates a muon and because the ice is so transparent, Ice Cube can detect the blue light of the nuclear reaction from hundreds of yards away. This data is then fed to scientists at 35 labs in seven countries for analysis. Scientists will be able to track the direction of the neutrinos that pass through Earth and reveal their cosmic source.

Antarctic ice is incredibly pristine and free from impurities and more importantly, it’s free from radioactivity, making it the perfect environment for neutrino detection. The detector faces a daunting problem though-muons produced by cosmic rays outnumber those from neutrinos by a ratio of a million-to-one, so the detector has to use earth as a filter. Because neutrinos are the only known particles that can pass through the earth (and our bodies too!) Ice Cube scientists know collisions can’t be from bombarding cosmic rays.

But, to shield their instruments from the natural background radiation at the surface, the top layer of detectors had to be placed 1.4 kilometers deep in the ice. Once placed, the ice melted back around the 5,000 sensors and froze them in place for the next 25,000 years, or roughly the amount of time it will take for that chunk of ice to migrate to the ocean.

Also, in a Physcial Review Letters article last month, cosmologists from University College in London placed tighter restraints on just how small neutrinos might be. Because neutrinos are one of the most abundant particles in the universe and are flying all around at nearly the speed of light, cosmologists think they must play a large role in the cosmos. Until we understand them better though, it’s hard to know how large that role is.

Using a survey of how galaxies are distributed across the universe, the group was able to determine an upper limit on the neutrino’s mass. They created and studied the largest 3D map in existence (containing 700,000 galaxies), called MegaZ, by using the redshift of galaxies in the Sloan Digital Sky Survey to determine their distances.

The group combined their survey information with temperature fluctuation measurements from the Cosmic Microwave Background radiation to determine that the mass of a neutrino must be less than one billionth the mass of a hydrogen atom.

The paper, titled Upper Bound of 0.28eV on the Neutrino Masses from the Largest Photometric Redshift Survey, refers to the upper limit as “one of the tightest and cleanest constraints on the neutrino mass from cosmology or particle physics.”

The results also indicate that current to next-generation experiments to determine the mass of a neutrino, such as the German instrument KATRIN, are unlikely to detect anything.

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