# Tagged Questions

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### Counting higher-order corrections in “ABC theory”

I am trying to understand how to enumerate higher-order Feynman diagrams. In his book on Elementary Particle Physics, Griffiths considers a simple "ABC toy theory" which has: three (scalar, massive)...
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### Why does not Bhabha scattering contain u-channel diagram?

$e^+e^-\rightarrow e^+e^-$ is called Bhabha scattering. Let us only consider the tree level Feynman diagrams of this process. Apparantly, there are s-channel and t-channel diagrams as shown in the ...
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### Problem understanding the symmetry factor in a feynman diagram

I am trying to understand a $1/2$ in the symmetry factor of the "cactus" diagram that appears in the bottom of page 92 In Peskin's book. This is the diagram in question (notice that we are in $\phi^4$ ...
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### Why doesn't a renormalizable $\phi^4$ theory have odd diagrams?

I've been reading Zee's QFT textbook and trying to follow some lecture notes online whenever I can't grasp something. I really don't understand one thing regarding the renormalization of theories, ...
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### If you are only interested in deriving Feynman diagrams can you skip path integrals and just compute greens functions?

I've been reading about the path integral approach to quantum field theory and I noticed that at the end you are just computing greens functions that you could have started computing in the beginning. ...
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### How do you prove that $L=I-V+1$ in $\lambda\phi^4$ theory?

It is known that the number of loops in $\lambda\phi^4$ theory is given by the formula $$L=I-V+1$$ where $L$ is the number of loops, $I$ the number of internal lines and $V$ the number of vertices. ...
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### First-order EM Feynman diagram?

Is there any 1st order electromagnetic Feynman diagram? I.e. a process whose probability is just $\propto \alpha_{EM}$? If not, is there any physical reason why? We always need at least two particles ...
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### Order of Feynman diagrams for electroweak processes?

I want to compare two Feynman diagrams and be able to say which one describes a process that is more likely to happen. As far as I understand, this is done by considering the order of the diagram. ...
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### Perturbation theory : quadratic external field

I'm trying to derive the explicit form of S-matrix of an interaction Hamiltonian $$H' = \frac{1}{2} \lambda \left[ \int d^3 x \rho({\vec x}) \phi({\vec x}, t)\right]^2\tag{1}$$ Even though the ...
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### How to tell the order of a Feynman diagram?

How can we know the order of a Feynman diagram just from the pictorial representation? Is it the number of vertices divided by 2? For example, I know that electnro-positron annihilaiton is first ...
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### Feynman Parametrization in muon magnetic moment

I am calculating the muon magnetic moment due to Electroweak interactions in one loop diagrams involving $W$ bosons. While referring a particular research article by John S. Curiale, titled Weak ...
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### Feynman-like diagrams for photon/electron interaction and electron recollision

Feynman diagram is a useful and powerful tool to represent path integrals. But, most of its applications seem to be focused on relativistic particule creation/annihilation. As far as I can ...
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### Is there a mistake in a QFT textbook?

I tried to calculate one of the problems in the textbook Gauge Theory of Elementary Particle Physics by Ta-Pei Cheng and Ling-Fong Li. On page 248 you can find the following calculation of a loop ...
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### Decay Feynman diagram

$$D^0 \to K^+ +\pi^- + \pi^0 + \pi^0$$ cannot occur by first order weak interactions. Could anybody help me confirm this? What I currently have is that the initial charm goes to anti-strange and the ...
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### How to count and 'see' the symmetry factor of Feynman diagrams?

Could somebody explain how one can derive the symmetry factor both by counting possible contractions and by looking at the symmetry of a diagram. Consider for example this diagram in $\phi^4$-theory ...
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### The one-loop contribution to a time ordered product of conserved currents

In two dimensions one can define for a Lagrangian describing free Dirac fermions with $N$ associated flavours by $$\mathcal{L}=i\bar{\psi}_i\gamma^\mu \partial_\mu \psi^i$$ and associate vector ...
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### Fast and slow modes, and the vanishing of certain diagrams during re-normalization

In the middle of pg. 452 of Atland and Simonss Condensed Matter Field Theory, they state the following: Terms of $\mathcal{O}(\phi _{\text{s}}^3\phi _{\text{f}})$ do not arise because the addition ...
Suppose I have the following Lagrangian density: $$\mathcal L = -\frac{1}{2} \sum_{i = 1}^N \left [ \partial_\mu \phi_i\partial^\mu \phi^i +m^2 \phi_i^2\right ] + \frac{g}{2N}\sum_{i=1}^N \sum_{j=1}^... 1answer 271 views ### Feynman rule for propagator with derivatives Suppose you have an interaction term of the form$$\mathcal{L}_{int} = \frac{h g}{3!}\phi^3\partial^2\phi where $h$ and $g$ are both couplings. Now if I draw a diagram of the form given in the ...
In his book Quantum Field Theory - A Tourist Guide for Mathematicians, Gerald Folland introduces the on-shell renormalization scheme for the $\phi^{4}$-scalar field theory. According to my ...