All Questions
Tagged with quantum-chromodynamics quantum-electrodynamics
59
questions
2
votes
1
answer
280
views
Is there a simple explanation of why coupling constants run with $\log(E)$?
The inverse coupling constants run with $\log(E)$, where $E$ is the energy or four-momentum.
Some coupling constants increase, some decrease with $\log(E)$.
Is there a simple argument that explains ...
user85598
1
vote
0
answers
46
views
Extensions of the BCFW recursion relations
In their "original" form the BCFW recursion relations are used to calculate tree-level (color-ordered) $n$-gluon amplitudes. However is it possible to extend this formalism to more general ...
- 387
0
votes
0
answers
41
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Structure function in DIS calculation step
There is one step in the calculation that I am not understanting, but there are many things to present so one can understand and help me:
i)$k^2$ and $k_T^2$ are small, so can be neglected;
ii)$k^\mu=\...
1
vote
1
answer
131
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What happens to an electron when it radiates a photon?
I recently came across this Feynman diagram:
For a more simplistic diagram, I suppose even this would be adequate:
As you can see in these diagrams, they radiate these virtual photons. The virtual ...
- 126
2
votes
1
answer
80
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Is Compton Scattering the "Abelian limit" of $qg \rightarrow qg$?
I have calculated the average over initial and sum over final states of the squared amplitude for both Compton scattering $e^-\gamma \rightarrow e^-\gamma$ (QED) and quark-gluon scattering $qg \...
- 321
0
votes
0
answers
49
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$e^+e^- \to p^+p^-$ cross-section?
Does anyone know of a calculated or measured cross-section somewhere for $e^+e^-$ annihilation into a proton-antiproton pair, say at the LEP? Seems to me that it would be rather strongly energetically ...
- 417
0
votes
1
answer
271
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How do I compute the amplitude for this QCD diagram?
Studying the following scattering process at a tree-level:
$$\bar{q}^i (p_a) + q^j (p_b) \to \gamma(k_1) + \gamma(k_2)$$
Considering the “reduced” amplitude that is obtained by stripping away the
...
- 753
2
votes
0
answers
155
views
Can one distinguish QED (or QCD) from QED (or QCD) plus higher field powers?
The Lagrangian of QED is based on minimal coupling; it contains a term proportional to $F_{\mu\nu}F^{\mu\nu}$ for the electromagnetic field.
The QED Lagrangian is the simplest Lagrangian with U(1) ...
- 684
3
votes
0
answers
305
views
Is it a coincidence that the maximum number of protons in a stable nucleus is on the order of $\alpha_s / \alpha_{EM}$?
The equivalent to the fine-structure constant for the strong interaction is about $\alpha_s \approx 0.5$ at the proton rest mass energy (and it runs pretty slowly with energy, so it's on this order ...
- 48.4k
1
vote
0
answers
96
views
What is the real problem in the free body problem?
Ions are produced when an EM neutral atom gains EM charge by gaining or losing electrons, by collision with other charged particles or photons.
The study of such collisions is of fundamental ...
- 29k
2
votes
1
answer
814
views
Topological theta-term as a background electric/magnetic field?
The topological $\theta$-term in the Schwinger model (1+1-dimensional QED) can be interpreted as a background electric field, as explained in Chapter 7.1.2 of Tong's lecture notes. The same holds true ...
- 1,783
5
votes
3
answers
5k
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Most trivial neutral pion decay
Literature states neutral pion decay by QED cannot occur directly because the pion is uncharged.
However, I cannot see why Photons are not allowed to interact with the Quarks directly.
Please ...
- 59
2
votes
1
answer
461
views
Why is the fine structure constant the probability for photon emission by a charged particle?
I see in page 31 of Martin, Hanzel "Quarks and Leptons", that the fine structure constant is the probability for photon emission by a charged particle. Also I read from Lubos Motl's answer on this ...
- 700
2
votes
0
answers
106
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How to get group U(1) from SU(N)?
I have read that the unitary group is somehow given by the direct product $U(N)=U(1)*SU(N)$ and it follows that for $N$ going to zero we get just $U(1)$.
How it can be possible? What does it mean $SU(...
- 479
3
votes
1
answer
692
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Wilson Loops and Confinement in QED
In [1] Kenneth G. Wilson proposed a mechanism for confinement using lattice paths what leds him to the concept of Wilson loop. It seems to me that he is using mainly a single abelian field. He says
...
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