Slashdot Mirror
Fermilab Confirms Evidence of 4th Flavor Neutrino
eldavojohn writes "We've only had evidence for three kinds of neutrinos so far, but a recent test at Fermilab involving an antineutrino beam has reinforced a Michigan researcher's earlier experiment suggesting a fourth flavor. What's really odd about this is that a prior neutrino test (carried out as part of project MiniBooNE) did not result in indications of such strange oscillations. According to the researcher, 'The simplest explanation involves adding new neutrino-like particles, or sterile neutrinos, which do not have the normal weak interactions.' But this could also be an unknown or misunderstood effect. A Los Alamos National Laboratory scientist added that an explanation of this strange anomaly could result in understanding 'matter asymmetry of the universe, or why the universe is primarily composed of matter, rather than antimatter.' The results are published in the Physical Review Letters."
What has been found is an excess of certain events (namely anti-muon-neutrino to anti-electron-neutrino oscillations), where "excess" is defined relative to the current best-established model. So what this experiment (if correct) shows, is that the current model is not good enough.
From the PRL paper:
The source of the excess remains unexplained, although several hypotheses have been put forward
One of those hypotheses is additional neutrino flavours, but this finding is not evidence for that.
Well, IAAFPP (I Am A Former Particle Physicist, now no longer active in the field), and you have to be careful what you mean by "neutrino". In the Standard Model, neutrinos are partners to the charged leptons: electron, muon, or tau lepton. By "partner to", I mean connected (in a sense) by the weak force, which is the only non-gravitational force that acts on them (being neutral, they are immune to the electromagnetic force, and being leptons, they don't feel the strong force). Neutrinos are also very light, having near-zero mass.
This is what the Standard Model calls a neutrino. And there are, in fact, only 3 kinds. This was shown pretty convincingly by LEP at CERN. And it's also enough to discredit Heim's Theory (which no one really took seriously in the first place).
What this story is suggesting is that there may be a different kind of neutrino -- a so-called "sterile neutrino" -- that doesn't even feel the weak force. This isn't part of the Standard Model, but it is possible in certain extensions of the SM. This kind of neutrino doesn't act the same way as the SM neutrinos; it's a different beast, and comes about through a different part of the mathematics.
Your reasoning looks pretty sound to me; I don't think there is a fundamental reason to assume that right-handed neutrinos don't exist. I think the main reason people make that assumption is that there is no experimental evidence for it. It appears that the weak force only acts on left-handed particles
You're right in that a right-handed neutrino would interact only gravitationally. But if they exist, how did they get created in the first place? That creation process had to involve some combination of the other 3 forces -- gravity doesn't allow for particle creation or decay.
Another thing is that if it were massive (and it would have to be), it would have to have a left and right-handed component, and be invariant under Lorentz transformations. (One way to think about it is this: If it's moving in a certain direction, you could look at it from a reference frame moving even faster in that direction, and it would appear to be going the other way. This would change it from a right-handed to a left-handed particle, which would mean it could interact with the weak force, etc. etc. So it would have to be a mixture of both left- and right-handed components - you can't have a purely right-handed neutrino with a non-zero mass).
It also turns out (mathematically) that you can construct a (sterile) neutrino by using only left-handed fields, and still make it behave as if it had a right-handed component. This is the so-called "Majorana spinor". So you don't really need to invoke right-handed neutrinos, you can get the same result using just the left-handed fields.
So what is the problem with this reasoning? And could the sterile neutrino from this story be actually such a right-handed neutrino?
First: it cannot be dark matter because neutrinos are too light and hence move too fast. The result is that the WMAP cosmic microwave background would be blurred out far more than it is so we know that, whatever the dark matter is, it is slow moving and so not a light neutrino.
Second: MiniBoone has interesting results but have not BY ANY STRETCH of the imagination confirmed the existence of a 4th gen of neutrinos. Their signal is only 0.6% incompatible with background. To claim evidence the standard in the field is 3 std. dev. (or 0.27%) and to claim a discovery it is 5 sigma (0.000057%). Effects like this go away all the time and can easily be caused by errors. This is not a guarantee that theirs will but, to make claims like this you need solid, statistical evidence and they do not yet have that.
Third: we already know that right handed neutrinos exist because the neutrino has a non-zero mass. Any mass term in the Lagrangian mixes left and right helicity states. Effectively what this means is that if you have a left handed neutrino but chase after it faster than it is moving (which you can do because it has a mass) it will be a right handed neutrino to you. So, if there is anything interesting happening here, it is not a "normal" right handed neutrino.
Isn't Hawking radiation a process where gravitation creates particles?
Not really. It's an electroweak process that actually creates the particle-antiparticle pair.
Maybe a Higgs particle decays into a right-handed neutrino and something else?
No. "Decay" implies a weak interaction. And the weak force only interacts with left-handed particles (or more precisely with the left-handed fields, or components, of a particle).
The Higgs field can couple the left- and right-handed fields of a particle. But when you're talking about "Higgs particle decay", that's a weak interaction, which is only left-handed.
IIUC, if left-handedness depended on the frame of reference, then whether an electron (which very clearly has mass) can interact weakly would also depend on the frame of reference, and that doesn't make sense to me.
That's why you can't have a purely left- or right-handed massive particle. Any massive particle (like an electron) has to have both a left-handed and a right-handed (chiral) component. It also has to be invariant under Lorentz transformations, meaning that as you change reference frames, the particle looks the same. Only massless particles can be purely left-handed or right-handed, and for them chirality and helicity are equal. But not for massive particles.
By the way, the evidence of neutrino oscillations means that the three Standard Model neutrinos must have some non-zero mass, which means they're not purely left-handed. They were once thought to be purely left-handed, but that was when they were thought to be massless. Now we know that they're more like electrons, with a left- and right-handed component.