Day 8.8Bubble 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 bythe 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) -1B) 0C) +1D) not enough information

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) -1B) -2C) +1D) +2

3) Is momentum conserved at 1?

A No, therehas 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.

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

5) A branching has appeared at point 2. What are the charges of the particles leaving this point?

A)Two positive particlesB)Two oppositely charged particles, the positive is the lower trail

C) Two negative particlesD)Two oppositely charged particles, the negative is the lower trail

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

7) At point 5 there is a kink in the trail to the left. This means that the incomingcharged particle

A)interacted with a neutral particle.B)interacted with a positive particle

C)decayed into a positive and a neutralD) decayed into a negative and a neutral

8) At point 3 there is a kink in the trail to the right. This means that the incomingcharged particle

A)interacted with a neutral particle.B)interacted with a positive particle

C)decayed into a positive and a neutralD) decayed into a negative and a neutral

9) Two charged particles appear at point 4. Compare the charges and momenta of the two particles.

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 particle. Where did it come from?

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?

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.

2)Look at the kink further up at point 2. What are the charges of the particles produced? Explain.

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?

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?

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.

2)Sketch the pair creation near the bottom of figure 7, page 716. How are charge and momentum conserved?

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?

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?