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I am so tired of seeing massless neutrinos being described as a "prediction" of the Standard Model, and finite neutrino masses as beyond-Standard Model physics
by T-A 5y ago
I am so tired of seeing massless neutrinos being described as a "prediction" of the Standard Model, and finite neutrino masses as beyond-Standard Model physics or even a "mystery". This is especially disappointing coming from a supposedly serious magazine like Symmetry.
Neutrinos were originally hypothesized in order to solve a problem which did not require them to have mass, and for a long time after they were actually observed, their measured masses remained within error bars straddling zero. It therefore made perfect sense to model them as massless.
But to actually include neutrino masses in the Standard Model is trivial, and was done long ago.
The most straightforward way to do it is to give them quark-like mass terms. This requires introducing a right-handed partner for each known (left-handed) neutrino, which some people don't like because right-handed particles don't partake in weak interactions, and weak interactions are the only (known) neutrino interactions (apart from gravity), so you end up with undetectable particles.
The main alternative is to use Majorana mass terms, making neutrinos their own anti-particles, which some people don't like because it deviates from the pattern of all other fermions in the Standard Model.
A third way is to say "it's both", typically involving the seesaw mechanism, which some people don't like because it requires unfashionable GUT-style beyond-Standard Model physics.
Point is, there is neither a failed "prediction" nor a great "mystery" here. There is uncertainty about which kind of mass term we should use for neutrinos, because the experimentally observable differences between the alternatives are really, really tiny.
- nxpnsv 5y agoThe mystery part is: what is the correct way to include the mass term.
- T-A 5y agoNo mystery at all, you add it to the massless part of the Standard Model Lagrangian. :)
- supergarfield 5y ago> This requires introducing a right-handed partner for each known (left-handed) neutrino, which some people don't like because right-handed particles don't partake in weak interactions, and weak interactions are the only (known) neutrino interactions (apart from gravity), so you end up with undetectable particles. I'd be curious to hear more about this. What mechanism forces you to add right-handed partners? Is it some conservation property? Are calculations too difficult without them? Maybe it's also not clear if there really is a difference between saying "right-handed neutrinos exist but are undetectable" and "right-handed neutrinos are fake particles added for ease of modeling". Edit: Wikipedia also says > The neutral-current Z^0 interaction can cause any two fermions in the standard model to deflect: Either particles or anti-particles, with any electric charge, and both left- and right-chirality, although the strength of the interaction differs. So could right-handed neutrinos be detected this way?
- T-A 5y ago> What mechanism forces you to add right-handed partners? A Dirac mass term (the kind used for all other Standard Model fermions) involves both left-handed and right-handed particles: https://en.wikipedia.org/wiki/Sterile_neutrino#Mass https://en.wikipedia.org/wiki/Sterile_neutrino#Mass So you can't have one without both. (BTW, the linked section says "there are no Dirac mass terms in the Standard Model's Lagrangian", but it should really say that the Dirac mass terms in the Standard Model Lagrangian arise as a consequence of the Higgs mechanism.) > Wikipedia also says I can't find that quote, but I guess it's about experimentally observed particles. It would not apply to a right-handed neutrino: https://en.wikipedia.org/wiki/Sterile_neutrino#Properties https://en.wikipedia.org/wiki/Sterile_neutrino#Properties
- not2b 5y agoI've seen another argument, but I lack the competence to assess its validity: if a (left-handed) neutrino has mass, it moves at less than the speed of light, which means you can pass it and look back at it. You'd then see it as having reversed spin. But that might be based on a classical physics analogy that doesn't hold.
- SaberTail 5y agoThat's correct. The more jargon-y way to say it would be that for massive particles, helicity isn't Lorentz invariant. It's an approximation that helicity equals chirality (which is what matters for weak force interactions), but this approximation is pretty good for particles moving close to the speed of light (which neutrinos tend to do, due to their low mass)
- T-A 5y agoYou're thinking about helicity, not chirality: https://en.wikipedia.org/wiki/Neutrino#Chirality https://en.wikipedia.org/wiki/Neutrino#Chirality
- bopbeepboop 5y agoThis is wrong. We know that either: a) there’s mysterious undetectable particles, or b) that something beyond the standard model is happening, as in your second and third points. There’s a failed “prediction” that all fermions have similar mass terms, and that failure suggests either something strange (undetectable particles), something strange (fermions without consistent mass mechanisms) or something strange (novel physics). I think that qualifies as a mystery.
- Iwan-Zotow 5y ago"either something strange (undetectable particles)" There is nothing strange here. Undetectable in this context means it would be only detectable by gravitation detectors - all masses/energies have gravitation changes.
- AnIdiotOnTheNet 5y ago> This requires introducing a right-handed partner for each known (left-handed) neutrino, which some people don't like because right-handed particles don't partake in weak interactions, and weak interactions are the only (known) neutrino interactions (apart from gravity), so you end up with undetectable particles. For anyone else wondering: yes, this does make said right-handed (aka sterile) neutrinos a candidate for dark matter, assuming some of them are much heavier than their left-handed counter parts to account for the 'cold' properties of dark matter that we observe.
- AnimalMuppet 5y agoIf I understand this correctly, the "right hand" means antiparticles? Do I have that right? And if so, then I have two questions. 1. Antiparticles don't participate in the weak force? So if I had antimatter, and I made a nucleus of some kind, then it couldn't beta decay? If so, does this say anything about the matter/antimatter asymmetry in the universe? 2. At various times, I have seen references to anti-neutrinos. They seemed to say that what made them "anti" was simply that the spin was in the opposite direction relative to the direction of motion compared to a "regular" neutrino. Were they wrong? And if they were right, what about the direction of spin should make them unable to participate in the weak force?
