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Is it experimentally ruled out that right-handed neutrinos are actually antineutrinos, and left-handed antineutrinos are neutrinos ?

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A massive right handed neutrino turns left handed if you chase it fast so that the velocity turns around, and in the rest frame, such a particle would not be handed. –  Ron Maimon May 14 '12 at 2:40
    
To deal with the issue that Ron brings up replace "right handed neutrino" with "neutrino having positive helicity" and "left handed neutrino" with "neutrino having negative helicity". –  dmckee May 14 '12 at 3:57
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up vote 12 down vote accepted

Short answer: Unknown

Slightly longer answer: the situation you describer would obtain if neutrinos were Majorana particles (and thus not Dirac particles). It is favored by theorists because it feeds into a nice explanation of why the neutrinos are so light by comparison to the other massive particles.

Experiments are underway that might settle the question by detecting so called "neutrino-less double beta decay" reactions which are forbidden for Dirac neutrinos but allowed for Majorana neutrinos.

June 2012: One of the neutrino-less double beta decay experiments, EXO, has announced modestly significant limit on the possible masses of the neutrinos if they are Majorana particles. This limit excludes some of the theoretically favored scenarios. Right now the significance is too low for a lot of fanfare, but this might be a the first indication of a surprise.

January 2013: KamLAND-Zen has reported a slight improvement on the EXO result as of the end of their first run, with purification of the working material planned before resuming data taking.

July 2013: EXO released another paper last month, setting a much beter measurement of the decay with neutrinos:

$$ T_{1/2}^{2\nu\beta\beta} = \left( 2.172 \pm 0.017\text{(stat)}\pm 0.060\text{(sys)} \right) \times 10^{21} \text{ years}\,,$$

but they do not update their limit for neutrino-less double beta decay, so still no definitive answer on Majorana/Dirac.

August 2013: GERDA has published a report on a low significance null result in the July 17 issue of Phys. Rev. Lett. (also as arXive:1307.4720). Like the EXO and KamnLAND-ZEN results this isn't a strong result by itself, but the combination of all three is starting to look pretty significant.

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The short answer should be "theoretically yes, experimentally inconclusive", this is the content of the rest of the post. –  Ron Maimon Jul 31 '12 at 7:56
    
The seesaw mechanism - to which you appeal when you say the Majorana nature of Neutrinos may explain their tiny masses - needs right-handed neutrinos as separate fields from the left-handed ones. It is the right-handed neutrinos that then have the Majorana character. So it is wrong to say that right-handed neutrinos are simply anti-neutrinos. While right-handed neutrinos and right-handed anti-neutrinos would be indistinguishable, the difference can be made for the left-handed ones, as they are part of an $SU(2)$ isospinor. –  Neuneck Jul 9 '13 at 6:19
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