Why this self-loop diagram is not included in $\phi^4$-theory of Peskin & Schroeder? Consider a $2\rightarrow2$ scattering process in $\phi^4$-theory.
On p. 326 in the book of Peskin & Schroeder, they consider the 3 1-loop corrections in the parenthesis:

My question is:
Why don't they include below self-loop diagram?

Is this 0? Why?
 A: Peskin & Schroeder (P&S) on p. 326 are discussing a connected, amputated 4-pt amplitude$^1$ ${\cal M}_4$. It is clear from various places in the P&S textbook [see e.g. eq. (10.21)] that ${\cal M}_4$ is an amputated amplitude. Now OP's self-loop diagram can be understood as an amputated diagram, so OP's question is still a valid question. 
However, P&S explain on p. 113-114 that such diagrams should be cut away. This is e.g. to avoid over-counting when we attach connected propagators $G_c$ to the external legs of the amputated amplitude ${\cal M}_4$. [Technically, self-loop diagrams in principle contribute to the connected propagator $G_c$, but are often cancelled via renormalization conditions, see e.g. eqs. (10.28) & (10.29) on p. 328.] 
--
$^1$ By the way, the last explicit diagram in P&S is not a 4-loop diagram: It denotes an interaction counterterm, cf. fig. 10.3 on p. 325 :)
A: if you separate the new diagram you added to 2 diagram it means:


*

*diagram of the first order

*diagram with a 1 particle that scatter with itself and continue in its trajectory


the probability for a particle to continue its trajectory when it only "interacts" with itself is 1 due to conservation of momentum.
therefore the probability is the same and the physical meaning is the same as the first order diagram
