Sterile Neutrinos

They're back at the top of the news again, and who's to say they won't stick around this time? Time to catch up.

From Wikipedia:

A sterile neutrino is a hypothetical neutrino that does not interact via any of the fundamental interactions of the Standard Model except gravity.^{[note 1]} It is a right-handed neutrino or a left-handed anti-neutrino.

Contents 1 Properties 2 Detection 3 See also 4 Notes 5 References 6 External links

Properties

Such a particle belongs to a singlet representation with respect to the strong interaction and the weak interaction and has zero weak hypercharge, zero weak isospin and zero electric charge. The left-handed anti-neutrino has a B-L of 1 and an X charge of 5. Sterile neutrinos would still interact via gravity, so if they are heavy enough, they could explain cold dark matter or warm dark matter. In some grand unification theories, such as SO(10), they also interact via gauge interactions which are extremely suppressed at ordinary energies because their gauge boson is extremely massive. They do not appear at all in some other GUTs, such as the Georgi-Glashow model (i.e. all its SU(5) charges or quantum numbers are zero).

Detection

Sterile neutrinos may mix with ordinary neutrinos via a Dirac mass^{[citation needed][clarification needed]}. The sterile neutrinos and ordinary neutrinos may also have Majorana masses. In certain models^[which?], both Dirac and Majorana masses are used in a seesaw mechanism, which drives ordinary neutrino masses down and makes the sterile neutrinos much heavier than the Standard Model interacting neutrinos. In some models^[which?] the heavy neutrinos can be as heavy as the GUT scale (~10¹⁵ GeV). In other models^[which?] they could be lighter than the weak gauge bosons W and Z as in the so-called νMSM model where their masses are between GeV and keV. A light (with the mass ~1 eV) sterile neutrino was suggested as a possible explanation of the results of the LSND experiment. On April 11, 2007, researchers at the MiniBooNE experiment at Fermilab announced that they had not found any evidence supporting the existence of such a sterile neutrino.^[1] More recent results and analysis have provided some support for the existence of the sterile neutrino.^[2]

See also

• LSND
• MiniBooNE
• Neutrino oscillation

Notes

1. ^ Real neutral particle

References

1. ^ First_Results (PDF)
2. ^ Scientific American: "Dimensional Shortcuts", August 2007

External links

• Experiment Nixes Fourth Neutrino (April 2007 Scientific American)

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Spacecraft has closest encounter ever with comet

By the CNN Wire Staff

November 4, 2010 11:15 a.m. EDT

(CNN) -- A spacecraft survived the closest encounter ever with a comet on Thursday, tracking it just 435 miles (700 kilometers) from the comet's nucleus.

Mission control at NASA's Jet Propulsion Laboratory in Pasadena, California, broadcast live coverage of the event on NASA Television's Media Channel.

The agency's EPOXI spacecraft is expected to acquire data with two imagers and an infrared instrument of a comet named Hartley 2 as it traveled at speeds of more than 27,000 mph (43,450 kph).

Scientists are still working to determine if there was any damage to the spacecraft as the comet passed by. They hope to learn more about comets from images detailing the close approach.

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"Those early images may not be the 'money shot,' but we on the science team will prize them just as well, as they will help us further understand the nature of comets," EPOXI principal investigator Mike A'Hearn, of the University of Maryland, College Park, said in a NASA statement.

The images are expected to depict the comet nucleus as little more than a point of light with a fuzzy coma, a gaseous cloud, surrounding it.

Five years after NASA launched an 800-pound projectile into a comet in an effort to study its contents, the same spacecraft that launched the missile tracked Hartley 2 on Thursday.

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Has Fermilab Discovered The Source of Dark Matter ? (4th family of neutrinos)

Physics experiment suggests existence of new particle

November 2, 2010

(PhysOrg.com) -- The results of a high-profile Fermilab physics experiment involving a University of Michigan professor appear to confirm strange 20-year-old findings that poke holes in the standard model, suggesting the existence of a new elementary particle: a fourth flavor of neutrino.

The new results go further to describe a violation of a fundamental symmetry of the universe asserting that particles of antimatter behave in the same way as their matter counterparts.

Neutrinos are neutral elementary particles born in the radioactive decay of other particles. The known "flavors" of neutrinos are the neutral counterparts of electrons and their heavier cousins, muons and taus. Regardless of a neutrino's original flavor, the particles constantly flip from one type to another in a phenomenon called "neutrino flavor oscillation."

An electron neutrino might become a muon neutrino, and then later an electron neutrino again. Scientists previously believed three flavors of neutrino exist. In this Mini Booster Neutrino Experiment, dubbed MiniBooNE, researchers detected more oscillations than would be possible if there were only three flavors.

"These results imply that there are either new particles or forces we had not previously imagined," said Byron Roe, professor emeritus in the Department of Physics, and an author of a paper on the results newly published online in Physical Review Letters.

"The simplest explanation involves adding new neutrino-like particles, or sterile neutrinos, which do not have the normal weak interactions."

The three known types of neutrino interact with matter primarily through the weak nuclear force, which makes them difficult to detect. It is hypothesized that this fourth flavor would not interact through the weak force, making it even harder to find.

The existence of sterile neutrinos could help explain the composition of the universe, said William Louis, a scientist at Los Alamos National Laboratory who was a doctoral student of Roe's at U-M and is involved in the MiniBooNE experiment.

"Physicists and astronomers are looking for sterile neutrinos because they could explain some or even all of the dark matter of the universe," Louis said. "Sterile neutrinos could also possibly help explain the matter asymmetry of the universe, or why the universe is primarily composed of matter, rather than antimatter."

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