Computing QED amplitudes in a collider

In summary, the conversation discusses the possibility of processes involving four particles interacting in quantum electrodynamics (QED) and whether these interactions should be taken into account when computing the cross section. It is noted that getting four particles to interact is very difficult and the effective interaction region is smaller for leptons. The exercise provided asks how likely it is for four particles to be within 1 fm of each other and compares it to the likelihood for two particles.
  • #1
eoghan
207
7
Hi there,

While reviewing the theory of Feynman diagrams for QED, a question came into my mind. In the textbooks, one usually deals with processes involving two incoming particles. But I could imagine a process where four particles are interacting (e.g. attached picture) and this can give a contribution that is of the same order as a loop diagram with only two interacting particles. Since in a collider two beams of particles collide I can expect to have interactions between any even number of particles. So in order to compute the cross section should one compute also these interactions? Or for some reasons the diagrams with more than two particles cancel away?
 

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  • #2
It is hard enough to get two particles to collide. Having four particles interact is practically impossible.
 
  • #3
Exercise for you: The typical size of the interaction region of hadron collisions is 1 fm2. Ignoring correlations, how likely is it that four particles are within 1 fm of each other? How does that compare to two particles?
For leptons the effective interaction region is even smaller.

In practice even the “~0“ answer is an overestimate as particles in a beam don’t get that close.
 

Related to Computing QED amplitudes in a collider

1. How are QED amplitudes computed in a collider?

QED (Quantum Electrodynamics) amplitudes in a collider are computed using Feynman diagrams, which represent the possible interactions between particles. These diagrams are used to calculate the probability of a given interaction occurring within a collider.

2. What is the significance of computing QED amplitudes in a collider?

Computing QED amplitudes in a collider allows for predictions to be made about the behavior of particles and their interactions. This is important for understanding the fundamental laws of nature and for making accurate predictions in particle physics experiments.

3. How are QED amplitudes affected by the energy of the particles in a collider?

The energy of the particles in a collider has a significant impact on QED amplitudes. As the energy increases, more complex interactions become possible, and the amplitudes become larger. This allows for the study of higher energy phenomena and the discovery of new particles.

4. What challenges are faced when computing QED amplitudes in a collider?

One of the main challenges in computing QED amplitudes in a collider is dealing with the large number of possible interactions that can occur. The calculations can also become very complex and require advanced mathematical techniques. Additionally, experimental uncertainties and theoretical assumptions can also affect the accuracy of the calculations.

5. How do QED amplitudes in a collider contribute to our understanding of the universe?

Studying QED amplitudes in a collider allows us to test and refine our understanding of the fundamental laws of nature. It also provides valuable insights into the behavior of particles and the structure of the universe. These studies have led to the discovery of new particles and have helped to further our understanding of the origins and evolution of the universe.

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