All Questions
Tagged with quantum-electrodynamics perturbation-theory
37
questions
3
votes
1
answer
130
views
Weisskopf and self-energy
I am working my way through the 1934 paper by Weisskopf on the self-energy of the electron and is much helped by the English translation found here. I do have some difficulties with section 2 of this ...
- 91
1
vote
0
answers
55
views
$Z_1=Z_2$ and its relation to vertex renormalization in QED
I have been working on the full renormalization of scalar QED with self-interactions, following the steps of Schwartz’s treatment on spinor QED (Chap 19). I have 3 main questions regarding this:
Need ...
- 162
3
votes
1
answer
92
views
How can I count diagrams (in this scalar QED example) at a particular order without drawing all the Feynman diagrams?
Here's a 3-loop diagram for light-by-light scattering in scalar QED (from Schwartz textbook question 9.2):
The question 9.2 asks approximately how many other diagrams contribute at the same order in ...
- 1,783
1
vote
0
answers
72
views
Confusion about the dressed propagator
When considering interactions, the free propagator $S_0(p)$ of fermions for example gets "dressed" due to the self-energy of the fermion. The complete propagator then becomes
$$
S(p)=\frac{/\...
- 172
4
votes
1
answer
166
views
If the fine-structure constant was very large could positronium have negative mass?
Positronium is an atom with one electron and one positron. It's mass is 1.022MeV which is almost twice the electron mass: The ground-state (1S orbital) binding energy of -6.8eV reduces the total mass ...
- 6,531
1
vote
0
answers
61
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Why Quantum Field Theory series expansion is called Asymptotic? [duplicate]
I once saw the Freeman Dyson's article, referring the Expansion series occurring in Quantum Field Theory is asymptotic in nature;
It will deviate from actual physical phenomena when too many terms are ...
- 321
1
vote
1
answer
476
views
Feynman rules for scalar QED [closed]
Can i have a derivation for the Feynman rules of scalar QED?
I don't understand the ones i find online such as the one in the link https://canvas.harvard.edu/files/936391/download?download_frd=1&...
- 11
20
votes
1
answer
2k
views
What theory are we actually working in when "using" QED?
My understanding of QED is roughly that (please correct me if I have made a mistake, I am not very experienced in QFT):
Firstly, QED is not exact. At a certain energy scale, we have electroweak ...
- 301
2
votes
2
answers
177
views
How can QFT perturbation be used for electron-positron scattering?
I'm studying scattering and perturbation theory in QFT from Peskin and Schroeder book. After all the calculations and theory developing, they made a calculation for $$e^-e^+\to\mu^-\mu^+$$ scattering. ...
- 97
0
votes
2
answers
201
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Two question about evaluate electron self-energy in Peskin & Schroeder Book Charpter 7.1
(I attached the e-book link beneath)
First question is on P.220 the equation (7.27):
$$\delta m=m-m_0=\Sigma_2(p\!\!/=m)\approx\Sigma_2(p\!\!/=m_0).\tag{7.27}$$
Why taking $\Sigma_2(p\!\!/=m)\approx\...
1
vote
0
answers
210
views
Wick's theorem on a four-point QED Green function in 2nd-order perturbation theory
Problem 13.1 in Mandl & Shaw's QFT. I need to calculate the second order contributions to the four point Green function
$$ \langle A^{\mu}(x_1)A^{\nu}(x_2)\psi(x_3)\bar{\psi}(x_4) \rangle, $$
...
- 311
2
votes
0
answers
187
views
Bremsstrahlung in peskin and schroder
I'm studying chapter 6 of Peskin and Schroeder on bremsstrahlung. Here is the Feynman diagram on page 182
My questions are:
Why are there two photon lines attached to the dark disk? (I circled the ...
- 789
0
votes
1
answer
268
views
Feynman diagram with different particles
I'm studying Peskin and Schroeder chapter 5. At the beginning of section 5.1, the book tries to compute S matrix of $e^+e^-\rightarrow \mu^+\mu^-$. Using the Feynman from section 4.8, we can draw a ...
- 789
4
votes
2
answers
1k
views
I don't understand Dyson's argument for divergence of perturbative QED
Dyson's argument that the perturbative expansion for QED must diverge:
[...] let
$$F(e^2)=a_0+a_1e^2+a_2e^4+\ldots$$
be a physical quantity which is calculated as a formal power series in $e^2$ by ...
3
votes
1
answer
154
views
When can you trust first-order QED?
In QED, we use the perturbation expansion. Usually, we use the first-order perturbation expansion, and sometimes the second. I want to know, physically, what conditions need to be satisfied for us to ...
- 188