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Until the past decade or so, two mysteries bedeviled physicists. One was the attempt to catch neutrinos. These elusive particles are so light, less than 1/7 the mass of an electron, they pass through everything virtually unhindered. They are passing through you, the Earth, everything in the Universe at all times.
Until 1998, we thought neutrinos had zero mass, but the massive underground detectors built to capture them by then showed they did have that slight mass mentioned above. The detectors also proved they were affected by the weak force, the force that causes radioactive decay and permits quarks to morph into antiparticles, among other phenomena. One of these detectors is shown below. It is the Sudbury Neutrino Observatory (SNO) in Sudbury, England. Two thousand meters below the ground, this 12-meter sphere is filled with heavy water and surrounded with light detectors.
Up to that time, the standard model predicted three types of neutrinos—the Muon, Tau, and electron neutrinos. That weak force can cause the various types to switch from one to the other as they course through the Universe.
Since 1998, astronomers and astrophysicists have found strong evidence of a fourth neutrino. Theorists had insisted that, since the three known types did have mass, however small, there had to be a fourth type, heavier than the other three. The orbiting observatory Wilkinson Microwave Anisotropy Probe (WMAP), since 2001, has been mapping tiny fluctuations in the radiation left over from the Big Bang. In data just released last January, there is evidence that a fourth neutrino existed in the early Universe.
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Dark Matter Found?
More recently the Chandra X-ray observatory detected faint pulses of X-rays from sections of the sky thought to contain Dark Matter. These pulses were exactly what would be expected if sterile neutrinos were decaying into lighter neutrinos. This is the second mystery that has bedeviled physicists—what is Dark Matter?
Unfortunately, the WMAP and Chandra data seem to contradict each other in one respect. Chandra indicates a sterile neutrino heavier that the other three—perfect as the Dark Matter culprit. WMAP, on the other hand, indicates a lighter particle. This might be explained by the fact that WMAP was looking at remnants of the big bang. It may have been detecting the switch of heavy sterile neutrinos to 'normal' light neutrinos.
Experiments at both FermiLab and CERN have shown evidence of the existence of the sterile neutrino, but have not solved the WMAP and Chandra contradictions.
Still, observations from the orbiting observatories do indicate that sterile neutrinos appear to exist in astronomical (pun intended) numbers throughout the Universe. Heavy or light, there may be enough of them to answer the question—What is Dark Matter?
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A Neutrino Hit on the Standard Model
Sterile Neutrinos not only may hold the answer to the Dark Matter quandary. They may rewrite the Standard Model, since it does not predict a fourth neutrino. To accommodate this strange new particle, our model of the subatomic zoo will have to undergo significant revision.
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Nature: Hunt for the sterile neutrino heats up http://www.nature.com/news/2010/100317/full/464334a.html
Cornell University: Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Interpretation http://arxiv.org/abs/1001.4538v1
Cornell University: Dark Matter Search Using Chandra Observations of Willman 1, and a Spectral Feature Consistent with a Decay Line of a 5 keV Sterile Neutrino http://arxiv.org/abs/0912.0552
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Neutrino Detector: NASA http://arxiv.org/abs/0912.0552