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Name: Joe
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In the answers in your archives it is stated that neutrinos do not have magnetic moments. Yet the information I find on-line says that because they have mass they also have a magnetic moment, like the neutron. I know new discoveries are made all the time, so which is correct? I am now thoroughly confused.

Hello Joe,

Neutrinos are a wonderful subject within particle physics because we are actively learning more about their properties, both experimentally and theoretically. Our understanding of their properties has changed as better observations are made and better explanations/descriptions offered. Neutrino mass is a great example of this. In fact, to my outsider's knowledge of particle physics it seems like neutrinos have probably be the area of most progress within particle physics in the past couple of decades. (It is worth saying that I study condensed matter physics myself and thus should not be treated as an expert on particles which are far from my area of expertise!)

The long standing thought on the matter was that neutrinos have no observable mass and could be treated as massless. This was correct within the data available for many decades. Attempts to both directly and indirectly observe or measure the mass of a neutrino all came back with "no measurable mass." This did not mean zero mass, but just that it seemed to always be zero to within our ability to detect. However, recently (1999 I think) neutrinos have been observed to change their type (or flavor as it is commonly called) with time. I do not want to go into the details of this, but in order for this to occur, there must be a difference between the mass of the different neutrino flavors. A difference in mass requires that the masses are not all 0. It is furthermore quite expected that if some of the flavors have mass, then they all must. It makes little sense to have some with mass and others without (though perhaps some of the people that actually study neutrinos could disagree with that statement and point out why it would in fact make sense).

Anyhow, we are able to observe the neutrinos changing flavor and this allows us to measure a mass difference between the neutrinos. If you then follow that conclusion that neutrinos must all have mass, then it is possible for them to have a magnetic moment! I believe the magnetic moment would be linearly proportional to mass of the particle. The estimated value of this magnetic moment is very small and below our ability to currently detect. However, perhaps at some point in the future scientists will figure out a means to test it. The history of our understanding of neutrino properties offers a great deal of insight into how science works and how ideas evolve over time. In the strict sense that people used to say "neutrinos are massless" were wrong. However the longer version of saying "we think they are massless, cannot measure any mass, and for all our relevant calculations and data they appear massless, therefore we will treat them as massless," was entirely correct. It is only with more modern experiments that we have been able to begin to unravel this mystery. The initial postulate for the existence of neutrinos was another great example of science, but that is taking us too far away from your question.

So 15 years ago the short answer was, "Neutrinos have no mass and no magnetic moment, at least within our ability to measure." Now we are able to observe that neutrinos must have some mass, albeit we have not measured it directly, only the difference in the masses and that the mass of the lightest neutrino is in fact quite small. Since they have mass, then they should have magnetic moments too! That answer may continue to evolve (I hope so!) as we learn more.


Michael S. Pierce

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