- T-A 5y ago> If I understand this correctly, the "right hand" means antiparticles? Do I have that right? No, it's about chirality: https://en.wikipedia.org/wiki/Chirality_(physics) https://en.wikipedia.org/wiki/Chirality_(physics)
- AnimalMuppet 5y agoOK, then: Why would chirality determine whether a particle can interact with the weak force? (ELI5, probably, or at least ELI20, if you can.)
- SaberTail 5y agoThis is more of an empirical thing. We observe effects such as parity violation that can be best explained by an interaction that only works on left-handed particles.
- goatlover 5y agoSo the theoretical thing would be to account for the observed effects.
- T-A 5y agoUltimately it's an experimental fact - a very surprising one when discovered: https://en.wikipedia.org/wiki/Wu_experiment https://en.wikipedia.org/wiki/Wu_experiment In theoretical terms, the equation used to describe all massive fermions known at the time (like the electron) is built out of four-component quantities called Dirac spinors: https://en.wikipedia.org/wiki/Dirac_spinor https://en.wikipedia.org/wiki/Dirac_spinor A Dirac spinor can be viewed as composed of two simpler, two-component quantities called Weyl spinors: https://en.wikipedia.org/wiki/Weyl_equation#Weyl_spinors https://en.wikipedia.org/wiki/Weyl_equation#Weyl_spinors You can try to describe a fermion using only one Weyl spinor, but then it turns out that you can't build mass terms (unless you're willing to violate special relativity). The massless equation you can write down with a Weyl spinor has two plane wave solutions with opposite helicity, left and right. A Dirac spinor combines a left-handed and a right-handed Weyl spinor; the mass term of the Dirac equation "mixes" them, in the sense that if you start out with a Dirac spinor having its left-handed component set to 0, the mass term will cause it to grow at a rate proportional to mass. If you set the mass to exactly zero, you're left with two uncoupled Weyl equations, one for the left-handed component, one for the right-handed one. Knowing this, and faced with the experimental fact that weak interactions make a distinction between left and right, you write down separate weak interaction terms for the left- and right-handed components of your Dirac spinors. Eventually it turns out that the simplest choice, having only the left-handed components participate in those interactions, is the best fit to experiment: https://en.wikipedia.org/wiki/Weak_isospin#Relation_with_chirality https://en.wikipedia.org/wiki/Weak_isospin#Relation_with_chi... ¯\_(ツ)_/¯
- 0PingWithJesus 5y agoI think you're correct to say a lot of people simplify what the problem is with neutrino mass. In principle it seems like there is no problem, you just add a mass term for the neutrino just like any other particle. Just b/c at first we didn't expect that term to be there doesn't mean it's a problem to now, or that original expectation was all that meaningful. And again, as you point out, there are a couple of potential ways to add that mass term in, either the "normal" way with a right handed neutrino, or with some fancy see-saw majorana term, or some combination thereof. The issue is though, right now the standard model is at least ambiguous in terms of the majorana mass term. If it ends up the neutrino gets its mass from only the "normal" mass term, then why doesn't it have a majorana mass term? There's no current symmetry that says there can't be a majorana mass? If the neutrino's majorana mass is zero, then you'd probably have to introduce a symmetry into the standard model that says majorana particles can't exist. But if the neutrino does end up having a non-zero majorana mass term then that means the neutrino is a majorana particle, and can undergo lepton number violating processes (e.g. neutrinoless double beta decay). Again, that's new physics. So no matter how you give the neutrino mass, you're gonna have to modify the standard model in some "significant" way to accommodate. Either by specifically saying majorana particles can't exist, or by allowing for lepton number violating processes. Now you could say, well then it might the case that majorana particles don't exist b/c that would require lepton number violating processes, so I don't need to introduce a new symmetry, I can just take advantage of one that's already lying around. That might be a valid claim to make...I'm not sure. I think the issue with that comes down to the difference between lepton number a global vs accidental symmetry in the standard model.
- unholiness 5y agoHonest question from a physics novice: Would it be wrong to say the same is roughly true of all of the first 4? I have heard it is plausible that Dark Matter is merely another particle that fits in the standard model. I've heard it plausible that Dark Energy is e.g. a WIMP or other new particle in the standard model. And, on more-matter-than-antimatter, I imagine _some_ explanations of baryon asymmetry could come from outside the standard model but others (boundary condition, mirror anti-universe) would be fully standard-model-compatible, right? That would leave only #5 as a mystery: Why is gravity as we know it in general relativity so different (weaker) than the force that a standard-model graviton would predict?
- T-A 5y ago> I have heard it is plausible that Dark Matter is merely another particle that fits in the standard model. I've heard it plausible that Dark Energy is e.g. a WIMP or other new particle in the standard model. Whoever told you that was getting dark matter and dark energy mixed up. Dark matter could be "another particle", possibly a sterile neutrino: https://en.wikipedia.org/wiki/Sterile_neutrino#Sterile_neutrinos_as_dark_matter https://en.wikipedia.org/wiki/Sterile_neutrino#Sterile_neutr... Dark energy however is definitely something else. WIMPs in particular are hypothetical dark matter particles: https://en.wikipedia.org/wiki/Weakly_interacting_massive_particles https://en.wikipedia.org/wiki/Weakly_interacting_massive_par...