* Astronomy

Physics Bridges Long Distances to Study Tiny Particles

A remote monitoring station allows McMicken physicists to actively participate in Fermilab's MiniBooNE experiment.

These extremely tiny particles - many times smaller than the electrons that orbit individual atoms - are thought to play a role in everything from solar energy to radioactive decay to the "dark matter" that makes up most of the universe.
Neutrinos' small size and the difficulty in isolating them for study has made it hard for physicists to fully understand how they relate to the other subatomic particles that form the building blocks of atoms and molecules. But an experiment being conducted by researchers from the University of Cincinnati and 17 other universities could unlock critical secrets about the nature of neutrinos.

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The detection of extra dimensions beyond the familiar four--the three dimensions of space and one of time--would be among the most earth-shattering discoveries in the history of physics. Now scientists at the Fermi National Accelerator Laboratory in Batavia, Ill., are designing a new experiment that would investigate tantalizing hints that extra dimensions may indeed exist.
While observing a beam of muon neutrinos generated by one of Fermilabs particle accelerators, the MiniBooNE researchers found that an unexpectedly high number of the particles in the low-energy range (below 475 million electron volts) had transformed into electron neutrinos. After a year of analysis, the investigators have failed to come up with a conventional explanation for this so-called low-energy excess. The mystery has focused attention on an intriguing and very unconventional hypothesis: a fourth kind of neutrino may be bouncing in and out of extra dimensions.

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Title: The Highest Energy Neutrinos
Authors: Francis Halzen

Measurements of the arrival directions of cosmic rays have not revealed their sources. High energy neutrino telescopes attempt to resolve the problem by detecting neutrinos whose directions are not scrambled by magnetic fields. The key issue is whether the neutrino flux produced in cosmic ray accelerators is detectable. It is believed that the answer is affirmative, both for the galactic and extragalactic sources, provided the detector has kilometre-scale dimensions. We revisit the case for kilometre-scale neutrino detectors in a model-independent way by focussing on the energetics of the sources. The real breakthrough though has not been on the theory but on the technology front: the considerable technical hurdles to build such detectors have been overcome.
Where extragalactic cosmic rays are concerned an alternative method to probe the accelerators consists in studying the arrival directions of neutrinos produced in interactions with the microwave background near the source, i.e. within a GZK radius. Their flux is calculable within large ambiguities but, in any case, low. It is therefore likely that detectors that are larger yet by several orders of magnitudes are required. These exploit novel techniques, such as detecting the secondary radiation at radio wavelengths emitted by neutrino induced showers.

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Low-energy neutrinos from the Sun have been spotted by researchers for the first time. Detecting these solar particles confirms theories about how nuclear fusion occurs within the Sun, and how neutrinos oscillate between different types.

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