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03: Bubble Chamber Detective
Physicists discovered hundreds of different ‘elementary’ particles using bubble chambers which are
large vessels filled with liquid hydrogen in a uniform magnetic field. Identical particles are injected into
the chamber where they interact with protons in the liquid hydrogen to form new particles which may
or may not decay in the image. The following rules will allow you to analyze photographs of such events:
 Charge is conserved.
- Only moving charged particles leave a trail. Neutral and stationary particles cannot be ‘seen’.
- The charged particles either have a charge of +1e or -1e.
- The charges can be determined by the directions they curve according to the right hand rule.
 Momentum is conserved.
- The magnetic field changes the direction of the momentum
- The radius of the curved trail is proportional to momentum.
 Changes in the trails are evidence for changes in particles
- Either the particle has combined with a proton to form new particles.
- Or the particle has decayed to form new particles.
mv 2
FM  qvB 
R
mv  qBR
Part 1: CERN’s Big European Bubble Chamber 1970’s
Figure 1 is a photograph showing seven kaons entering a bubble chamber from the bottom. There is a
constant magnetic field into the page. Answer each question and provide a brief justification.
1. The paths of the kaons are curving slightly to the right. What is the charge of the kaons?
a) -1
b) +1
c) 0
d) not enough information
2. At point Q there is a branching. This means the kaon has interacted to form new particles and the
trails of two of them can be seen leaving this point. What is the charge of the particle on the right?
a) -1
b) +1
c) 0
d) not enough information
3.
At point P a kaon arrived and two charged particles left. How was charge conserved? The kaon
a) decayed into two oppositely charged particles.
b) decayed into two identically charged particles.
c) interacted with a proton to produce two oppositely charged particles.
d) interacted with a proton to produce two identically charged particles.
4. It looks like momentum was not conserved at point P, but it must be. What else was produced?
a) a charged moving up and to the left.
b) a charged particle moving up and to the right.
c) a neutral particle moving up and to the left.
d) a neutral particle moving up and to the right.
5.
Two particles have appeared at point Q. What are the charges of the particles leaving this point?
a) Two positive particles
b) Two oppositely charged particles, the positive is the lower trail
c) Two negative particles
d) Two oppositely charged particles, the negative is the lower trail
6. Momentum is always conserved. What must be happening at point Q? Two particles formed from
a) a stationary charged particle.
b) a stationary neutral particle.
c) a charged particle moving up and to the left.
d) a neutral particle moving up and to the left.
7. At point T there is a big kink in the trail. This means that the incoming charged particle has
a) interacted with a neutral particle.
b) interacted with a positive particle
c) decayed into a positive and a neutral
d) decayed into a negative and a neutral
8. Two charged particles appear at point S. How do their charges and momenta compare?
9. There is a very slight kink in the track at point R. What has happened here? Describe the charges and
momenta of the particles involved.
10. At point U, the two trails overlap as one particle passes underneath the other. If you join this point
with point S where they were created, you get the direction of the original neutral particle. Trace
this back. Where did the neutral particle come from?
11. Extend the curved trails from point Q to find their intersection point. Where did the neutral particle
that decayed to form these particles come from?
Part 2: Brookhaven National Laboratory’s Bubble Chamber: 1964
Figure 2 is an historic photograph. It provided the first evidence for the omega minus particle – a
prediction of the quark model which led to the Nobel Prize in 1969 for Murray Gell-Mann. Negative
kaons enter at the bottom. The magnetic field is into the page. The essential trails have been darkened.
1. Look at point V. Identify the charges and momenta of the particles interacting at this point.
2. Look at point W. Identify the charges and momenta of the particles interacting there.
3. At point Y there is a pair of oppositely charged particles created from a neutral particle. The curves
have been extended. Where did this particle come from?
4. At points X and Z, pairs of oppositely charged particles have been produced from neutral particles.
Estimate the origin of these particles by extending the bisector of the ‘vee’. These lines should
intersect at one point with the line you drew for question 3. What happened at this point?.
5. Reconstruct the interactions by labeling the visible and invisible particle trails on the photograph.
Begin at point V with a kaon (K-) colliding with a proton (p) to produce the omega minus (-), a
neutral kaon (K0) and a positive kaon (K+). The omega decays at point W into a neutral xion (0) and
a negative pion (-). The xion then decays into a neutral lambda (0) and a neutral pion (0). The
neutral pion decayed almost immediately into two photons () which decay at points X and V into
electron-positron (e-, e+) pairs. The lamda decays at point Y into a proton and a negative pion.
6. Nicholas Samios was the researcher who did the original analysis of this photograph. It was 3:00 in
the morning and he wanted to phone his boss with the news, but his analysis was based on four
invisible trails. What would you do? How would you explain what you’d found over the phone?
U
T
S
R
Q
P
Figure 1: 4.2 GeV Kaons from CERN. The magnetic field is into the page. The trails
of seven kaons can be seen entering the photograph from the bottom.
Z
Y
X
W
V
Figure 1: This is one of the most famous bubble chamber pictures. It is from Brookhaven
National Laboratory and it provided the first evidence for the omega minus particle that
was predicted by the quark model. Eleven kaons enter at the bottom and the magnetic
field is into the page.