What kinds of divergences for a given interaction

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Discussion Overview

The discussion revolves around the nature of divergences in interaction terms within Lagrangian and Hamiltonian frameworks, focusing on how these divergences manifest in various diagrams, including their dependence on the number of vertices and loops. The scope includes theoretical considerations related to renormalization and power counting methods.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant inquires whether the divergence type of a single interaction term can be immediately determined from its diagrams.
  • Another participant notes that the level of divergence is diagram-specific and generally increases with the number of loops in the diagram.
  • A question is posed about the relationship between the divergence of single-vertex interactions and those with multiple vertices, specifically if a quadratic divergence in a single-vertex diagram implies a quartic divergence in a two-vertex diagram.
  • It is suggested that a power counting procedure can be applied to assess divergences, which depends on various factors including the number of derivatives, internal lines, vertices, loops, and external lines.
  • A reference to T. Muta's work is provided as a resource for further understanding these methods.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between divergences in single-vertex and multi-vertex interactions, indicating that the discussion remains unresolved regarding this specific aspect.

Contextual Notes

The discussion highlights the complexity of determining divergences based on interaction terms and the various factors that influence these divergences, which may depend on specific definitions and assumptions within the theoretical framework.

copernicus1
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Can you look at an interaction term in your lagrangian or hamiltonian, like L_{\rm int} or H_{\rm int}, and say immediately how its diagrams will diverge (as in quartic, quadratic, linear, log, etc.)?
 
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The level of divergence depends on the particular diagrams. An interaction term in the Lagrangian is just one vertex that may compose your diagram. For example, in general, diagrams with higher number of loops have higher degree of divergence.

From the interaction term you can usually determine if you theory is renormalizable or not by looking at the dimensions of the coupling. For example, theories with dimensionless couplings are renormalizable.
 
Great thanks. Is there a relationship though between the divergence in the single-vertex interaction and the interactions with higher numbers of vertices? Like, if a single-vertex diagram has a quadratic divergence, would a two-vertex diagram have a quartic divergence?
 
If you assume that your theory only has one kind of interaction vertex then you can always perform a power counting procedure in a general fashion. This procedure clearly depends on you vertex but in order to do that you need to consider:

1) The number of derivatives contained in your vertex
2) The number of internal lines
3) The number of vertices in a given diagram
4) The number of loops
5) The number of external lines

Note that some of these quantities can be related with each other. If you want to see a very neat application of these kind of methods you can look into T. Muta - "Foundation of Quantum Chromodynamics", in particular Ch. 2.5.
 

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