Day 8.8
Bubble Chamber Detective:
13.6
In the early 60’s physicists detected hundreds of different ‘elementary’ particles formed by high energy
collisions in particle accelerators using bubble chambers. These are large vessels of liquid hydrogen in a
uniform magnetic field. Charged particles passing through the hydrogen leave a visible trail of bubbles.
The following rules will allow you to analyze photographs of such events.
1) 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.
2) Momentum is conserved.
 The momentum causes the particles to move in circular paths governed by F = qvB = m v2/r.
 A tight curve indicates a particle with very little momentum. The momentum is directly
proportional to the radius of curvature. (mv = qBr = p, the charge and field are constant.)
 All of the charged particles are accelerating and therefore radiating energy. However, this is
only noticeable for electrons and positrons whose masses are so small. Their paths spiral in.
3) A change in the path means there has been an interaction. Particles can
 collide elastically with the protons or electrons of the hydrogen (not very interesting)
 collide inelastically with a proton to form new particles. Mass is not conserved, E = mc2.
 decay to form new daughter particles
Part 1: CERN’s Big European Bubble Chamber 1970’s
1) This photograph shows a beam of 7 kaons entering a bubble chamber from the bottom of the page. The
kaons are curving slightly to the right. The magnetic field is into the page. What is the charge of the kaons?
a) -1
b) 0
c) +1
d) not enough information
The paths curve very slightly to the right. The field is into the page so they must be negatively charged.
2) One of the kaons has interacted at point 1 where there is a branching. What is the charge of the particle
that leaves this point and moves diagonally up and to the right?
a) -1
b) -2
c) +1
d) +2
It curves left, so it must be positive.
3) Is momentum conserved at 1?
a) No, there has been a gain in momentum to the right.
b) Yes, a negative particle has been produced that moves up and to the left.
c) Yes, a positive particle has been produced that moves up and to the left.
d) Yes, a neutral particle has been produced that moves up and to the left.
Momentum is always conserved. There must be another moving particle. We can’t see it, so it is neutral.
4) How is charge conserved at 1? The kaon
a) produced two charged particles, with charges of -2 and +1.
b) has interacted with a proton
c) has split its charge between two charged particles.
d) has interacted with an electron
According to our rules, there are no +2 charges. The kaon cannot split each charge because there are no
fractional charges. We have a positive charge coming out of the interaction so there had to be one at the start.
The kaon interacted with one of the protons of the liquid hydrogen. It was at rest and so there was no trail.
The proton and the kaon produced two new particles – one is negative and one is positive.
5) A branching has appeared at point 2. 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
They curve in opposite directions, so they must have opposite charges. The negative is on the bottom.
6) How is momentum conserved at 2? Two particles
a) with equal and opposite momentum appeared from nothing.
b) appeared from nothing.
c) with equal and opposite momentum appeared from a neutral particle moving up and to the left.
d) appeared from a neutral particle moving up and to the left
The particles can’t come from nothing. The net momentum is up and to the left, so there must have been a
particle with momentum up and to the left that is invisible – therefore neutral. The two particles have similar
momentum but not equal. The bottom one curves more, so it has less momentum.
7) At point 5 there is a kink in the trail to the left. This means that the incoming charged particle
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
There are no neutral particles in the liquid hydrogen that it could react with. We see a negative particle going
in and out, so there can’t be a positive particle involved. This is a decay into a neutral and a new negative
particle. The new negative particle has much less momentum than the first and so it curves more.
8) At point 3 there is a kink in the trail to the right. This means that the incoming charged particle
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
This is a decay, similar to the one at point 5, but the change in momentum is less noticeable and you might
not have noticed that there was an interaction. The neutral particle must be moving up and to the left to
conserve momentum.
9) Two charged particles appear at point 4. Compare the charges and momenta of the two particles.
The one on the left curves more so it must have less momentum. It is negative.
10) At point 6, the two trails overlap as one particle passes underneath the other. If you join this point with
the point where they were created, you get the direction of the original neutral particle. Where did it come
from?
It came from point 3, where we predicted a neutral particle was emitted in question 7.
11) Extend the curved trails from point 2 to find their intersection point. You can do this by tracing the curve
onto a piece of paper, cutting along the curve and using this as a guide. Where did the neutral particle that
formed these particles come from?
It came from point 1, where we predicted a neutral particle was emitted in question 3.
