Re: How is a neutrino beam generated in a particle accelerator ?

Hello James,

I'll answer your question first with a diagram. Then I'll talk a bit about what is going on. Here's a typical muon neutrino beam:

high-    hits       pions    eventually, the pions decay:
energy   graphite   come     pi --> muon + neutrino
proton   target     out                    
beam       ||| . "                        
           ||| . :              \|/         |=======|
---->------>bang!--->--------> >poof< ----->|=======|---------> except the
           ||| * .              /|\         |=======|           neutrinos,
           ||| ...                         the beam passes      which
           |||                             through hundreds     make it
	                                   of feet of iron,     through. 
                                           lead, dirt, which    
                                           everything crashes
                                           into and stops ...

That's how you get a neutrino beam! There are a few points of particle physics to point out.

• 1) High energy proton beams are pretty standard, that's what we know how to accelerate . One neutrino experiment I know of, NOMAD, used 450 GeV protons ... you can read about proton accelerators on the Web.
• 2) Now, 450 GeV protons hitting a target (NOMAD used beryllium, TOSCA will use graphite, but anything would work.) make a huge spray of garbage. Pions, gamma rays, antiprotons, etc. However, it's all generally moving forward in a beamlike manner (the total forward momentum of each pile-of-collision-debris is 450 GeV, by conservation of momentum) ... and, right after the target, you can use magnets to steer and focus this beam. If you want, you can keep just the (say) 100 GeV positive pions in the beam, so you can get a pure, "monoenergetic" pion beam out of the potpourri. Or you can use the whole beam. Notice that later on, you can't steer neutrinos with magnets, so you'd better decide right now what direction you want the beam to go!
• 3) Now, positive pions can decay in a large number of ways, but 99.9877 percent of the time it's going to be to a muon and a muon neutrino. The presence of the neutrino is an instance of something called "lepton family number conservation". Typical lifetime is 26 nanoseconds! So you want to give them several tens of meters to cruise while they're decaying. CHORUS gives them 300 meters, since there are longer-lived particles in their mix as well (kaons).
• 4) And after the pions decay, you've got a beam of the desired neutrinos ... but also lots of high-energy muons, which would make your ultra-sensitive neutrino detectors light up like the fourth of July. So you want to stop them; the only way to stop them is by putting stuff in the way. The neutrinos can pass through this stuff, while the muons will interact, slow down, and eventually stop and decay. Some experiments run the beam through the side of a mountain, through a big dirt pile ... one famous experiment at Brookhaven chopped up a decommisioned Navy battleship, and built a 14-meter thick wall of solid steel! And some experiments, looking for neutrino behavior over very large distances, shoot their beam through some part of the planet ... say, start a beam in Illinois and detect it in Minnesota, or Geneva to Trieste, or some such.

Oh, and techniques are fairly similar if you want electron-neutrino or tau-neutrino beams ... you'd want something other than the pion decay to produce them, but that's easy enough.

A good article on neutrino experiments: here Some beam-experiment names and locations: BooNE and DONUT at Fermilab, NOMAD and CHORUS at CERN in Europe, K2K at KEK in Japan, all of which have Web pages. The Particle Data Group can tell you more about neutrinos, too.

Good question ... the original answer, after all, was worth a Nobel prize.

-Ben Monreal