> Based on what we've observed, and our current standard model of particle physics, only one type of neutrino can be massless.
Is the standard model complete to the point where we can predict how many types of neutrinos exist and what their properties should be?
I always thought that the standard model as a set of equations (a model) that fits observed data, without venturing far into “why this model is the governing principle for our universe”. That is, it is not able to explain things like “why an electron comes with two heavier varieties”.
Are neutrinos somehow different in a way that we can understand them to the point where we know things like “only one type of neutrino can be massless”?
We can measure the mass differences between the neutrinos pretty well through these oscillation experiments, but this also doesn’t tell us which the mass hierarchy. It could be bottom up or the other way round.
In principle, one neutrino could be massless and the mass differences we’ve measured so far would still be correct.
Aside from this, pretty much anything is on the table. Neutrinos being their own anti particles? Maybe. Fourth generation of neutrinos? Could be.
We observe neutrino oscillations through a variety of channels. We first observed fewer (electron) neutrinos from the sun than expected, suggesting they were oscillating to other flavors. And this has been further observed in neutrinos produced in the atmosphere by cosmic rays, neutrinos produced by decays of particles in beams, and neutrinos from nuclear reactors.
The best explanation, and the one that fits the standard model, is that the pure "flavor" (electron, mu, tau) neutrino states are mixtures of pure "mass" states. And from those different channels, which look at different energies and flavors of neutrinos, we can work out what those mixtures are.
When you go through all the math, it turns out the oscillations depend on the differences of the squares of the masses of the pure mass states. And we observe oscillations that tell us that two of these differences are nonzero. That is, if there are mass states 1, 2, and 3, then we know that (mass 1)^2 - (mass 2)^2 is nonzero, and (mass 3)^2 - (mass 2)^2 is also nonzero. So this implies that at least two of them must have nonzero masses.
To explain that I need to explain symmetry breaking.
Consider a pencil. The equations for a balanced pencil are completely symmetric with the point of symmetry being balanced on its tip. However an actual pencil is never to be found balanced on its tip it is always lying on a side. Therefore a perfectly symmetric theory may describe a real world situation which is not in the least symmetric.
The Standard Model is exactly such a theory. It is in principle completely symmetric, and there is no particular reason in it why an electron would weigh less than a proton rather than more. However it posits a number of free parameters, whose actual values are set by fields, which are therefore fixed throughout the observable universe. Each field is carried by a particle. And we've found all of those particles for the Standard Model, thereby confirming the existence of the fields. And our measurements of the properties of those particles reveals the values of the free parameters, and our theory gets better.
How many parameters? As https://en.wikipedia.org/wiki/Mathematical_formulation_of_th... says, the traditional Standard Model has 19 parameters which we've measured. Thus while the theory itself says nothing about why the electron would have 2 heavier varieties, it does say that there should be 3 varieties on that particle, and the measured parameter values say what the masses are very precisely.
Proposals to include the complexities of the neutrino add another 7 parameters which we've so far not managed to measure. But we're working on it.
they would stuck in that type then right? which is not what we see in mid - long baseline experiments, so I think all three types are massive?
As an analogy, you could think of a photon with circular polarization traveling through a vacuum. If you measured its linear polarization you would sometimes find that it is up-down and other times left-right. The photon is massless but is still able to oscillate between these two linear polarizations because it is propagating with a mixture of these two polarizations.