So their rest mass is not zero?
Many years ago, when I first heard of neutrinos, everybody talked of them as ghostly chunks of nothing that, like photons, had no rest mass or electric charge and moved with the speed of light, carrying momentum and energy, but, unlike photons, had a very hard time interacting with anything.
Several years ago I began hearing that neutrinos have mass. As of now, we are told it is no longer disputed that they have mass. Neutrinos have non-zero rest mass.
They are not all the same
There are neutrinos and antineutrinos, of course, but this is not all.
For many years now it has been known that there are three types of neutrinos: the electron neutrino, the muon neutrino, and the tau neutrino. They are named after their corresponding charged leptons (the electron, the muon, and the tau). You may refer to neutrino types as “flavors” if you have a taste for them.
As for their chirality, all neutrinos are left-handed and (you guessed it!) all antineutrinos are right-handed. I find this pretty odd.
Getting used to neutrinos
I am a conservative sort of person. For instance, when I was told that the decay of Z bosons dictates that there can be no more than three types of neutrino, I felt content. We had them all in the bag: the e, the mu, and the tau! Yet some people now say other neutrinos may exist that have some special quality (“sterility”, what else?) that renders the Z boson decay constraint irrelevant for them.
People have a fertile mind. I say to them: Please give me a break! It has taken me quite a while to get used to regular neutrinos in the first place.
The question of mass (1)
As I said, it took me a while to get used to neutrinos. But in due course I even came to like them. I came to like them as I thought they were: having no electric charge, no mass, and only the weakest disposition to interact.
Now we are told that there is compelling evidence of “oscillations” between neutrino types which imply that their mass differences cannot be zero. In other words, their masses (more precisely, their three eigenvalue mass states) must be different from each other. Therefore, they cannot all be zero.
The question of mass (2)
Suddenly people are happy because, among other things, neutrino oscillations provide an explanation to the long-standing mystery of fewer solar neutrinos being detected here on Earth than expected. I am happy for that, too.
But I am unhappy that while their non-zero masses have to be tiny, as current upper bounds demand, no credible lower bounds seem to exist. I am uncomfortable with the absence of a significant time lag between photons and the supposedly massive neutrinos from supernova 1987A on their very long flight to Earth.
The question of mass (3)
The well-known masses of the electron, muon, and tau increase in this order. From what I have read, it appears that the minuscule mass states of their corresponding neutrinos ought to increase in the same order, but, alas, an inverted order cannot be ruled out.
Conservative as I am, it has occurred to me that perhaps the smallest neutrino mass state could be zero, after all, with the other two being different positive mass states! Is this possible? If you know the answer, please let me know.
And now the speed of neutrinos is in question!
This is a small 10 December 2011 addendum to my thoughts of 10 May 2008.
Recent experiments appear to have detected muon neutrinos traveling faster than light! I have no idea of what new theory people will come up with in order to explain this finding if (against my expectation) it can be confirmed.
For now I am having a little fun saying that things may be easier to explain if one admits that in the relativistic equations the mass of the muon neutrino is imaginary…
(Editorial note: In 2012, a few months after this addendum was written, it was finally determined that imperfect equipment used in the OPERA experiments had caused the neutrino speed measurements to be in error.)
A. B. Carleial Consulting