# Interpreting the various tracks in a hydrogen bubble chamber

• songoku
In summary, we don't have any information on the energy of the incoming ##p^-##, and the momentum conservation for the center of mass requires a large fraction of that to go into the products forward motion. The annihilation adds almost 2 GeV distributed over at least 8 particles (zero charge particles are invisible in a cloud chamber), so the expected answer is that the incoming ##p^-## has the highest kinetic energy of all particles shown. But I don't feel very convinced.
songoku
Homework Statement
Look at the figure, which shows tracks in a hydrogen bubble chamber. One anti-proton hits a proton in the chamber. The resulting tracks show the tracks of various pairs of particles (##\pi^{+}## / ##\pi^{-}## , ##e^{+}## / ##e^{-}## ) which are created in the collision.

(a) Identify the incoming anti-proton track.

(b) How did you identify the incoming anti-proton? What does this tell you about its kinetic energy compared to the resulting particles?

(c) How does the picture illustrate the conservation of matter/energy?

(d) Identify any pair of particles (e.g. ##e^{+}## / ##e^{-}##). How do you know that the tracks show you a pair of identical but opposite particles?

(e) How does the picture illustrate the conservation of charge?

(f) How does the picture illustrate the conservation of momentum in two dimensions?
Relevant Equations
None

(a) I don't know how to identify which one is the anti-proton track. The anti-proton hits proton at point in the figure where all the lines are coming together but how to identify which one is anti-proton?

Do I need to know a certain rule regarding the color of each particle?

Thanks

Hehe, the particles have no colour. The tracks have been given a colour afterwards, to help you.
A bubble chamber is usually placed in a strong magnetic field, perpendicular to the plane of the picture.

##\ ##

songoku
BvU said:
Hehe, the particles have no colour. The tracks have been given a colour afterwards, to help you.
A bubble chamber is usually placed in a strong magnetic field, perpendicular to the plane of the picture.

##\ ##
Okay so the magnetic field will cause magnetic force to act on the particle and the particle will move in a somewhat circular trajectory but how to identify which one is anti-proton (since all charged particle will be deflected)?

Thanks

There is one track that changes curvature; what does that mean for the momentum ?

songoku
BvU said:
There is one track that changes curvature; what does that mean for the momentum ?

Sorry I still don't know which track that changes curvature? Is it: A1 to A2 or B1 to B2 or C1 to C2 or D1 to D2 or none of them?

Momentum should be conserved in this case so changing curvature would mean there is collision between particles.

Or maybe you mean the momentum of certain particles would change (possibly decrease) due to the change in curvature?

Thanks

Hint:
Why did you not label one or two tracks?

songoku
songoku said:
somewhat circular trajectory
Yes, in particular: ##r\propto mv##

songoku said:
Sorry I still don't know which track that changes curvature? Is it: A1 to A2 or B1 to B2 or C1 to C2 or D1 to D2 or none of them?
A1 is the straightest track; the exercise composer also hasn't given it a colour ...
So my money is on A1 as answer to part (a). It curves to the right, so green appears to indicate negative.

Now I humbly must admit my 'changes momentum' wasn't very well thought through
(even though very advanced in years, I only worked with near zero COM momentum in colliders ) :
If the ##p^-## and the ##p^+## annihilate, then A1 --> A2 for the ##p^-## is nonsense.

The hypothesis A1 --> A2 for the ##p^-## leaves us searching for a forward ##p^+## which is absent.
(D1 as outgoing ##p^+## has too much transverse momentum)

Reverting to the problem statement:
(b) We have no information on the energy of the incoming ##p^-##. Momentum conservation for the center of mass requires a large fraction of that to go into the products forward motion. The annihilation adds almost 2 GeV distributed over at least 8 particles (zero charge particles are invisible in a cloud chamber). I suppose the expected answer is that the incoming ##p^-## has the highest kinetic energy of all particles shown. But I don't feel very convinced.

I'm sorry I can't offer much help on (c) and (d).

There are four outgoing green tracks of negative and four of positive particles. No neutral particles that decay within the visible region - let's assume all products are seen in the picture. At least we see charge conservation is satisfied (e).

For (f) we would need to add the momenta of all eight tracks. The sum should be the incoming ##p^-## momentum.

