Download Part 1: CERN`s Big European Bubble Chamber 1970`s

Bubble Chamber Detective:
Note to teachers: This activity is intended to be used with students who have already studied momentum,
circular motion and the force of a uniform magnetic field on a charged particle. This activity can be used to
review these ideas and show an application of them in modern physics research. The students do not need to
know the details of a bubble chamber or what a kaon or a quark is. The first part has students analysing a
photograph using a series of multiple-choice concept questions. Students should be given an opportunity to
compare their answers and discuss their reasoning in small groups. This first part should take almost an hour.
The second part is a similar exercise but without the assistance of multiple-choice answers. This could be
assigned to the students as homework and should take half an hour.
Part 1: CERN’s Big European Bubble Chamber 1970’s
Answer each question and write a brief justification.
1) a) -1: The kaons appear to be travelling straight, which suggests a neutral particle. However, a neutral
particle would not leave a trail. If you look carefully you can see that the kaon trails curve very slightly to the
right. This is easiest to see if you use a straight edge or look at the page tilted away from you. Using the right
hand (or left hand) rule indicates that they are negative.
2) c) +1: The particle curves in a direction opposite that of the kaons and therefore must be positive. However,
this seems to be violating the conservation of charge. This is clarified in the next question.
3) c) The kaon has interacted with a proton: A positive was produced and all the particles have just one charge,
so there must have been a positive particle before the interaction. This was one of the protons in the liquid
hydrogen. It was invisible because it wasn’t moving. The kaon has a negative charge so the particle produced
on the left must be negative. If you look closely, you can see that it does curve slightly to the right.
4) c) a neutral particle moving up and to the left. Conservation of momentum requires that there be a particle
moving up to the left. We can’t see it so it must be neutral.
5) d) Two oppositely charged particles, the negative is the lower trail: They curve in opposite directions, so
they must be oppositely charged. The bottom one curves like the kaon, so it must be negative.
6) d) Two particles appeared from a neutral particle moving up and to the left. The two particles have
opposite charges so they had to come from a neutral particle to conserve charge. The neutral particle had to be
moving or else the charged particles would be moving directly opposite each other to conserve momentum.
7) d) decayed into a negative and a neutral: It can’t have interacted with a neutral particle because the liquid
hydrogen only provides electrons and protons as targets – not a). (Normal water has a tiny amount deuterium
which could provide neutrons, but it has been removed from this water. There is also the negligible possibility
of an interaction with a neutral particle created in an interaction elsewhere.) It can’t have interacted with a
charged particle or else both of the two daughter particles would be charged and leave visible trails - not b or
c).
8) The one on the left is negative and the other is positive. The one on the left curves more so it has less
momentum.
9) The analysis of this decay is similar to what happened at point X. A negative particle has decayed into a
negative and a neutral particle. It is harder to see because the negative particle in this case has more
momentum and curves very little.
10) If you extend the line, you will see that the neutral particle that decayed came from point X, a point where
we know a neutral particle was formed.
11) It comes from point W. Questions 10 and 11 are very important to show students how particle physicists are
able to infer the existence of invisible particles through conservation laws. This same sort of analysis led Pauli
to propose the existence of the neutrino in 1930 to explain beta decay.
The next exercise gives students further practice and less guidance. It uses the photograph that first confirmed
the existence of the omega minus particle predicted by the quark model and led to a Nobel Prize in 1969 for
Murray Gell-Mann.
Part 2: Brookhaven National Laboratory’s Bubble Chamber: 1964
1) There is a charged particle moving in almost the same direction as the original kaon. It curves to the left, so
it must be positive. This positive charge indicates that the kaon interacted with one of the protons of liquid
hydrogen. Another charged particle branches to the right. It must be negative to conserve charge from the
kaon. There also must a third neutral particle somewhat to the left to conserve momentum.
2) There is a negative particle going to the right. It must be negative to conserve charge and because it curves
clockwise. There must also be a neutral particle to conserve the original momentum. Its momentum will have
a significant component up the page to conserve the original momentum.
3) When you join the intersection points and extend the line, you find that the neutral particle did not come
from the kink at point W as you would expect. There is not enough evidence to determine where along this
line the particle was produced.
4) It doesn’t look likely. If we assume that the neutral particle from W decayed to produce just a positive and a
negative particle – and no extra neutral particle – then the momentum of the product particles doesn’t
match, especially for point X. However, if there also were neutral particles produced it would be possible to
conserve momentum.
5) The intersection point will vary using this technique, but is should be up and to the right of point W.
6) Students do not need to know the properties of these particles. They also show up in the Particle Zoo activity.
7) This question requires that the students clearly explain the steps of the analysis using the four neutral trails.
A complete analysis also requires a knowledge of properties of the particles and how they can decay,
something which the students can’t be expected to know. Information about this is available on CERN’s
website. This question also lets the students reflect on science as a human endeavor with bosses, gambles
and payoffs.
For more information and bubble chamber images go to
http://teachers.web.cern.ch/teachers/archiv/HST2005/bubble_chambers/BCwebsite/index.htm