# Feynman diagram interpretation - 2 electrons in and 2 electrons out

• catkin
In summary, the conversation discusses a physics problem from a textbook on the topic of particle interactions. The question asks for the nature of the interaction shown in Figure 9.5 and for three properties of the exchange particle. The conversation leads to the conclusion that the interaction is an electromagnetic force interaction and the exchange particle is a photon. There is no transfer of charge in the interaction and it is not necessary to know the direction of motion of the exchange particle.
catkin

## Homework Statement

Hello

The question is from "Advanced Physics" by Adams and Allday. It is from section 9 "The Physics of Particles", "Practice Exam Questions". It is question 11, part c. The level of this study is between school and University.

Figure 9.5 represents a simple particle interaction. State the nature of the interaction and describe what happens to the particles involved. State three properties of the exchange particle.

[Figure 9.5 is in the attachment]

None.

## The Attempt at a Solution

The particles in the interaction are leptons (electrons) so it must be a weak force interaction.

It looks like nothing happens to the particles involved! The same particles are present before and after the interaction (possibly at different energies).

It is not clear which way the exchange particle moves and there is no transfer of charge.

Because there is no transfer of charge and the weak force is acting, I guess the only unidentified particle must be the uncharged weak force exchange particle, the Z0.

I further guess that the directionless nature of the reaction means the exchange particle must be one that is its own anti-particle; again the Z0.

I don't like guessing -- it means lack of understanding

As for the three properties, I don't think we are required to recall actual figures so how about: massive (hence very unstable), uncharged, own anti-partcle?

Best

Charles

#### Attachments

• Figure 9.5.png
4 KB · Views: 393
catkin said:

## The Attempt at a Solution

The particles in the interaction are leptons (electrons) so it must be a weak force interaction.

Nope. The squiggly line represents a photon. W and Z bosons are represented by straight dashed lines.

It looks like nothing happens to the particles involved! The same particles are present before and after the interaction (possibly at different energies).

When two billiard balls collide, the same balls are present before and after the interaction. Does that mean that nothing happens to them??

It is not clear which way the exchange particle moves and there is no transfer of charge.

You don't need to know the direction of motion of the exchange particle. But you're right, there is no transfer of charge.

Thanks Tom :-)

Duh! I didn't consider that it might be an electromagnetic force interaction! We've been so focussed on weak and strong I missed the obvious.

Best

Charles

## 1. How do Feynman diagrams represent the interaction of 2 electrons going in and 2 electrons going out?

Feynman diagrams use lines and vertices to represent the paths and interactions of particles. In the case of 2 electrons going in and 2 electrons going out, the diagram would show 2 incoming electron lines meeting at a vertex, which then splits into 2 outgoing electron lines.

## 2. What does the direction of the arrows on the lines in a Feynman diagram represent?

The direction of the arrows on the lines in a Feynman diagram represents the flow of time. The arrow pointing towards the vertex indicates the initial state of the particle, while the arrow pointing away from the vertex indicates the final state of the particle.

## 3. How are Feynman diagrams used to calculate the probability of a particle interaction?

Feynman diagrams are used in a technique called Feynman diagrammatic expansion, which involves summing over all possible diagrams to calculate the probability of a particle interaction. This is based on the fact that the probability amplitude of a particle interaction is the sum of the probability amplitudes of all possible paths or diagrams.

## 4. Can Feynman diagrams be used to represent interactions between more than 2 particles?

Yes, Feynman diagrams can represent interactions between any number of particles. Each additional particle would be represented by an additional line and vertex in the diagram.

## 5. How do Feynman diagrams relate to the fundamental forces of nature?

Feynman diagrams are used to visualize and calculate the interactions between particles, which are governed by the fundamental forces of nature. Each of the fundamental forces (gravity, electromagnetism, strong and weak nuclear forces) has a corresponding set of Feynman rules that dictate how the particles interact in a diagram.

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