Teacher Notes: decay Bubble Chamber picture

The beam is made of 10 GeV/c particles blasting through the CERN 2m Hydrogen Bubble Chamber. The beam tracks are the almost straight, parallel lines across the picture.

First you want to find out about the beam and the magnetic field. The bubble chamber shows tracks only for charged particles, and the magnetic field makes the charged particles curve in opposite directions for opposite charges. Try printing the picture and then hold it so that you are looking along the tracks at a very flat angle. The beam tracks will be the most obvious tracks. You may see that they curve slightly.

Look for the electrons. Pay attention to the spirals that look as if they are starting on the tracks of the beam particles. These electrons are being knocked out of hydrogen atoms, and they move forward in the same direction as the beam. You see one large spiral to the left and several small ones coming off the beam. So, the beam in this picture is coming in from the left, and the magnetic field makes negative charges curl in a counter-clockwise direction.

What is the interaction? This is a bit tricky, because from this camera angle the center of the picture is complicated. Sort out the background tracks you can ignore. (If you had the two other camera angles to compare, it would be easier.) The two slightly curving tracks and the straight track between them that cut across the entire field are not part of the interaction we want to follow. They come from an interaction upstream and simply happen to be in the right place to confuse you.

The interaction you are interested in starts with the short curved track in the center that decays into three tracks. This is a , but not a beam particle. How do you know it isnt formed by a beam interaction at the point where it appears? You can see that the beam continues straight on and out the far side of the picture.

What produced this ? One possibility is that a beam particle hit the wall, forming a neutral . The doesnt leave any track, but it is still moving at near light speed. Eventually it interacts with one of the bubble chamber protons to produce a new. But now it is a comparatively low momentum particle, and it curves in the magnetic field. Thetrack ends by disintegrating, or decaying, into three tracks.

64% of the time, disintegrates directly into; 21% of the time into and. Only 6% of the disintegrations produce three particles:

Amazing! You now have more particles than you started with. More importantly, this isnt just a question of one big lump of matter shattering into several smaller ones, like smashing s big rock into pebbles. If you write down the quarks for the and the three pions, you find that you started with only two quarks, and you end up with six:

Have you violated any conservation laws? What about Conservation of Mass? Conservation of Energy? Check out the masses of your particles:

493.67 MeV 3(139.57) MeV + extra energy

Aha! The easily has enough energy in its rest mass to make three pions.

The pions curve in the magnetic field. You can tell from the curvature whether each one has a positive or a negative charge. Has charge been conserved?

The and convert intoand . These travel only a very short distance in the chamber. Why such a short track? The difference in mass, , is small, which leaves only a small kinetic energy, 34 MeV. A heavy particle with low energy can have only a very low velocity and low momentum, and it will not travel far in the bubble chamber. (For a full development of the energy balance, see: Energy and Momentum Conservation in Pion Decay)

The two then decay into positrons, . How do you know that the positrons have positive charge? The disappears without producing anything. What happened? It probably hit the glass window in the bubble chamber.

Have the particles violated any obvious laws? Is momentum conserved? Try drawing tangents to the initial direction of the pions. You dont have measurements for this picture, but the direction of the sum of the momenta for the three pions agrees with the direction of the momentum of the .

Look at one of the decays. How peculiar. Suddenly the muon takes off at a sharp angle to the track of the pion. This is a clue. When you see a particle moving out from the site of an interaction or a decay, carrying some momentum in one direction, it means that something else, another particle, carried momentum in the opposite direction. Here it must be a neutral particle (you already have accounted for the charges), and neutral particles, remember, leave no tracks in the bubble chamber. In this case neutrinos are the invisible particles taking the opposite momentum:

and

Then, when the muons decay to electrons and positrons, you see only the charged electron or positron, but not the neutrino or antineutrino:

and

With their near zero mass, those and have high velocity, and because they will not likely to be interacting, they are moving out into the galaxy. The and lose their energy by radiation and spiral to a stop in the bubble chamber,

.