# How are neutrino beams emitted at CERN?

As far I know they come from accelerator collisions, but I have read confusing things like magnetically focused. How could neutrinos be guided magnetically if they aren't affected by the electromagnetic field?

I would like to have a better idea of how neutrinos are emitted.

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So mostly we have $$k^- \to \mu^- + \bar{\nu}_\mu$$ $$\pi^- \to \mu^- + \bar{\nu}_\mu$$ and several other channels (or the charge conjugates, of course (or not and because the horn selects for one sign)); end-state muons subsequently decaying as $$\mu^- \to e^- + \nu_\mu + \bar{\nu}_e ,$$ but we arrange the decay beam line so that few of them do this before reaching the beam stop (which means that few of the products end up in the final beam as decays from rest are isotropic).
Well, all the non-neutrinos are taken out when they get to the 700 km of rock part...but it is a little of both. Consider the hadron decay in the COM---the muon gets a small velocity and the antineutrino a big---then boost back to the lab frame and the muons continue almost undisturbed while the antineutrinos have a bigger envelope. What I don't recall right off is how the $\nu_\mu$ is preferred in the muon decay (but the above argument does not apply so much because the electron is also light relative a muon). –  dmckee Sep 23 '11 at 19:09