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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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The basis of all neutrino beams is a less exotic (protons most of the time) beam smashing some mundane target and making scads of assorted particles---many of them charged. Those charged particles are focused (and possible subjected to a second filtering for energy by using collimators and more magnets, though this step is not being done at CERN), then they are allow to fly for a while during which time many of them decay, and the decay products include neutrinos which are well collimated in the beam direction by the Lorentz focussing. The un-decayed particles are stopped with a thick pile of something that the neutrinos go right through.

The particles that are most interesting for this purpose are those that decay only by weak processes. Both because it takes more time to focus them that strong-decays allow, and because weak interactions are necessary to make neutrinos.

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).

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When you say that the kinematics select for nu_mu, do you mean that it selects nu_mu over /bar{nu_mu}, or that both are selected over non-neutrinos? –  Colin K Sep 23 '11 at 18:51
    
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
    
Ah...after a little poking around on the web I am reminded. You stop the beam before many of the muons decay in flight, and their at-rest decay contributes little to the beam. –  dmckee Sep 23 '11 at 19:12
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