Inquiring Minds

Questions About Physics

Neutrino Spin

How does one determine the spin of neutrinos? Since they have no charge.

I assume he is thinking of doing some kind of Stern-Gerlach experiment and it is true that you cannot use this method to measure the spin of an uncharged particle (actually, technically, you need a magnetic moment which is related to the product of the spin and the charge). What you can do is collide neutrinos with particles of known spin and examine the distributions of the remnants or, even more simply, look at the decays of particles with known spin. For example, if a zero spin particle decays into a spin 1/2 particle and a neutrino, then you know that the neutrino must be spin 1/2 since there are no other combinations that conserve spin (An aside, add two spin 1/2 particles with no orbital angular momentum between them and the parent can be either spin 0 or spin 1. In this case, however, you know the parent was spin 0 so you know the neutrino spin is 1/2).

And has anyone generated from the computer, what a neutrino collison might look like?

Not only has it been done on a computer (one generally simulates such events as part of the detector design process), many, many experiments have real examples of neutrino collisions. Here at Fermilab, there are event displays of neutrino events that you can get from the web! The URL is and the experiment is E815 which looks at reactions involving muon neutrinos. I think it is not obvious what is displayed in these events (that is, they are obvious to any experimental particle physicist) but I would be happy to answer questions about them. I hope this is useful. Feel free to give this person my email address if they want clarification or more information. Take care,

Scott Menary
NuMI, WH12W, MS #220
Fermi National Accelerator Lab

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last modified 2/11/1998