In order to compute the scattering probability that two particles of type 1 (associated to ##\phi_1(x)##) which come from the far past with the momenta ##p1## and## p2##, to scatter and evolve into two particles of type 2 (associated to ##\phi_2(x)##) with the momenta ##p3## and ##p4## , I am going to apply the momentum space scattering rules, I'm just first of all trying to establish the number of distinct Feynman diagrams of ##<0|T( \phi_1(x_1)\phi_1(x_2)\phi_2(x_3)\phi_2(x_4))|0>##, where the interacting Hamiltonian is: ##H_{int}= g/4 \phi_1^{2}(x)\phi_2^{2}(x)##(adsbygoogle = window.adsbygoogle || []).push({});

QUESTION:

The solution lists these two Feynman diagrams only , see attachment 'dia 1'

But I am also getting this diagram, see attachment 'dia 2'

Where solid lines are associated to ##\phi_1(x)##, dotted to ##\phi_2(x)##, and ##z## and ##w## are the internal variables I am integrating over.

Is there a reason this should be excluded? I know that 'bubble/vacuum' diagrams are excluded - disconnected diagrams with no connection to external points, but I'm unsure here since it's connected...

Many, many thanks in advance.

On another note, my lecture notes describe the contribution from the ##g^{0}## term as 'trivial' since it describes non-interacting particles. so yeh in this case it's ##x1, x2## and ##x3, x4## contracted since you can not contract different fields, however, what is meant by 'trivial' in this sense? Because I know that diagrams that have no connections to external points are not included since they are vacuum contributions and will just cancel out anyway, however here it is a diagram solely between external points, so whilst it doesn't describe any interaction, it still needs to be included right?

There is the 'amputate external legs' rule which in my lecture notes is stated as equivalent to 'moving propagators to external points'...does this cover it? I don't really understand this rule? If so, or anyway, any links or help on this appreciated, ta.

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# I Scattering, 4 point correlator, number of distinct Feynman diagrams

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