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Neutrinos are elementary particles, to our current knowledge they do have rest mass, but they are the lightest particles (with rest mass).

What we observe in neutrino experiments?

I do understand that they are EM neutral theoretically, but experimentally it is not obvious.

Neutrinos interact with oridnary matter weakly, so just detecting them is hard.

The mass of the neutrino is much smaller than that of the other known elementary particles.[1] The weak force has a very short range, the gravitational interaction is extremely weak, and neutrinos, as leptons, do not participate in the strong interaction. Thus, neutrinos typically pass through normal matter unimpeded and undetected.[2][3]

https://en.wikipedia.org/wiki/Neutrino

Now since they interact with ordinary matter weakly, and they travel as close as you can get to lightspeed, the only way to tell if they are EM neutral is if we tried to deflect them in a EM field, and they flew straight. But since even detecting them is hard, how can we tell whether they interact with an EM field, and get deflected or not?

Neutrinos can interact via the neutral current (involving the exchange of a Z boson) or charged current (involving the exchange of a W boson) weak interactions.

https://icecube.wisc.edu/outreach/neutrinos

The reason I am asking is because if the neutrino has very little rest mass, and flies near light speed, and only has a fraction of the elementary charge (like 10^-10e), that it weakly interacts with the EM field too, it could be hard to detect its charge (EM interaction) at all.

Question:

  1. How can we experimentally tell whether the neutrino is EM neutral?
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    $\begingroup$ “do understand that they are EM neutral theoretically, but experimentally it is not obvious.” How do you figure that? That they are uncharged is one of the experimentally obvious things. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Dec 1, 2019 at 23:12
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    $\begingroup$ So you are now changing the question to basically ask whether charge is quantised and conserved? $\endgroup$
    – ProfRob
    Commented Dec 1, 2019 at 23:17
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    $\begingroup$ That wouldn’t change the nature of the interactions, just the rate. They still show up the same way in detectors. And we have plenty of large, low-background experiments where they could be seen. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Dec 1, 2019 at 23:26
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    $\begingroup$ “Though quarks are confined, so we cannot tell experimentally their charge.” Simply nonsense. This measurement was first done decades ago. It’s not an easy measurement to describe at the pop-sci level, but the results are solid science. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Dec 1, 2019 at 23:29
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    $\begingroup$ "is there any document": yes, you've already found the PDG's annotated bibliography. Now read the papers it links to. $\endgroup$
    – rob ♦
    Commented Dec 1, 2019 at 23:32

3 Answers 3

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If neutrinos were charged, you could stop them with a thin sheet of aluminium or a few metres of air, like beta electrons with similar energies.

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  • $\begingroup$ So basically what you are saying is that in this case the only difference between a beta electron and a neutrino with same energy, is the EM charge? Otherwise they are identical? The reason I am asking is because it is said that the neutrinos interact with ordinary matter weakly, and you say that this is just because they only interact through the weak interaction? It has nothing to do with their low rest mass and almost speed of light speed? $\endgroup$
    – Árpád Szendrei
    Commented Dec 1, 2019 at 23:02
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    $\begingroup$ @ArpadSzendrei how do you propose to make them charged, but avoid them interacting like charged particles? Beta electrons have low mass and move relativistically. $\endgroup$
    – ProfRob
    Commented Dec 1, 2019 at 23:04
  • $\begingroup$ They could interact so weakly because of their low rest mass and near speed of light speed, that the EM field could not deflect them (nor could they be absorbed)? An electron, though low energy, has a bigger rest mass, and needs to have low speeds (low kinetic energy), and it has the elementary charge. It cannot fly through an aluminum sheet correct. But if the neutrino has low rest mass, flies near light speed, and has only fraction of the elementary charge then it could pass through, without interaction. $\endgroup$
    – Árpád Szendrei
    Commented Dec 1, 2019 at 23:07
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    $\begingroup$ I can’t spell his name with all of the accents. $\endgroup$
    – Dale
    Commented Dec 1, 2019 at 23:58
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    $\begingroup$ @Dale and in my comment above, you'll see it works just fine without accents. $\endgroup$
    – ProfRob
    Commented Dec 2, 2019 at 6:55
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If neutrinos were charged, then beta decays (the problem whose solution required the proposal of the neutrino) wouldn't conserve electric charge.

If neutrinos were charged, they would emit Cherenkov radiation in matter. They don't. (But particles which are scattered by neutrinos do. The IceCube detector in Antarctica detects neutrinos by looking at Cherenkov radiation from their charged scattering products.)

Charged neutrinos would create a trail of electron-ion pairs in matter, by interacting with the strong electric fields inside of atoms. But they don't. A useful search term is "minimum-ionizing particle."

For most neutral particles, the Particle Data Group summary includes an upper limit on the magnitude of the charge, and an annotated bibliography for that limit. You should read the one for neutrinos.

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  • $\begingroup$ All correct. What I do not get is, what if the neutrino is so low mass, and flies near speed of light, and only has a fraction of the elementary charge, that it weakly interacts with EM fields too. Is that a possibility at all for a neutrino to have for example only 10^-10 the elementary charge? $\endgroup$
    – Árpád Szendrei
    Commented Dec 1, 2019 at 23:10
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    $\begingroup$ To learn if it's possible for a neutrino to have charge $10^{-10}e$, follow my instructions in the final paragraph. $\endgroup$
    – rob ♦
    Commented Dec 1, 2019 at 23:12
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    $\begingroup$ If you want to know what would happen if neutrinos had a very small charge, you should read the papers that set those limits to see what effects they were looking for. $\endgroup$
    – rob ♦
    Commented Dec 1, 2019 at 23:23
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    $\begingroup$ Excellent answer. I appreciate the reference. I am sure that people in the field know it already, but I didn’t. Thanks! $\endgroup$
    – Dale
    Commented Dec 1, 2019 at 23:57
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    $\begingroup$ @ÁrpádSzendrei Note that it doesn't say between $10^{-4}$ and $10^{-15}$. Different experiments set different limits. The best one says the charge must be less than $10^{-15}$. $\endgroup$
    – Chris ♦
    Commented Dec 5, 2019 at 0:41
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[...] the only way to tell if they are EM neutral is if we tried to deflect them in a EM field, and they flew straight. But since even detecting them is hard, how can we tell whether they interact with an EM field, and get deflected or not?

Our detectors detect electromagnetic interactions directly. You wouldn’t try to detect the neutrino twice; you’d just detect the actual interaction. Reducing the charge doesn't affect the nature of these interactions, just the rate, so they continue to show up in standard detectors.

And there is, of course, a limit on the exclusion provided by null experimental measures. That fact is—in and of itself—vastly uninteresting. It only becomes interesting if you have a reason to wonder about the inaccessible range. Random spitballing doesn't generally count as "a reason".1

Now, I have no idea what the currently available lower limits are, but the first scheme that comes to my mind for setting one is examining the up-down asymmetry of elastic-scattering rates in a large, low-background, imaging detector such as Super-K. In that measurement you are simply using the body of the Earth as an absorber and looking for rate reductions.2 By setting a sufficient energy threshold for the measurement you can remove flavor considerations and thereby reduce your dependence on calculation of the matter effect. You do, however, have to disentangle the effects of weak interactions other than forward scattering and the theoretical uncertainty on that background probably sets the limits for the measurement.

1 By which I mean you won't get funding on that basis.

2 In an earlier post I made an estimate of how much absorbtion to expect from Venus.

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