Bubble Chamber Detective: Brookhaven National Laboratory’s Bubble Chamber 1964
This is a very important historic photograph from 1964 from Brookhaven National Labs. Once again
negative kaons enter at the bottom and the magnetic field is into the page.
1) Look at the branching at point 1. What are the charges of the particles produced? Explain.
This is very similar to point 1 in the first photo. A kaon has interacted with a proton to produce a positive, a
neutral and a negative particle.
2) Look at the kink further up at point 2. What are the charges of the particles produced? Explain.
This is similar to points 3 and 5 in the first photo. A negative particle has decayed to produce a neutral and a
negative particle which kinks strongly to the right.
3) At point 4 there is a pair of oppositely charged particles created from a neutral particle. Extend the
curves until they meet and determine the direction the particle came from. Estimate the direction of
the total momentum at points 3 and 5. These three neutral particles came from one neutral particle.
Trace the three momenta back until they meet at one point. Where did this particle come from?
It is hard to determine graphically, where the neutral particles of points 3 and 5 come from, but you can make
an estimate and all three paths must intersect at one point. It is near point 2 – but it is not at point two.
4) In 1964, in the middle of the night at Brookhaven National Laboratory, Grusan Radojicic looked at
this photograph. He thought he might have found the first evidence of the omega minus particle that
was predicted by Gell-Mann’s quark model. (It is the short path going from point 2 to your
intersection point.) He had to decide whether he should wake up his boss with the news. His evidence
was based on the four invisible paths. Would you make the call? How would you explain what you’d
found over the phone?
This is from an email of Grusan’s
“ The event defied all statistical probability in the completeness of detected tracks. Moreover, the
event was in a good orientation for measurement. I immediately measured it on the scanning table
using all 4 stereo views. The measuring device was just a ruler and a set of approximate optical
constants of the chamber. By the early morning, I had the mass of the new particle and as a check, the
masses of the Xi and the K in the same event. It was the unmistakable Omega- of Gell-Mann.
Early in the morning I rang Nick Samios who rushed to the Lab. During the morning, we telephoned
Maurice Goldhaber (the Lab director) who immediately came to see us. I showed him my results. He
was impressed but he realised that one needed more events. Nick told him about the poor quality Kbeam and the slow progress in our struggle to improve it. Maurice asked what help we needed
immediately. Nick said that it would be important to get a higher intensity of the proton beam on our
target. Maurice telephoned from our office to the AGS and instructed them to give a huge increase to
our beam (I have forgotten how big an increase). That slowed down some other experiments but we
obtained 3 more events within a week or 10 (?) days. Then the discovery was published.

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p
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e
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e
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The negative kaon interacted with a proton to form a neutral kaon, positive kaon and an omega minus.
The omega minus went a very short distance and then decayed into a negative pion and a neutral xion.
The xion went a slightly longer distance and then decayed into two photons and a neutral lambda.
The photons decayed into electron-positron pairs and the lambda decayed into a proton and a negative pion.
The textbook gets into some very mathematical analyses of bubble chamber photographs. You will not
have to do this on the exam. Instead, answer the following;
1) Sketch a kaon-proton interaction from Figure 4, page 715 and explain how charge and momentum are
conserved at each point. Include paths of the invisible neutral particles.
At point1, the kaon decays into a pion and a sigma that move in almost opposite directions. It can
only be exactly opposite it there was no momentum to start with. The kaon was moving up and left.
In order for the momenta to add to the original, each of the two daughters must have more momentum
than the original.
2) Sketch the pair creation near the bottom of figure 7, page 716. How are charge and momentum
conserved?
A neutral particle with upward momentum decayed into two oppositely charged particles. These particles
gained equal and opposite momentum from some of the matter changing into kinetic energy.
3) Look at the cover. The curly trails are from electrons and positrons.
a) Sketch a few and add arrows to show which way they are moving. How do you know this?
You know that the particles lose energy and spiral in, not out. Also the main direction of movement is
down from the top. Interactions cause trails to spread – not converge.
b) If the magnetic field is into the page, which are the electrons?
c) Why do these particles spiral in and not the other particles?
They are the lightest particles. The magnitude of the forces will be the same for all charged particles
because they only have single charges. Therefore the amount of acceleration is inversely proportional
to their mass. The electrons and positrons are much lighter than the other particles - 100 to 1000
times. Acceleration of charged particles results in EM radiation and loss of energy.