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I am confused. I know Feynman Diagrams looks like the ones here. But then I saw these, look in page 17[1247] of this paper.

Any idea?

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student1
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    The paper is behind a paywall. – Danu Sep 22 '13 at 18:21
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    The picture shows all connected Feynman diagrams to a given order with no external legs. A good discussion of them can be found in Peskin and Schroeder, "An Introduction to Quantum Field Theory", section 4.4. – zkf Sep 22 '13 at 18:28
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    Such Feynman diagrams with no external legs are called Vacuum bubbles. – Qmechanic Sep 22 '13 at 18:32
  • OK, so these are normal Feynman Diagrams w/o external legs. But how are the "propagator" represented in such diagrams? Normally it is a curvy internal line (e.g. photon in QED). – student1 Sep 22 '13 at 19:33
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    One may always choose different conventions for the shape of the lines for various particle species, can't he? – Luboš Motl Sep 22 '13 at 20:09
  • Of course. My point was to be sure these are indeed just the 'normal' Feynman diagrams. So in the paper, straight lines actually indicate propagators, right? – student1 Sep 22 '13 at 20:10
  • In the first diagram, for example, there is only two loop. Where is the propagator there? – student1 Sep 22 '13 at 21:31
  • Yes, there are propagators. See, for instance, the 2-loop vacuum bubble graph page $32$ (Problem 5) of this paper, and you will see the detailed expression of the graph. – Trimok Sep 23 '13 at 07:16

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