Phi^4 theory

  • Thread starter Kalimaa23
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  • #1
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Greetings,

I stumbled across two question that I have no idea on how to answer them.

1) The interaction term in a scalar field theory is [tex]-\frac{\lambda}{4!} \phi^4[/tex]

Why should lambda be positive? (they say look at the energy of the ground state...)

2) Write down the Feynman rules for phi^4

I have no clue as to how you get the two intersecting lines that give is -i lambda vertex. I see where the propagator comes from though.

Any thougths?
 

Answers and Replies

  • #2
dextercioby
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For #2,the vertex is the 4 point OPI Green function,namely

[tex] (2\pi)^{4}\delta^{4}\left(p_{1}+p_{2}+p_{3}+p_{4}\right)\tilde{\Gamma}_{1}^{(4)}\left(p_{1},p_{2},p_{3},p_{4}\right) =-\lambda (2\pi)^{4}\delta^{4}\left(p_{1}+p_{2}+p_{3}+p_{4}\right)[/tex]
[tex]\Longrightarrow \tilde{\Gamma}_{1}^{(4)}\left(p_{1},p_{2},p_{3},p_{4}\right) =i\cdot \ \mbox{vertex} [/tex]

Consider the function [itex] V(\phi)=\frac{m^{2}}{2}\phi^{2}+\frac{\lambda}{4!}\phi^{4} [/itex].

Get its minimum & see what constraint needs to be imposed on [itex]\lambda [/itex] to ensure the minimum to be 0 or greater.

Daniel.
 
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  • #3
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Great! Thanks alot!
 
  • #4
dextercioby
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I hope you know how to get the other rules,using the connected Green functions in various orders of perturbation theory.

Daniel.
 

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