On Tue, 17 Aug 1999, John Clark wrote:
> Joseph C Fineman <jcf@world.std.com> Wrote:
>
> >Antineutrinos exist in any case.
> >[...]
> >They are distinguished from neutrinos by their helicity
> >(direction of spin relative to their direction of motion). When
> >a neutron decays, for example, it yields a proton, an electron,
> >and an antineutrino. However, it seems to me that if neutrinos
> >have rest mass, this distinction must in principle lapse, since
> >you can then choose a frame of reference in which the helicity
> >is reversed.
>
> You can't turn a right hand glove into a left one in any frame of
> reference, at least not if you stay in 3 dimensions; you'd have to
> flip it around in the fourth spatial dimension.
No doubt, but a neutrino isn't like a glove -- or, more appositely, like a screw. Its handedness is determined by its spin _relative to its momentum_. If its rest mass is zero, that is an unambiguous definition, because there is no frame of reference in which the momentum is reversed. But if the neutrino as a rest mass, then it has a rest frame, in which its momentum is zero, and the helicity is not even defined. Its spin, presumably, can be in any direction whatever. You can then give it either a right-hand or a left-hand thread by choosing the direction in which your frame of reference moves with respect to it.
> Besides, the two particles act differently. A antineutrino is
> absorbed (rarely) by a proton which turns into a neutron and a
> positron (also called a anti electron), a neutrino is absorbed
> (rarely) by a neutron which turns into a proton and a electron.
Well, maybe so; but suppose I decelerate a neutrino to rest and then accelerate it to a near-c velocity in the other direction. What do I have, then? A phony antineutrino -- a particle with the antineutrino's helicity but the neutrino's lepton number?
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