What is the energy dependence of the Equivalent photon approximation?

In summary: This is because the form factor is describing the response of a scattering system to a point-like probe.
  • #1
ribella
5
0
Hi,
What is the energy dependence of the Equivalent photon approximation? For this approach to be valid, what is the maximum center of mass-energy. As know, this approach is an energy-dependent approach. Can this approach be used to calculate, for example, at a center of mass energy of 100 TeV? Is there any problem with the approach in terms of physics?
 
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  • #2
For what type of process do you have in mind?

A probe of the elastic form factor of a nucleon/nucleus? Or something like the inelastic photon pdf of a nucleus?

Typically this is governed by the probe energy / exchange energy Q^2 through the photon its self. If this energy scale is sufficiently small compared to other scales in the scattering process, then it is a decent approximation. If there are other energy scales such as a fermion mass that is mf~Q, then the EPA (assuming the photon is essentially on shell) neglects power corrections of the form Q/mf which can be important for a precision theory description.
 
  • #3
Thank you for your answer. For example, let's assume that the center of mass energy for the gg->e-e+ process (Here, g is EPA photon) is 100 TeV. In this case, is the EPA approximation valid for the incoming photons at these energies?
 
  • #4
I would not be confident that the EPA is a good approximation in this case.

(Do you mean the photon photon CoM or the hadronic one? That wasnt clear to me)

Even if the CoM is high, I think it is possible that the cross-section may still be dominated by low-Q (corresponding the virtuality probe of the nucleus/nucleon that is giving you these EPA photons).

Depending if you are considering a nucleus or a nucleon form factor (im not sure what you have in mind exactly) these distributions-the photon flux-may peak at small Q (eg at values below the electron mass). If that were the case, terms like Q/m_electron power corrections are absent in the EPA approach.

Note that a form factor like woods-saxon peaks as Q tends to zero (ie when you resolve the whole photon field of the nucleus).
 

1. What is the Equivalent photon approximation (EPA)?

The Equivalent photon approximation (EPA) is a theoretical model used in high-energy physics to describe the interaction between a high-energy particle and a target material. It assumes that the high-energy particle can be treated as a beam of equivalent photons, which are virtual particles that have the same energy and momentum as the original particle.

2. How is the energy dependence of the EPA determined?

The energy dependence of the EPA is determined by the energy of the high-energy particle and the properties of the target material. As the energy of the particle increases, the number of equivalent photons also increases, leading to a higher probability of interaction with the target material.

3. What is the significance of the energy dependence of the EPA?

The energy dependence of the EPA is significant because it allows us to predict the behavior of high-energy particles when they interact with different target materials. It also helps us understand the underlying physics of these interactions and can be used to improve the accuracy of experimental results.

4. How is the EPA used in practical applications?

The EPA is used in various practical applications, such as in particle accelerators and medical imaging techniques. In particle accelerators, the EPA is used to calculate the probability of particle interactions with the accelerator's target material, while in medical imaging, it is used to determine the energy and intensity of X-rays produced by high-energy particles interacting with the patient's body.

5. Are there any limitations to the EPA?

Yes, there are limitations to the EPA. It is a simplified model that does not take into account all possible interactions between high-energy particles and target materials. It also assumes that the target material is homogeneous and does not consider the effects of the particle's trajectory or the target's atomic structure. Therefore, the EPA may not accurately predict the behavior of high-energy particles in all situations.

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