Feynman rules and decay process

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Discussion Overview

The discussion revolves around the application of Feynman rules to specific decay processes involving charm quarks (c) and their antiparticles (c-bar). Participants explore whether certain decay channels are allowed, focusing on interactions mediated by the strong and electroweak forces.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that the decay of cc to tau++tau- is possible via the strong force.
  • Another participant counters that this decay is incorrect as leptons do not interact via the strong force, proposing instead that it could occur through electroweak interactions involving a virtual Z-boson or photon.
  • Regarding the decay of cc to cu and cu, one participant believes this cannot happen, while another argues that it can occur if one of the initial particles releases a gluon to produce a pair of u quarks.
  • A later reply indicates that the cc to cu and cu decay is kinematically forbidden if the particles are at rest relative to each other, but may be allowed if there is sufficient energy to overcome the mass barrier of the u quarks.

Areas of Agreement / Disagreement

Participants express differing views on the allowed decay processes, with no consensus reached on the mechanisms or conditions under which these decays may occur.

Contextual Notes

Some discussions hinge on the definitions of decay and the energy conditions required for certain processes, which may not be fully resolved in the conversation.

jc09
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Hi I need help to understand Feynman rules for decay for exams. In past paper there is the following question as whether the following are allowed.

cc decays to tau++tau-. From What I can see this is possible via the strong force is this correct?

The next is cc decays to cu and cu. From what I can this this can't happen but is this correct?

In these question c is the antiparticle
 
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The second process can happen.
One of the initial particles, say c, can release a gluon which can produce a pair of u quarks.
 
jc09 said:
Hi I need help to understand Feynman rules for decay for exams. In past paper there is the following question as whether the following are allowed.

cc decays to tau++tau-. From What I can see this is possible via the strong force is this correct?

This is certainly wrong: the tau is a lepton and leptons do not interact via strong force.
IMHO, this decay is possible via electroweak interactions: c and c-bar annihilate into a virtual Z-boson or photon, which in turn decays into the pair of leptons.

jc09 said:
The next is cc decays to cu and cu. From what I can this this can't happen but is this correct?

In these question c is the antiparticle

This is kinematically forbidden (energy conservation): the outgoing particles would be in sum heavier than the decay parent.
 
As far as ccbar goes, it depends on what you mean by "decay". If both are at rest with respect to each other, then ccbar -> c ubar + u cbar is forbidden. If they have enough energy to overcome the 2m_u barrier you need to start that reaction, then you're fine and it's allowed.
 

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