##\ ##

songoku
Tom.G said:
Hint:
Why did you not label one or two tracks?
No particular reason. I just thought what I did would be wrong because I labeled the tracks randomly. I mean the correct pair could be A1 and B1 or A1 and C1 instead of A1 and A2

BvU said:
A1 is the straightest track; the exercise composer also hasn't given it a colour ...
So my money is on A1 as answer to part (a).
Is this a general rule, the straightest track would be the initial particle? Or maybe this is an educated guess?

BvU said:
It curves to the right, so green appears to indicate negative.
By "curves to the right", do you mean curves clockwise? And do you assume the direction of magnetic field is into the plane of the picture?

BvU said:
If the ##p^-## and the ##p^+## annihilate, then A1 --> A2 for the ##p^-## is nonsense.
If they annihilate, two photons should be created and we would not be able to detect the photons in the bubble chamber?

BvU said:
The hypothesis A1 --> A2 for the ##p^-## leaves us searching for a forward ##p^+## which is absent.
(D1 as outgoing ##p^+## has too much transverse momentum)
When anti-proton hits proton, does the proton at rest initially?

BvU said:
Reverting to the problem statement:
(b) We have no information on the energy of the incoming ##p^-##. Momentum conservation for the center of mass requires a large fraction of that to go into the products forward motion. The annihilation adds almost 2 GeV distributed over at least 8 particles (zero charge particles are invisible in a cloud chamber). I suppose the expected answer is that the incoming ##p^-## has the highest kinetic energy of all particles shown. But I don't feel very convinced.

I'm sorry I can't offer much help on (c) and (d).
Actually you have helped, a lot.

For (c), I am thinking something like the total kinetic energy of red and green tracks should be equal to the total KE of anti-proton and proton

For (d), I am thinking B1 and D2 because they have somewhat similar radius but opposite curvature

Thanks

songoku said:
Is this a general rule, the straightest track would be the initial particle?
The straightest track is the track with the highest momentum

songoku said:
And do you assume the direction of magnetic field is into the plane of the picture?
The problem statement says it's a ##p^-##

songoku said:
When anti-proton hits proton, does the proton at rest initially?
For all practical purposes, yes. (It only has some minimal thermal kinetic energy)

songoku
BvU said:
The straightest track is the track with the highest momentum The problem statement says it's a ##p^-##For all practical purposes, yes. (It only has some minimal thermal kinetic energy)
Ah ok ok

Now I have something to build my argument upon.

Thank you very much for the help and explanation BvU, Tom.G

BvU and Tom.G

## 1. How does a hydrogen bubble chamber work?

A hydrogen bubble chamber is a device used for detecting and visualizing subatomic particles. It is filled with superheated liquid hydrogen, which is kept just below its boiling point. When a charged particle passes through the chamber, it ionizes the hydrogen atoms, causing them to form tiny bubbles along its path. These bubbles can be photographed and studied to determine the properties of the particle.

## 2. What types of particles can be detected in a hydrogen bubble chamber?

A hydrogen bubble chamber can detect various types of particles, including electrons, protons, neutrons, and their antiparticles. It can also detect more massive particles, such as muons and alpha particles.

## 3. How are the tracks in a hydrogen bubble chamber interpreted?

The tracks in a hydrogen bubble chamber are interpreted by analyzing the size, shape, and direction of the bubbles formed by the particles. The curvature of the track can also provide information about the particle's charge and mass. By comparing these characteristics with known particles, scientists can identify and study the particles that passed through the chamber.

## 4. What can the tracks in a hydrogen bubble chamber tell us about particle interactions?

The tracks in a hydrogen bubble chamber can provide valuable information about the interactions between particles. By studying the tracks, scientists can determine the type of particle that was produced, the energy of the interaction, and the type of force involved. This can help in understanding the fundamental building blocks of matter and the underlying laws of physics.

## 5. What are the advantages of using a hydrogen bubble chamber in particle physics research?

Hydrogen bubble chambers have been an essential tool in particle physics research for decades. They have several advantages, including high sensitivity, high resolution, and the ability to detect a wide range of particles. They also provide a visual representation of particle interactions, making it easier for scientists to interpret the data and make new discoveries about the subatomic world.

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