In this picture, apart from the thick cosmic ray coming in from the top (How do we know it is coming in? It will be discussed later.), the `motion' is roughly from the bottom to the top. What we are looking at is a sideshow - a small electromagnetic shower - on a picture taken during a high energy neutrino experiment in the Fermilab 15-foot (diameter) bubble chamber. The bubble chamber liquid is neon-hydrogen mixture.
The bubble chamber is inside a huge superconducting magnet, and the momentum of the tracks is proportional to the curvature.
(The discussion below is inconsistent in that it is partly aimed at PPEP physicists and partly at our intended audience down the line!)
You can tell that the magnetic field is up out of the paper by noticing two things:
(1) on the right-hand side of the picture, half-way up, a track starts off (at a point we'll call C) moving to the right and then curls to the left. This is an electron that has been knocked out of an atom by a photon travelling from the left (can you see where this photon comes from; we will discuss it later?).
Even on this scanned image there are two more such lone tracks, and they too curve to the left. (Compton Effect)
(2) Near the bottom left corner of the picture there is another electron track which curls to the left; notice that this begins on another track which is comparatively straight (higher momentum). The curly track is an electron that has been given a `snick' by the high momentum tack; such electrons are known as `delta rays'.
There are other delta rays in the picture - can you find them? They all curve to the left.
The clearest is on the cosmic ray - and is the signature that tells us its direction of motion.
There are several examples in the photograph of two tracks - one positive and one negative - emerging from a point parallel to each other (with zero opening angle!), before curving (like the electrons we've been describing) in opposite directions.
Qu: What can we infer from the fact that the opening angle is zero?
Ans: That the rest mass of the photon is zero.
What we are seeing is a high energy gamma ray materialising (in the presence of a neon nucleus; for more details see Phys. Educ. 27 (1992)) into an electron and an `anti-electron' or positron. - matter and anti-matter! We shall refer to this as an `e+e- pair' from now on.
Consider the e+e- pair on the left side of the picture (2/3 the way up the picture). Looking backwards along the original line of flight of the e+ and e- one sees that this line is tangential to a curling electron; this tells us that the photon from which the e+e- pair materialised was `radiated' from the accelerating electron. (Bremsstrahlung.)
If you now look at the e+ partner of the radiating e- we have just discussed, you will see that it also curls up - as a result of emitting a photon. It is not difficult to imagine that this photon, emitted tangentially (like mud off a wheel!) caused the Compton electron that we discussed earlier, beginning at the point C. (To convince oneself, it is necessary to look at another view of the same interaction, and check that the Compton electron is in the same place relative to the spiralling e+.)
Can you find evidence for another bremsstrahlung photon?
GTJ; 12 March 1998