I've been reading about the T2K experiment where a "neutrino beam" is being sent from the J-PARC accelertor in Takio to the Super-Kamiokande detector in Japan. So if neutrinos only interact weakly, how do they create a beam of them? At J-PARC they create them by firing high-energy protons at a graphite target. You can't focus them magnetically or electrostatically, so how do they focus them into a beam? I've searched around and there seems to be something to do with target geometry in the production, but I haven't seen anything on how this works. Anyone know or have an ideas where I might look? I can see how they could create a cone because of conservation of momentum, but a beam?