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I learned from Wikipedia that neutrinos "are not affected by the electromagnetic forces". How was this identified experimentally?

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Experimentally it has been found that neutrinos travel in a straight line through matter, unless they interact directly with nuclei on the way into a spate of particles. Their path is not affected by the magnetic field of a bubble chamber for example. (The OPERA neutrinos travel kilometers in a straight line unaffected by the magnetic field of the earth).

They leave no ionization trace in matter, as the other particles in the bubble chamber event.

Thus they are not charged.

They can interact electromagnetically with other charged particles through higher order Feynman diagrams, but as this would require a first weak vertex, the probability of interaction electromagnetically is very low and can be ignored to all practical purposes.

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Thank you very much. But correct me if I'm wrong: the bubble chamber can only detect electrically charged particles. I'm testing the conjecture that neutrinos bear magnetic charge instead of electric charge. Are there any other experimental evidences other than bubble chamber experiments? – Murod Abdukhakimov Mar 24 '12 at 8:53
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all neutrino experiments. bubble chamber are the ones that are visual. There is no magnetic charge, i.e. they are not monopoles. If they were they would also ionize like the electric charge. It is one of the reasons we know there are no magnetic monopoles in the particle interactions. It would be an ionizing track with a funny behavior to the magnetic field (attracted up or down with respect to the plane of B, rather than a circular path). – anna v Mar 24 '12 at 9:32
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Neutrinos carry no electric charge and hence can't interact by electromagnetism.

I don't know if anyone has attempted to measure the charge on a neutrino, but you can tell immediately that they carry at most a very very small charge since they interact so weakly with matter. If they carried a significant charge they would be strongly scattered by matter just like all the other charged particles.

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I'm sure they carry no electric charge. But I guess it is not the same as "not affected by the electromagnetic forces". – Murod Abdukhakimov Mar 23 '12 at 18:30
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No electric charge means they don't feel the EM force, just like no colour charge means a particle doesn't feel the strong force. – John Rennie Mar 23 '12 at 18:37
@Murod You may be thinking of composite particles like the neutron which are net neutral and are still effected by electromagnetic forces. However, the neutrino is believed to be fundamental, in which case no charge does mean no electromagnetic interaction. – dmckee♦ Mar 23 '12 at 19:03
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They can interact with electromagnetism through higher order diagrams, after a first weak vertex: a neutrino_e in a virtual loop of e-W+, which closes again into a neutrino_e . e- and W+ within the loop can interact with other charged particles to higher order by a photon exchange. In general charge is quantized, down to 1/3 and 2/3 for quarks, but still quantized. The neutrino has charge 0. – anna v Mar 23 '12 at 20:02
@anna v: Just to add for people who don't know this, an interaction like that would involve several factors of the weak coupling constant and would be a very small interaction compared to something like an electron scattering off of a $\vec E$ field. – Jerry Schirmer Mar 23 '12 at 20:49
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