Lab frame versus center of momentum frame

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Discussion Overview

The discussion revolves around the analysis of Compton scattering in different reference frames, specifically the lab frame versus the center of momentum (CM) frame. Participants explore the reasons for the conventional choice of frames in various experimental contexts and the implications for calculations such as cross-sections.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants suggest that the choice of the lab frame for analyzing Compton scattering is primarily due to convenience.
  • Others question the feasibility of having stationary muons in experiments, indicating a challenge in achieving such conditions.
  • It is noted that while the center of mass frame is often easier for analysis, results must eventually be translated back to the lab frame for practical measurement.
  • A participant proposes that if electrons are relativistic, muons could be considered at rest when confined by electric or magnetic fields.
  • There is a query about whether cross-sections calculated in the center of mass frame can be directly converted to lab frame cross-sections or if a complete recalculation is necessary.
  • One participant mentions that cross-section formulas can typically be expressed in terms of invariants and kinematic factors, which may simplify the conversion between frames.
  • A reference to Mandelstam variables is made as a relevant concept in this context.

Areas of Agreement / Disagreement

Participants express differing views on the practicality and implications of using different frames for analysis, indicating that multiple competing perspectives remain without a clear consensus.

Contextual Notes

The discussion highlights the complexities involved in frame transformations and the assumptions related to the conditions of particles in various experimental setups.

jdstokes
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Why is it conventional to analyse compton scattering in the lab frame (fixed target) whereas all other processes I've seen are analysed in the center of momentum (colliding beam) frame.

Are there any experiments where one would collide electrons with stationary muons e.g.?
 
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Convenience, mostly.

To answer your second question, how do you get stationary muons?
 
It is often far easier to analyze things in the center of mass frame. The measurement equipment, however, is in the lab frame, so at some point the results of the analysis have to be recast to the lab frame.
 
If the electrons are relativistic then muons confined by electric of magnetic fields could be considered at rest?
 
So If I calculate a cross-section in the center of mass frame, is there any way of converting this directly to a Lab cross-section, or do I have to go back to the Feynman amplitude for the process and redo the kinematics at that point.
 
You can usually write a cross-section formula in terms of invariants times simple kinematic factors, making going between the cm and the lab relatively easy.
 
Ahh yes, the old Mandelstam variables, good point.
 

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